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Computer Network
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CHAPTER 1
INTRODUCTION
Networks
A network is a set of devices (often referred to as nodes or station) connected by
communication links. A node can be a computer, printer or any other device capable of
sending and/or receiving data generated by other nodes on the network.
Goals of Computer Networks
Resource sharing
All programs, remote equipments and especially data can available to anyone on the
network. We can share printer, floppy drive, CD ROM drive, CD writer, hard disk etc. that
are attached to the network.
High reliability
Files could be replicated on two or three machines, so if one of them is unavailable (due to
some hardware breakdown), the other copies of the files could be used. On the other word,
the presence of multiple machines means that if one of them is failed, the other machines
can take over its work. This goal is most important, since hardware failure is one of the
most common real-life problems.
Saving Money
Microcomputers have a much better cost/performance ratio than large computers. That is
why most of the system designers prefer to build client-server model consisting of
microcomputers. In client-server model, data stored on one or more shared file server
machines and others are called client machines. Typically there are many clients using a
small number of servers.
In this model, communication takes form of a request message from the client to the server
asking for some job to be done. Then the server does the work and sends back the reply to
the client.
Client machine
Server machine
Communication Medium
A computer network can provide a powerful communication medium among widely
separated people. By using network one can easily communicate to another, who lives far
apart. One user can send a mail or email to another and can get reply immediately. This
technology makes it possible to have virtual meetings, which called videoconference,
among far-apart people. Anyone can access to information system about the arts, business,
cooking, government, health, history, hobbies, recreation, science, sports, travels and too
others topics. Many people can pay their bills; manage their bank account, download
software and also can shopping by online.
Scalability
To increase system performance as the workload grows, just adding more computers. In
client-server model new clients and new servers can be added as needed. Where as in
mainframe, when the system is full then it must be replaced by larger one, which is too
expensive.
Components of Network
1.
2.
Networks consist of many components; they are divided into two groups:
Hardware Components.
Software Components.
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Hardware Components
The basic hardware components of a network include three types of devices:
1. Transmission Media (Medium). Transmission media is a physical path by which a
message transfers from sender to receiver.
2. Access Devices. A device that can accept transmitted data in the network and also can
places data on the network.
3. Repeater. A Repeater is a device that accepts transmitted signals, amplifies them and
puts them back on the network.
The software components required in a network include the following:
1. Protocol. A protocol is asset of rules that controls data communication.
2. Device Driver. A device driver is a hardware-level program that controls a specific
device.
3. Communication Software. Communication software makes the available network
bandwidth actually usable.
Elements of a communication system
A typical communication system consists of an information source and a sink connected
by a communication system that transfers messages from transmitter to receiver over a
communication channel. As the raw message is unsuitable for direct transmission it is led
as input to a transmitter. The transmitter performs necessary signal processing such as
encoding and modulation, so that make it suitable for transmission. The function of the
transmitter is to couple the message onto the transmission channel in a form that matches
the transfer characteristics of the channel best.
Information
Source
Information
Sink
Transmitter
Receiver
Encoding &
Modulation
Decoding &
Demodulation
Channel
Noise
Block diagram of a typical communication system
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The communication channel is the path or medium for electrical or electromagnetic
transmission between transmitter and receiver. The transmitting message recovered by
receiver as output. The receiver performs demodulation and decoding.
Encoder (or simply coder) and decoder usually come as a single unit called CODEC.
During forward information flow (i.e. transmission) it acts as COder, while during reverse
flow (i.e. reception) it performs the function of a DECoder and hence the name. For analog
channel, mapping is required at the transmitter to convert digital signals into a suitable
waveform by modulation and back-mapping is required at the receiver to reconvert the
received waveform into digital data by demodulation. The respective modules are known
as modulation and demodulation and it perform by MOdulator and DEModulator
respectively which collectively called MODEM. Modem is the device responsible for
allowing a digital signal to be carried over an analog channel. It performs modulation at
the sending end and demodulation at the receiving end.
A complete block diagram of a data communication system
Classification of Networks
According to Transmission technology, there are two types of networks 1.
Broadcast Networks.
2.
Point-to-Point Network.
Broadcast Network
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Broadcast Networks have a single communication channel that is shared by all the
computers on the network. When a computer sends any message then it received by all the
others computers on the network. An address field within the massage specifies for whom
it is sent. If the message is sent for itself, it processes the message; if the message is
intended for some other computer, it is just ignored.
Broadcast Networks can be divided into static and dynamic, depending on how the channel
is allocated. In typical static allocation, time would be divided into discrete intervals and
run a round robin algorithm to allow each computer broadcast only when its time slot
comes up. Static allocation wastes capacity when a computer has nothing to transmit during
its allocated slot. In dynamic channel allocation, methods for a common channel are
centralized and decentralized. In centralized channel allocation method, there is a central
entity which determines who transmit next. In decentralized channel allocation method,
each computer decides for itself whether or not to transmit; there is no central entity.
Point-to-Point Network
Point-to-Point Networks consist of many connections between individual pairs of
computers. When a computer sends any message on this type of network then it may have
to first visit one or more intermediate computers. Often, multiple routes, of different
lengths are possible and therefore routing algorithms play an important role in this type of
network.
Network categories
Network categories can be specified based on size, ownership, the distance it covers and
its physical architecture. According to scale there are mainly three types of networks 1.
Local Area Network (LAN).
2. Metropolitan Area Network (MAN).
3. Wide Area Network (WAN).
Distance
10 m
100 m
1 km
10 km
100 km
1000 km
Location
Room
Building
Campus
City
Country
Continent
Example
Local Area Network
dodoMetropolitan Area Network
Wide Area Network
do-
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Local Area Network (LAN)
A Local Area Network is usually privately owned and
links in a single room, building or campus covering a
small geographical area (up to few thousand meters).
LANs connect workstations, peripherals, terminals and
other devices on the network. LANs run at high speed,
typically at 10 to 100Mbps (1 Mb equal to 1,000,000
bits) and at very low error. LANs Different types of
topologies are possible for LANs, like bus topology,
ring topology, star topology etc. LANs are distinguished from other types of networks by
their size, transmission technology and their topology.
However, since all the equipments are located within a single establishment, LANs
normally installed and maintained by the organization. Hence LANs are also referred as
private data networks.
(a)
(b)
(c)
Some topologies (a) Bus, (b) Ring, (c) Star
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•
•
•
LAN has no repeater/amplifier.
LANs can run at high speed and at very low error.
LANs are distinguished from other kinds of networks by their size, transmission
technology and their topology.
Metropolitan Area Network (MAN)
A Metropolitan Area Network (MAN) is designed to extend over an entire city. MAN is
basically a bigger version of a LAN and normally uses similar technology. It may be a
single network such as a cable television network or it may be means of connecting a
number of LANs into a large network so that resources may be shared LAN-to-LAN as
well as device-to-device. The high speed links between LANs within a MAN are made
possible by fiber-optic connections.
Metropolitan Area Network uses a standard called DQDB (Distributed Queue Dual Bus).
DQDB consists of two unidirectional buses (cables) to which all the computers are
connected, as shown in following figure. Each bus has a device that initiated transmission
activity, is called head-end. To transmit a message, a computer has to know whether the
destination is to the left of it or to the right of it. If the destination is to the right, the sender
uses the upper bus A and if the destination is to the left sender uses the lower bus
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used for user programs. The hosts are connected by a communication subnet or subnet.
The subnet carries messages from host to host. The subnet consists of two different
components:
1. Transmission Lines.
2. Switching Elements.
Transmission lines travel messages between hosts. The switching elements are expert
computers used to connect two or more transmission lines. These computers may be called
as router.
• WAN contain switching elements.
Direction
of Data flow
According to the direction of data flow communication between two devices; the data
transmission modes can be three types:
1. Simplex.
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2. Half-duplex.
3. Full-duplex.
Simplex
In simplex mode communication data can be transmitted in only one direction. That means
the communication is unidirectional. Only one of the two devices on a link can transmit
data, the other can only receive. Keyboards and monitors, mouse are examples of simplex
devices. The keyboard and mouse can only sent input; the monitor can only accept output.
Device
Device
Direction of data
Simplex
Half-Duplex
In half-duplex mode communication data can be transmitted in both direction, but not at
the same time. Each device can transmit and receive data; when one device is sending data,
the other can only receive and when second device is sending then first one can only
receive. In a half-duplex transmission, the entire capacity of a channel is taken over by
whichever of the two devices is transmission at the time. Walkie-talkies and radios are
examples of half-duplex devices.
Device
Device
Direction of data at time 1
Direction of data at time 2
Half-duplex
Duplex
In full-duplex (also called duplex) mode communication, both stations can transmit and
receive data simultaneously. In full-duplex mode, signals going in either direction share
the capacity of the channel. This sharing can occur in two ways: either the channel must
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contain two physically transmission paths, one for sending and the other for receiving; or
the capacity of the channel is divided between signals traveling in both directions. The
telephone network is an example of full-duplex communication. When two people are
communicating by a telephone line, both can talk and listen at the same time.
Device
Device
Direction of data at same time
Full-duplex
Data Transmission Modes
The transmission of binary data across a link can be able in serial mode and parallel mode.
In serial mode, 1 bit is sent with each clock tick; there are two types of serial transmission:
synchronous and asynchronous. In parallel mode, multiple bits are sent with each clock
tick.
Serial Transmission
In serial transmission data are transmitted one bit at a time over a single link. Since
communication within devices is parallel, so in serial transmission, parallel word should
be converted into serial bits at the sender end: this is known as parallel-to-serial conversion.
On the other hand at receiver end, serial bits should be converted into parallel word: this is
known as serial-to-parallel conversion.
•
•
•
Require only one communication channel.
Used in low speed data transmission.
Serial-to-parallel and parallel-to-serial conversion devices are required.
Serial transmission can be two types:
1. Synchronous Transmission
2. Asynchronous Transmission
Synchronous Transmission
In synchronous transmission, bits are sent one after another without start or stop bits or
gaps. It is the responsibility of the receiver to group the bits.
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Direction of data flow
00010000 11110111 11110110 11111011 11011
Sender
Receiver
Synchronous transmission
•
•
•
Generally use in high-speed data transmission.
This type of data transmissions occurs without gaps and starts or stop bit.
Regrouping the bits into meaning bytes is the responsibility of the receiver.
Asynchronous Transmission
In synchronous transmission, we send one start bit (0) at the beginning, followed by a byte
and one or two stop bits (1) at the end of each byte. There may be a gap between each byte.
This is also known as framing.
•
•
•
Generally use in low speed data transmission.
Send one start bit (0) at beginning of the byte and one or two stop bits (1) at end of each
byte.
There are variable–length gaps between each byte.
Direction of data flow
Stop bit
Data
Start bit
1 00010111 0
1 11111011 0
1 00010111 0
Sender
Gaps between data units
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Receiver
1
10011011
0
Asynchronous transmission
Parallel Transmission
In parallel transmission, a group of bits is sent simultaneously, with each bit on a separate
line. The advantage of parallel transmission is high-speed data transmission. In parallel
transmission required multiple wire to transmit data stream, what is why it is expensive,
parallel transmission is typically used at short distance.
In parallel transmission the entire stream is transmitted at one time.
• Use n communication channel to send n bits at one time.
• Parallel transmission is use at short distance.
• Used in high-speed data transmission.
• As there are required multiple lines, it is expensive.
Sender
1
0
1
1
0
0
0
1
Receiver
Eight lines are needed to
send eight bits together.
Parallel transmission
Baseband and Broadband Transmission
Bandwidth use refers to the ways of allocating the capacity of transmission media. The
total capacity or bandwidth can be divided into channels. A channel is simply a portion of
the bandwidth that can be used for transmitting data. The two ways of using the bandwidth
of transmission media are following:
1. Broadband.
2. Baseband.
Baseband Transmission
These transmissions use the entire bandwidth for a single channel. In baseband
transmission digital signal transmit at its original frequency without modulation. Baseband
is commonly used for digital signaling, but it can also used for analog signal. Signals flow
in the form of discrete pulses of electricity or light. Most Local Area Networks use
baseband signaling.
• Baseband transmission use digital signal over a single frequency.
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•
•
•
•
•
•
Signals flows in the form of discrete pulses of electrical or light.
The digital signal uses the complete bandwidth of a single channel.
Baseband systems use repeater.
No modem is required.
Used for a short distance.
Commonly use in Local Area Networks.
Broadband
These transmissions provide the ability to divide the entire media bandwidth into multiple
channels. Since each channel can carry a different analog signal, broadband networks
support multiple simultaneously conversations over a single transmission medium. Digital
signals are used to modulate a carrier signal whose frequency must be within the bandwidth
(BW) of the channel. While Baseband systems use repeater, broadband systems use
amplifiers to regenerate analog signals at their original strength. Since broadband signal
flow is unidirectional, there must be two paths for data flow in order for a signal to reach
all devices. Used for long distance communication using switching or leased lines from
public carriers.
•
•
•
•
•
•
Broadband systems use analog signaling and a range of frequencies. The signals are
continuous and discrete.
Signal flows across the physical medium in the form of electromagnetic or optical
waves.
Broadband signal flow is unidirectional. Therefore there must be to paths for data flow
in order for a signal to reach all devices.
Broadband systems use amplifiers to regenerate analog signals at their original
strength.
Always used in WANs.
Also used for some LANs based on cable TV technology.
Connection of Networks
A network is a two or more device connected together through links. Imagine any link as
a line drawn between two points. For communication to occur, two devices must be
connected in some way to the same like at the same time.
There are two possible types of connections:
1. Point-to-point.
2. Multipoint or Multidrop.
Point-to-point
A point-to-point connection provides a dedicated link between two devices. In pointtopoint connection, the capacity of the channel is reserved for the transmission between
those two devices. Point-to-point connections may use wire or cable but microwave or
satellite links are also possible.
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Computers
Link
Point-to-point connection
Multipoint
A multipoint (also known as Multidrop) connection is one in which more than two devices
share a link. In multipoint connection, the capacity of the channel is shared. When several
devices use the channel simultaneously, it is called spatially shared connection. When the
devices take turns, it is called timeshare connection.
Computers
Link
Multipoint connection
Network Architecture
Most of the networks are organized as a series of layers or levels. The number of layers,
the name of each layer, the contents of each layer and the function of each layer vary from
network to network. A five-layer network is illustrated in following figure.
A protocol is a set of rules and conventions that controls data communications. It
represents an agreement between the communication devices. Without a protocol, two
devices may be connected but not communicating themselves. A protocol defines what is
communicated, how it is communicated and when it is communicated. The key elements
of a protocol are syntax, semantics and timing.
Syntax
Syntax refers to the structure or format of the data, meaning the order in which they are
presented.
Semantics
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Semantics refers to the meaning of each section of bits. How is a particular pattern to be
interpreted and what action is to be taken on that interpretation?
Timing
Timing refers two characteristics: when data should be sent and how fast they can be sent.
The active elements in each layer are called entities. An entity is anything capable of
sending or receiving information. The entities must be agreed on a protocol.
The entities of the corresponding layers on different hosts are called peers. The peers are
communicating using protocol. The processes on each host that communicate at a given
layer are called peer-to-peer processes. Communication between different hosts is a peerto-peer process using protocols to a given layer.
No data are directly transferred from layer n on one host to layer n on another host. In fact,
each layer passes data and control information to its below layer, until the lowest layer is
reached. Below layer 1 is the physical medium through actual communication occurs.
The passing data and network information down through the layers of the sending device
and back up through the layers of the receiving device is made possible by an interface in
between each pair of adjacent layers. Each interface defines what information and services
a layer must provide for the layer above it.
A set of layers and protocols is called network architecture.
The communication of the top layer of the five-layer network is as follows:
• A message M is produced by an application process running in layer 5 and given
to layer 4 for transmission.
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• Layer 4 appends a header in front of the message to identify the message and
passes it to layer 3. The header includes control information, such as sequence numbers
to allow layer 4 on the destination host to deliver message in the right order (if the
lower layers do not maintain sequence). Headers may contain sizes, times and other
control fields.
• Layer 3 splits up the incoming messages into smaller units, called packets and puts
a layer 3 header to each packet.
• Layer 3 decides which of the outgoing lines to use and passes the packets to layer
2.
• Layer 2 adds a header to the front of each packet and also adds a trailer to the end
of each packet. Then layer 2 gives to the resulting unit to layer 1 for physical
transmission.
• At receiver host the message moves upward, from layer to layer, with headers being
stripped off as it progresses. None of the headers for layers below are passes up to layer
n.
Layer
Source Host
Destination Host
Interfaces and Services
The active elements in each layer are called entities. An entity can be a software entity or
a hardware entity. For example a process is a software entity where as an intelligent I/O
chip is a hardware entity. Entities in the same layer on different hosts are called peer
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entities. The entities in layer n implement a service that is used by layer n+1. In this case
layer n is called the service provider and layer n+1 is called the service user. Layer n may
use the services of layer n-1 in order to provide its service.
Layer N+1
ICI IDU
SDU
SAP
Interface
Layer N
ICI
SDU
SDU
Header N-PDU
SAP - Service Access Point
IDU - Interface Data Unit
SDU – Service Data Unit
PDU – Protocol Data Unit
ICI – Interface Control Information
OSI Reference Model
The OSI model is based on a proposal developed by International Standards Organization
as a first step towards international standardization of the protocols used in the various
layers. The model is called the ISO OSI (International Standard Organization Open
Systems Interconnection) Reference Model; it deals with connecting open systems-that
is systems that are open for communication with other systems. It is a seven-layer model.
Its main objectives are to:
1. Allow manufactures of different systems to interconnect equipment through
standard interfaces.
2. Allow software and hardware to integrate well and be portable on different systems.
The principles of OSI model are as follows:
1. A layer should be formed where a different level of abstraction is required.
Layer N+1
ICI IDU
SDU
SAP
Interface
Layer N
ICI
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SDU
SDU
Header N-PDU
2. Each layer should execute a well-defined function.
3. The function of each layer should be chosen with an eye toward defining
internationally standardized protocols.
4. The layer boundaries should be chosen to minimize the information flow across the
interfaces.
5. The layers should be large enough that different functions need not be put together
in the same layer out of requirement, and small enough that architecture does not
become cumbersome.
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The seven layers of ISI OSI reference model are1. Physical Layer.
2. Data Link Layer.
3. Network Layer.
4. Transport Layer.
5. Session Layer.
6. Presentation Layer.
7. Application layer.
Physical Layer
Physical layer is the bottom layer of the OSI Reference Model. The physical layer is
responsible for transmission of the bit stream. It accepts frames of data from data link layer
and transmits over a communication channel, one bit at a time. This layer is also
responsible for the reception of incoming bit stream. These streams are then passes on the
data link layer for reframing.
For transmission physical layer must perform the following tasks:
• Convert framed data from data link layer to a binary stream.
• Transmit data as a binary stream; that is one bit at a time.
For reception physical layer must perform the following tasks:
• Accept appropriately addressed streams.
• The binary stream pass up to the data link layer for reform into frames.
The physical layer is taking cared to transmitting raw bits over a communication channel.
It makes sure that when a transmitter sends a 1 bit then it is received by the receiver as a 1
bit, not as a 0 bit.
Physical layer defines electrical and mechanical specifications of cables, connectors and
signaling options that physically link two nodes on a network.
The main tasks of the physical layer are to provide:
• The transmission rate is defined by physical layer. That means this layer defines
the number of bits sent each second (Data rate).
• The physical layer defines the type of representation; how 0s and 1s are changed
into signals-electrical or optical (Representation of bits).
• The physical layer defines the characteristics of the interface between devices and
the transmission media. It also defines the type of transmission medium.
• The sender and receiver must use the same bit rate and their clocks must be
synchronized at the bit level.
• The physical layer also defines line configuration, physical topology, transmission
mode.
Data Link Layer
Data link layer is the second layer of the OSI Reference Model. The data link layer is
responsible for reassembling any binary streams received from the physical layer back into
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frames. The data link layer is also responsible for detecting and correcting any and all
errors.
The main tasks of the data link layer are to provide:
• Transferring data from the sending network layer to the receiving network layer
(servicing provided to the network layer).
• Data link layer implements an addressing system to handles the addressing problem
locally. This layer adds a header to the frame to defining the sender and receiver of
the frame (Physical addressing).
• Accept the bit stream from physical layer and packing into frames (Framing).
• Error detection and error correction of an erroneous frame (Error control).
• No data send faster than the receiver can handle the traffic (Flow control).
• When two or more devices are connected to the same link then it is necessary to
determine which device has control over the link at any given time (Access control).
The main task data link layer is to provide error free transmission. The sender break the
input data into data frames, transmit the frames sequentially and process the acknowledge
frames sent back by receiver. If there is an erroneous frame then data link layer protocol
can retransmit the frame.
Network Layer
The network layer is the third layer of the OSI Reference Model. The network layer is
taking cared to controlling the operation of the subnet. This layer is responsible for
establishing the route to be used between source and destination computers and take
responsible for the source to destination delivery of a packet possibly across multiple
networks. The network layer ensures that each packet gets from the original source to the
final destination.
The main tasks of the network layer are to provide:
• The connecting devices (called routers or switches) route or switch the packets
from the source host to the destination (Routing).
• When too many packets are present in the subnet at the same time, performance
degrades. This situation is called congestion. This layer control of such congestion
(Congestion control).
• The network layer implements an addressing system to help distinguish the source
and destination systems. This layer adds a header to the packet coming from the
transport layer and includes the logical addresses of the sender and receiver
(Logical addressing).
Transport Layer
The transport layer is the forth layer of the OSI Reference Model. The basic function of the
transport layer is to accept data from session layer and split up into smaller packets then
pass to the network layer; ensure that all of them arrive correctly at the other end. The
transport layer is responsible for end-to-end integrity of transmissions. The transport layer
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can detect packets that are discarded by routers and automatically generate a retransmission
request.
The main tasks of the transport layer:
• The transport layer header must include port address. The network layer gets each
packet to the correct computer; the transport layer gets the entire message to the
correct process on that computer (Port addressing).
• A message is divided into transmittable segments, each segment containing a
sequence number. These numbers enable this layer to reassemble the message
correctly (Segmentation and reassembly).
• The transport layer can be either connectionless or connection-oriented. A
connection-oriented transport layer makes a connection with the transport layer at
the destination computer and then delivering the packets. A connectionless
transport layer delivering each segment as a packet to the transport layer at the
destination computer (Connection control).
• This layer is responsible for flow control, which is performed end to end rather than
single link (Flow control).
• This layer is responsible for error control, which is performed end to end rather
than single link (Error control). The sending transport layer ensures that the entire
message arrives at the receiving transport layer without an error. For errors it
generates retransmission request.
Session Layer
The session layer is fifth layer of the OSI Reference Model. The function of session layer
is to manage the flow of communications during a connection between two computer
systems. It determines whether communications can be unidirectional or bidirectional. It
also ensures that one request is completed before a new one is accepted.
The main tasks of the session layer are to provide:
• Session Establishment
• Session Release
• Data Exchange
• Expedited Exchange
One of the services of the session layer is token management. It is ensure that both sides
do not attempt the same operation at the same time. There is a central entity called as token
which determines who transmit next. This token passes in round-robin fashion from side
to side and provides the transmission right among one of them. Suppose a side has data for
transmission, it can transmit the data only at the time when it gets the token and after
transmission it released the token.
Another session service is synchronization. When a computer trying to do a two-hour file
transfer to another computer with a one-hour mean time between crashes then after each
transfer is abort, the total transfer would have to start over again. And probably fail again
the next time as well. To solve the problem, the session layer provides a way to insert
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checkpoints into the data stream, so that after a crash, only the data transferred after the
last checkpoint have to be retransmitted.
Presentation Layer
The presentation layer is layer 6 of the OSI Reference Model. The presentation layer handle
the syntax and semantics of the information exchanged between two systems. This layer is
designed for data translation, encryption, decryption and compression. The presentation
layer is responsible for managing the way data is encoding. Not every computer system
uses the same data-coding scheme, As different computers have different codes for
presenting character strings (e.g. ASCII and Unicode), integers (e.g. one’s complement and
two’s complement) and so on, the presentation layer make it possible to for computers with
different representation to communicate.
At the sending computer, this layer translates data from a format sent by application layer
into a commonly recognized intermediary format. At the receiving computer, the
presentation layer translates the intermediary format into a format useful to that computer’s
application layer.
The presentation layer is also responsible for protocol conversion, translating the data,
encrypting the data, changing or converting the character set, and expanding graphics
commands. This layer also manages data compression to reduce the number of bits that
need to be transmitted.
A utility known as the redirector operates at this layer. The purpose of the redirector is to
redirect input/output operations to resource on a server.
The main tasks of the presentation layer are to provide:
• Translate data to a format that the receiving node can understand. For example from
EBCDIC to ASCII (Translation).
• Performs data encryption. Encryption means that the sender transforms the original
information to another form. Decryption transforms the message back to its original
form.
• Performs data compression. Data compression reduces the size of the information.
It is important in the transmission of multimedia.
Application layer
The application layer is the top layer of the OSI Reference Model. Application layer
provides the interface between applications and network’s services. Application layer
supports functions that control and supervise OSI application processes such as
start/maintain/stop application; allocate/de-allocate OSI resources, accounting, check point
and recovering. It also supports remote job execution, file transfer protocol, message
transfer and virtual terminal.
The main tasks of the application layer:
• Allows a user to access files in a remote host, to retrieve files from a remote host
for use in the local computer and manage or control files in a remote host locally (file
transfer and access).
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• Allows a user to log into a remote computer and access the different resources of
that computer (remote log-in).
• Mail services (e-mail forwarding and storing) and accessing the World Wide Web.
• Directory services provide distributed database sources and access for global
information about various objects and services.
CHAPTER 2
COMMUNICATION
Signal
Electromagnetic waves propagated along a transmission medium are called signals. Signals
can be two types i) Analog signal and ii) Digital signal.
Analog and Digital Signals
An analog signal has infinitely many levels of intensity over a period of time. As the wave
moves from value A to value B, it passes through and includes an infinite number of values
along its path.
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On the other hand, a digital signal can have only a limited number of defined values, often
as 1 or 0. The digital signal is a discrete signal with a limited number of values.
Periodic and Aperiodic Signals
A periodic signal complete a pattern within a measurable time frame called a period and
repeats that pattern over subsequent identical periods. The completion of one full pattern
is called cycle.
An aperiodic or nonperiodic signal changes without exhibiting a pattern or cycle that
repeats over time.
Analog and digital signals both ban be periodic or aperiodic. In data communication we
typically use periodic analog signals and aperiodic digital signals to sent data from one
point to another.
Analog Signal
Analog signal can be classified as
i.
Simple analog signal.
ii.
Composite analog signal.
A simple analog signal is a sine-wave, cannot be decomposed into simpler signals. A
composite analog signal is composed of multiple sine-waves.
Sine Wave
The sine wave is the most fundamental form of a periodic analog signal.
Mathematically describe a sine wave is s (t) =A sin (2π ft+φ ) where s
is the instantaneous amplitude
A is the peak amplitude,
f
is the frequency,
φ is the phase.
Characteristics of sine wave
i.
ii.
iii.
Amplitude.
Frequency.
Phase.
Peak amplitude
The peak amplitude of a signal represents the absolute value of its highest intensity,
proportional to the energy its carries. For electrical signals, peak amplitude is normally
measures in volts.
25
1s
t
T
Period and Frequency
Period refers to the amount of time, in seconds, a signal needs to complete one cycle.
Frequency refers the number of periods in one second. Period is the inverse of frequency
and frequency is the inverse of the period. f=1/T and T=1/f
Period is formally expresses in seconds and frequency is expressed in hertz (Hz). Phase
Phase describe the position of the waveform relative to time zero. Phase
is measured in degree or radians.
The Maximum Data Rate of a Channel
Noiseless Channel: Nyquist Data Rate
H. Nyquist proved that if an arbitrary signal has been run through a low-pass filter of
bandwidth H, the filtered signal can be completely reconstructed by making only 2H
samples per second.
If the signal consists of V discrete levels (number of signal levels used to represent data),
for a noiseless channel of H bandwidth, Nyquist’s theorem states: Maximum data rate = 2
H log2V bits/sec
Example:
A noiseless 5-kHz channel is transmitting a signal with two signal levels. The maximum
bit rate can be calculated as:
Maximum data rate = 2 × 5000 × log22 = 10000 bps.
We can say that the channel cannot transmit two-level signals only at a rate 10,000 bps and
not exceeding this rate.
Noisy Channel: Shannon Data Rate
26
The amount of thermal noise is measured by the ratio of the signal power to the noise
power, called the signal-to-noise ratio. For a noisy channel, the signal power denoted by S,
the noise power by N and the signal-to-noise ratio is S/N (or SNR). Typically, the ratio is
present in quantity 10 log10 S/N. These units are called decibels (dB). An S/N ratio of 10 is
10 dB, a ratio of 100 is 20dB, and a ratio of 1000 is 30dB and so on. Shannon’s
the
maximum data rate of a noisy channel whose bandwidth is H Hz and whose signal-to-noise
ratio is S/N is given by Maximum data rate = H log2 (1+S/N)
Example:
A 5-kHz channel is transmitting a signal with two signal levels and signal-to-noise ratio of
30 dB. The maximum bit rate can be calculated as:
Maximum data rate = 5000 × log2 (1+1000) = 5000 × 9.967 = 49836 bps. We can
say that the channel never transmit much more than 50,000 bps, no matter how many signal
levels are used and no matter hoe often samples are taken.
Line Coding
Line coding is the process of converting binary data, a sequence of bits, to digital signals.
For example, data, text, image, audio and video that are stored in computer memory are all
sequence of bits.
01011101
Line
Coding
Pulse Rate
The Pulse rate defines the number of pulse per second. If a pulse carries only 1 bit, the
pulse rate and the bit rate are the same. If the pulse carries more than 1 bit, then the bit
rate is greater than the pulse rate.
Bit Rate = Pulse Rate x log 2 L
where L is number of data levels of the signal.
Line Coding
Unipolar
Polar
Unipolar
Unipolar encoding uses only one voltage level.
Amplitude
27
Bipolar
0 1
0
1
1
0
1
Time
Polar
Polar encoding uses two voltage levels (positive and negative).
Polar
NRZ
NRZ-L
RZ
Manchester
Differential
Manchester
NRZ-I
NRZ (No return to Zero)
In NRZ encoding, the value of the signal is always either positive or negative.
NRZ-L (NRZ- Level):- In NRZ-L the level of the signal is dependent upon the state of bit.
A +voltage usually means the bit is 0 and –voltage means the bit is 1.
NRZ-I (NRZ- invert):- In NRZ-I the signal is inverted if a 1 is encountered. The change of
the voltage represents bit – 1 and no change represents bit- 0.
Amplitude
0 1
0
0
1
1
1
0
NRZ-L
Time
NRZ-I
Time
Return Zero (RZ)
In Return Zero encoding uses three values +, - and 0 values.
In RZ, the signal changes not between bits but during each bit.
Actually 1 = positive to zero.
0 = negative to zero.
28
A good digital signal must contain a provision for synchronization. The disadvantage
of RZ encoding is that it requires two signal changes to encode 1 bit and
therefore occupies more bandwidth.
Value
0
1
0
0
1
1
1
0
Time
Manchester Encoding
In Manchester encoding, the transition at the middle of the bit is used for both
synchronization and bit representation.
Amplitude
0
1
0
0
1
1
1
0
Time
Zero
One
Differential Manchester Encoding
In differential Manchester encoding, the transition at the middle of the bit is used only for
synchronization. The bit representation is defined by the inversion at the beginning of the
bit.
0
1
0
0
29
1
1
1
0
Amplitude
Time
Presence of transition at the beginning of the time means zero
Bipolar
In Bipolar encoding use three levels: +, 0 and - .
Amplitude
0
1
0
0
1
1
1
0
Time
The 1s are positive and negative alternately
Sampling
Pulse Amplitude Modulation (PAM)
One analog – to – digital conversion method is called pulse amplitude modulation. This
technique takes an analog signal, samples it, and generates a series of pulse based on the
result of the sampling. The term sampling means measuring the amplitude of the signal at
equal intervals.
PAM uses a technique called sampled and hold. At the given moment, the signal level is
read, and then held briefly. The sampled value occurs only instantaneously in the actual
waveform, but in generalized over a still short but measurable period in the PAM result.
The PAM is more useful to other areas of engineering than it is to data communication.
However PAM is the foundation of an important analog-to-digital conversion method
called pulse code modulation (PCM).
30
PAM Signal
Analog Signal
PAM
Pulse Code Modulation (PCM)
PCM modifies the pulses created by PAM to create completely digital signal. To do so,
PCM first quantizes the PAM pulses. Quantization is a method of assigning integral values
in a specific range to sampled instances.
The binary digits are then transformed to a digital signal by using one of the line coding
techniques. PCM is actually made up four separate processes: PAM, quantization, binary
encoding and Line coding. PCM is the sampling method used to digitize voice in T-line
transmission in the North American telecommunication system.
0001100001000…..
Binary data
Quantization
Analog Data
Quantized data
Line coding
PAM
127
-87
Digital data
Binary encoding
Sampled analog data
31
From analog signal to PCM digital code
Sampling Rate: Nyquist Theorem
According to the Nyquist theorem, the sampling rate must be at least 2 times the highest
frequency.
Sampling Rate (Nyquist Theorem) = 2 x fh where fh is the highest frequency
Modulation of analog signals
Modulation of an analog signal or analog-to-analog conversion is the representation of
analog information by an analog signal. Modulation is needed if the medium has a band
pass nature or if only band pass bandwidth is available to us. An example is radio. The
government assigns a Baseband bandwidth to each radio station. The analog signal
produced by each station is a low-pass signal, all the same range. To be able to listen to
different station, the low-pass signals need to be shifted, each to a different range. Modulate
is to mix a data signal onto a carrier and modify its characteristics for transmission in a
communication network.
Analog/analog
modulation
AM
FM
PM
Amplitude Modulation (AM)
In AM transmission, the carrier signal is modulated so that its amplitude varies with the
changing the amplitudes of the modulating signal. The frequency and the phase of the
carrier remain the same.
32
AM Bandwidth
The Bandwidth of an AM signal is equal to twice the bandwidth of the modulating signal
and covers a range centered on the carrier frequency .AM stations are allowed carrier
frequencies anywhere between 530 and 1700 KHz (1.7MHz). However, each station’s
carrier frequency must separate from those on either side of it by at least 10 KHz to avoid
interference. If one station uses carrier frequency 1100 KHz, the next station’s carrier
frequency can not be lower than 1110 KHz.
BWt=2 x BWm where
•
Amplitude modulation is easy to implementation.
33
•
•
•
It can be used both for analog and digital signal.
It is affected by the noise signal that may add up with the original signal.
The strength of the signal decrease with distance traveled.
Frequency Modulation (FM)
In FM transmission, the frequency of the carrier signal is modulated to follow the changing
voltage level (amplitude) of the modulating signal. The peak amplitude and phase of the
carrier signal remain constant, but as the amplitude of the information signal changes, the
frequency of the carrier signal changes correspondingly.
FM Bandwidth
The bandwidth of a FM signal is equal to 10 times the bandwidth of the modulating signal
and, like AM bandwidths, covers a range centered on the carrier frequency. FM stations
34
are allowed carrier frequencies any where between 88 and 108MHz. Station must be
separated by at least 200 KHz to keep their bandwidth.
BWt=10 x BWm where
• Frequency modulated wave is least effected by noise due to electrical disturbance.
• Frequency signal has a wide spectrum of frequencies and therefore needs much
•
higher bandwidth than amplitude modulation.
The number of FM signals is smaller than the number of AM signals one can
transmit over a channel with a fixed total bandwidth.
Modulation of Digital Data
Digital/analog
modulation
ASK
FSK
PSK
QAM
Bit Rate: - Bit rate is the number of bits transmitted during 1 second.
Baud rate: - Baud rate is the number of signal units per second.
Relation between bit rate and baud rate: - The baud rate is less than or equal to the bit rate
( baud rate<=bit rate ).
35
Bit rate equals the baud rate times the number of bits represented by each signal unit. The
baud rate equals the bit rate divided by the number of bits represented by each signal unit.
Example 1: An analog signal carries 4 bits in each signal unit. If 1000 signal units are sent
per second, find the baud rate and the bit rate.
Baud rate = number of signal units per second =1000 bauds per second.
Bit rate = baud rate X number of bits per signal unit = 1000 X 4 = 4000 bps.
Example 2: The bit rate of a signal is 3000. If each signal unit carries 6 bits, what is the
baud rate?
Baud rate = 3000 / 6 = 500 baud/s
Amplitude shift key (ASK)
In amplitude shift keying, the strength of the carrier signal is varied to represent binary 1
or 0 .Both frequency and phase of remain constant while the amplitude changes. Which
voltage represents 1 and which represent 0 are left to the system designer. The peak
amplitude of the signal during bit duration is constant, and its value depends on the bit (0
or 1). Unfortunately, ASK transmission is highly susceptible to noise interference. Noise
usually affects the amplitude; therefore, ASK is the modulation method most affected by
noise. A popular ASK technique is called on/off keying (OOK). In OOK one of the bit
values is represented by no voltage. The advantage is a reduction in the amount of energy
required to transmit information.
ASK.
Band width of ASK: - Band width requirements of ASK are calculated the formula
36
BW= (1+d) X Nbaud where BW= Bandwidth
Nbaud = Baud rate
modulation process (with minimum value 0)
d = it is a factor to related to
Relationship between baud rate and bandwidth in ASK
Frequency Shift Key (FSK)
In frequency shift keying, the frequency of the carrier signal is varied to represent binary
1 or 0. The frequency of the signal, during bit duration is constant, and its value depends
on the bit (0 or 1): both peak amplitude and phase remain constant. FSK avoids the most
of the problems from noise. Because the receiver looking for specific frequency changes
over a given number of periods. It can ignore the voltage spikes.
Band Width of FSK: - FSK shifts between two carrier frequencies, it is easier to analyze
as two coexisting frequencies .We can say that the FSK spectrum is a combination of two
ASK spectra centered on fc0 and fc1. The band width required of FSK transmission is
equal to the baud rate of the signal plus the frequency shift (difference between the two
carrier frequencies) BW= fc1-fc0+Nbaud
37
Relationship between baud rate and bandwidth in FSK
Phase Shift Key (PSK):- In phase shift keying, the phase of the carrier is varied to
represent binary 1 or 0. Both peak amplitude and frequency remain constant as the phase
changes. For example if , we start with phase of 0-degree to represent binary 0 ,then we
can change the phase to 180-degree to send binary 1.the phase of the signal during each
bit duration is constant , and its value depend on the bit (0 or 1)
2-PSK
PSK is not susceptible to the noise degradation that affects ASK or to the bandwidth
limitation of FSK. This means the smaller variations in the signal can be detected reliably
by the receiver .therefore instead of utilizing only two variations of a signal, each
representing 1 bit; we can use four variations and let each phase shift represent 2 bits.
4- PSK
38
Band Width of PSK: - The minimum band width required to PSK transmission is the same
as that required for ASK transmission.
Relationship between baud rate and bandwidth in PSK
Multiplexing
Multiplexing is the set of techniques that allows the simultaneous transmission of multiple
signals across a single data link. In a multiplexing technique system technique, n lines share
the bandwidth of one link.
39
Multiplexing
Analog
FDM
Digital
WDM
TDM
Frequency Division Multiplexing (FDM)
FDM is an analog technique that can be applied when bandwidth of a link is greater than
the combined bandwidths of the signals to be transmitted. In FDM, signals are generated
by each sending device module different carrier frequencies. These modulated signals are
then combined into a single composite signal that can be transported by the link. Carrier
frequencies are separated by sufficient bandwidth to accommodate the modulated signal.
Channels must be separated by strips of unused bandwidth (guard bands) to prevent signals
from overlapping.
FDM
Multiplexing Process
Below figure is a conceptual illustration of the multiplexing process. Each input devices
generated a signal. Inside the multiplexer, these signals are modulated onto different carrier
frequencies .The resulting modulated signals are then combined into a single composite
signal that is sent out over a media link that has enough bandwidth accommodate it.
40
Demultiplexing Process
In demultiplexer uses a series of filters to decompose the multiplexed signal into its
constituent composite signals .The individual signals are then passed to demodulator and
that separates them from their carriers and passes them to the waiting receivers.
A very common application of FDM is AM and FM radio broadcasting.
Wave Division Multiplexing (WDM)
WDM is designed to use the high data rate capability of fiber-optic cable. The optical fiber
data rate is higher than the data rate of metallic transmission cable. Using a fiberoptic cable
for one single line wastes the available bandwidth. Multiplexing allows us to connect
several lines into one. WDM is conceptually the same as FDM, except that the multiplexing
and demultiplexing involve optical signals transmitted through fiber-optic channels. The
idea is the same: we are combining different signals of different frequencies. However, the
difference is that the frequencies are very high.
Several input beams of light, each containing very narrow band of frequencies from
different sources are combined by multiplexer to make a wider band of frequencies. At the
receiver, the signals are separated by the demultiplexer.
WDM:
The technology of the WDM is very complex, the idea is very simple. In WDM we want
to combine the light sources into one single light at the multiplexer and do the reverse at
the demultiplexer. Combining and splitting of the light sources are easily handled by
prisms.
Prisms in WDM multiplexing and demultiplexing
41
One application of WDM is the SONET network in which multiple optical fiber lines are
multiplexed and demultiplexed.
Time Division Multiplexing
TDM is digital process that allows several connections to share the high bandwidth of a
link .Instead of sharing a portion of the bandwidth as in FDM, time is shared. Each
connection occupies a portion of time in the link.
The data flow of each connection is divided into units. The size of the unit can be 1 bit or
several bits. Combines one unit of each connection to make a frame; for n input connections
a frame is arranged into a minimum of n time slots, each slot carrying one unit from each
connection.
3T
3T
3T
A3
A2
A1
B2
B1
C3
C1
Data are taken from each
line every 3T seconds.
M
U
L
T
I
P
L
E
X
E
R
Channel
A3
C3
Frame3
A2 B2
A1 B1 C1
Frame2
Frame1
Each frame is 3 time slots. Each
time slot duration is T seconds.
Time Division Multiplexing (TDM)
42
D
E
M
U
L
T
I
P
L
E
X
E
R
A3
A2
B2
C3
A1
B1
C1
CHAPTER 3
TRANSMISSION MEDIA
Transmission media is a physical path by which a message transfers from sender to
receiver. Sender transmitted signals to receiver through transmission media. Transmission
Media are directly controlled by physical layer.
Transmission media can be divided into two categories: guided media and unguided media.
Guided media include twisted-pair cable, coaxial cable and fiber-optic cable. Unguided
media is wireless such as radio and lasers through the air.
Transmission media
Unguided media
Transmission
media
Guided media
Twisted-pair
cable
Coaxial
cable
Fiber-optic
cable
Air
Guided Media
Guided media are provided a channel from one device to another device, such as
twistedpair cable, coaxial cable and fiber-optic cable. Twisted-pair and coaxial cable use
metallic (copper) conductors that accept and transform signals in the form of electrical
current. Where as optical fiber is a glass cable that accepts and transports signals in the
form of light.
Twisted Pair Cable
A twisted pair consists of two insulated conductors (normally copper wires), usually about
1mm thick. The wires are twisted together in a helical form. Two parallel wires form a
simple antenna, then the noise or crosstalk will added with transmitted signals; which
43
results in a difference at the receiver. In twisted pair copper wires are twisted together to
reduce noise and crosstalk. In twisted pair, the number of twists per unit of length
determines the quality of the cable; more twists means more quality.
Twisted pair can be used for either analog or digital transmission. The bandwidth depends
on the thickness of the copper wires and the distance traveled. Because of performance and
low cost, twisted pair are widely used for many years.
Twisted-pair
cable
Unshielded Twistedpair cable (UTP)
Shielded Twisted
pair cable (STP)
The most common twisted pair cable used in communication is referred as unshielded
twisted pair (UTP). The IBM has produced a version of twisted pair cable called shielded
twisted pair (STP). Shielded twisted pair cable enclosed in a metal foil or braided-mesh
shield that protects against electromagnetic interference. As twisted air cable encased by
metal foil; it improved quality by preventing the penetration of noise or crosstalk. But it is
bulkier and more expensive than unshielded twisted pair that is why it used seldom outside
of IBM.
The twisted pair cable is mostly used in telephone system and in local area networks.
Twisted pair can transmit data several kilometers without any amplification. But for long
distance repeaters are needed.
The EIA (Electronic Industries Association) has developed standards to classify unshielded
twisted pair cable into seven categories.
Category
1
2
3
4
5
6
7
Bandwidth
Very low
<2 MHz
16 MHz
20 MHz
100 MHz
200 MHz
600 MHz
Data Rate
<100 Kbps
2 Mbps
10 Mbps
20 Mbps
100 Mbps
200 Mbps
600 Mbps
44
Digital/Analog
Analog
Analog/digital
Digital
Digital
Digital
Digital
Digital
Use
Telephone
T-1 lines
LANs
LANs
LANs
LANs
LANs
Categories are determined by cable quality, where as the category 1 is lowest and category
7 is highest quality.
The most common UTP connector is RJ45 (RJ means Registered Jack). The RJ45 is a
keyed connector that means the connector can be inserted in only one way.
• Twisted-pair cable consists of two insulated copper wires twisted together.
Twisting allows each ware to have approximately the same noise environment.
• Twisted-pair cable is used in telephone lines for voice and data communication.
Twisted-pair cable also used in Local Area Networks.
• Twisted-pair cables are two types Shielded Twisted–Pair (STP) and Unshielded
Twisted Pair (UTP).
• Twisted-pair cable is less expensive, that is why twisted-pair cable used for long
years.
Advantage of Twisted-pair Cable
1. The twisted-pair cable can easily install and can easily maintain.
2. The twisted-pair cable can be used for both analog and digital data transmission.
3. The twisted-pair cable is less expensive of transmission for short distance.
Disadvantage of Twisted-pair Cable 1.
Coaxial Cable
Coaxial cable (also known as coax) is another common transmission media in
communication system. Coaxial cable carries signals of higher frequency ranges than
twister-pair cable.
A coaxial cable has a central core conductor wire (usually copper) as the core, surrounded
by an insulating material. The insulator is encased by a cylindrical conductor of metal foil,
braid or a combination of two. The outer conductor is covered in an insulating sheath. The
whole cable is protected by a plastic cover.
Coaxial cable
45
Baseband Coaxial
cable
Broadband Coaxial
cable
Two kind of coaxial cable widely used:
1. Baseband Coaxial cable.
2. Broadband Coaxial cable.
Baseband coaxial cable, a 50 Ω cable, is commonly used in digital transmission. Baseband
coaxial cable is used in some local area networks.
Broadband coaxial cable, a 75 Ω cable, is commonly used in analog transmission.
Broadband coaxial cable is used in cable Television network and can be used in telephone
system.
Typically broadband cable runs a long distance; therefore require analog amplifiers. But
amplifiers can only transmit in one direction. So, a computer cannot transmit packet to
another computer if an amplifier be positioned between them. To solve these problem two
types of broadband systems have been developed:
1. Dual cable.
2. Single cable.
Head-end
Head-end
Amplifiers
Single cable.
Low frequencies
for inbound, high
frequencies for
outbound
communication.
Outbound
cable
Inbound
cable
Computers
(a)
(b)
Broadband networks. (a)Dual cable. (b) Single cable.
In dual cable system, two identical cables run in parallel. All computers transmit on cable
1 and receive on cable 2.To transmit data, a computer sends the data through cable 1, which
runs to a device called head-end at the root of the cable tree. The head-end than transmit
the data to cable 2.
46
In single cable system, different frequency bands are used for inbound and outbound
communication. The low-frequency band is used for communication from the computers
to the head-end, which then shifts the signal to the high-frequency and transmits to the
computers. In subsplit system, frequencies from 5 to 30 MHz are used for inbound and
frequencies from 40 to 300MHz are used for outbound communication. In midsplit system,
frequencies from 5 to 116 MHz are used for inbound and frequencies from 168 to 300
MHz are used for outbound communication.
A coaxial cable
Category Impedance Use
RG-59
75 Ω
Cable Television
RG-58
50 Ω
Thin Ethernet
RG-11
50 Ω
Thick Ethernet
RG-62
93 Ω
ARCnet LANs, IBM 3270 application
The most common type of connector for coaxial cable is the BNC (BayoneNeillConcelman). Three popular BNC connectors are the BNC connector, the BNC T
connector and the BNC terminator. The BNC connector is used in cable television to
connect the end of the cable to the TV set. The BNC T connector is used in Ethernet
networks to branch out a cable for connection to a computer or other devices. The BNC
terminator is used at the end of the cable to prevent the reflection of the signal.
•
•
•
•
•
Coaxial cable has following layers (starting from the center): a metallic rodshaped
inner conductor, an insulator covering the rod, a metallic outer conductor ( shield),
an insulator covering the shield and a plastic cover.
Coaxial cable has better shielding than twister cable and has excellent noise
resistance.
Coaxial cable can carry signals of higher frequency ranges than twisted-pair cable.
Bandwidth depends on the cable length.
Coaxial cable is used in cable TV networks and traditional Ethernet LANs.
The attenuation is much higher in coaxial cables than the twisted-pair cable.
Although the coaxial cable has a much higher bandwidth but the signal weakened
rapidly and needs frequent use of repeater.
Advantage of Coaxial Cable
47
1.
2.
3.
4.
5.
Bandwidth is much higher than twisted-pair cable. Although bandwidth
depends on the cable length.
Coaxial cable is well-shielded than twisted-pair cable and has excellent
noise immunity.
Coaxial cable can be used for both analog and digital transmission. For
analog, 75 ohm broadband coaxial cable is used and for digital, 50 ohm
Baseband coaxial cable is used.
Coaxial cable can cross longer distance at higher data rate.
The coaxial cable is relatively inexpensive than optical fiber cable and easy
to handle.
Disadvantage of Coaxial Cable
1. The attenuation is much higher than twisted-pair cable so needs repeaters
frequently.
Fiber Optics
A Fiber optics cable is made of very fine fibers of glass or plastic that accepts and transports
signals in the form of light. They consist of a glass core, roughly fifty micrometers in
diameter, surrounded by a glass "optical cladding" giving an outside diameter of about 120
micrometers. They make use of TIR (Total Internal Reflection) to confine light within the
core of the fiber.
Optical Fibers are optical waveguides. This means that wherever the fiber goes the light,
which is confined to the core of the fiber, also goes. So optical fibers can be used to make
light bend round corners. There are two types of bends macro-bend and micro-bend. An
optical transmission system has three components: the light source (LED or laser), the
transmission medium (Fiber-optics cable) and the detector (Photodiode). A pulse of light
indicates a 1 bit and the absence of light indicates a 0 bit. The detector generates an
electrical pulse when light falls on it. By attaching a light source to one end of an optical
fiber and a detector to the other end, a unidirectional data transmission system that accepts
an electrical signal, converts to light pulse and transmits it. At the receiving end the
incoming signal reconverts to an electrical signal.
The receiving end of an optical fiber consists of a photodiodes. The typical response time
of a photodiode is 1nsec so data rates will be 1 Gbps. A pulse of light must carry enough
energy to be detected.
Repeaters are needed only about 30km on long lines. The repeater convert the incoming
light to an electrical signal, regenerated full strength if it has weakened and retransmitted
as light.
There are two types of light sources for which fiber cables are available. These sources
lights are:
1. Light Emitting Diodes (LEDs).
2. Light Amplification bi stimulated Emission Radiation (Lasers).
48
Item
LED
Semiconductor Laser
Data rate
Low
High
Mode
Multimode Multimode or single mode
Distance
3 km
30 km
Lifetime
Long life
Short life
Temperature Sensitivity Minor
Substantial
Cost
Low
Expensive
A single fiber has a glass or plastic core at the center through which the light propagates.
In single-mode fibers, the core is 8 to 10 microns in diameter. In multimode fibers, the core
is 50 microns. The core is surrounded by a glass cladding with a lower index of refraction
than the core, to keep all the light in the core. Its diameter is usually 125 microns. Although
the cladding does not carry light, it is nevertheless an essential part of the fiber. The
cladding is not just a mere covering. It keeps the value of the critical angle constant
throughout the whole length of the fiber. For protection the cladding is covered in a thin
plastic jacket. Its diameter is usually 250 microns. Fibers are usually grouped together in
bundles protected by an outer sheath/jacket which is made of either PVC or Teflon. And it
is provided for protection against moisture, abrasion, crushing and other environment
dangers. In between the outer sheath and the plastic jacket are Kevlar strands to strengthen
the cable. Kevlar is a strong material used in the fabrication of bulletproof vests. An optical
fiber with its protection jacket may be typically 0.635 cm in diameter. Optical fibers are
defined by the ratio of diameter of their core to the diameter of their cladding.
Sheath Core
(a) ( b )
Core
(Glass) ( a) Side view of a single fiber. (b) End view of a sheath with three fibers
When the angle of indent is less than critical angle, the light is refracted and moves
closerSheath to the surface. If a light ray incident on the boundary at critical angle, the
light bendsCladding along to the interface. If the angle is greater than the critical angle,
the light ray reflected(Glass) Jacket Jacket Cladding internally. However, if the diameter
of core is reduced than the light ray can only moves
( Plastic )
in a straight line, without reflection.
Less Less Less dense dense dense
More
More
dense
More
dense dense
I
I
Less than critical angle,
refraction
I
equal to critical angle,
refraction
49
greater than critical angle,
reflection
Bending of a light ray
Cladding
Cladding
Light Source
Optical fiberCore
Propagation Modes
There are two types of modes for propagation of light along optical channel.
1. Multimode.
2. Single-Mode.
Multimode
In multimode fiber multiple rays from a light source (LED) moves through the core in
different paths. In multimode fibers, the core is 50 to 100 microns in diameter and 125
microns cladding.
Multimode can be in two forms:
1. Step-index.
2. Graded-index.
Cladding
Cladding
Multimode step-index Optical fiber
Cladding
Cladding
Multimode graded-index Optical fiber
Cladding
Cladding
Single-mode Optical fiber
50
Multimode Step-index
In multimode step-index propagation, the core density is constant and the light beam
moves through this constant density in a straight line until it reaches the interface of the
core and the cladding. The direction changes suddenly at the interface between the core
and the cladding. The term step index refers to the suddenness of this change.
Multimode Graded-index
In multimode graded-index propagation, the core density decreased with distance from
the center. Density is highest at the center of the core and lowest at the edge. As the index
of refraction is related to density; this causes a curving of the light beams. The word index
refers to the index of refraction.
Single-mode
In single-mode fiber as the core’s diameter reduce, the light can moves in a straight line.
As the core density is lower than multimode fiber, the critical angle that is close enough to
900, which make the propagation of beams almost straight. In this case, propagation of
different beams is almost identical and delays are negligible. All the beams arrive at the
destination together and can be recombined with little distortion to the signal. In
singlemode fibers, the core is 8 to 10 microns in diameter and 125 microns cladding.
Singlemode fiber uses an Injection Laser Diode (ILD) as a light source.
Type
Core(micron) Cladding(micron) Mode
50/125
50
125
Multimode Graded-index
62.5/125 62.5
125
Multimode Graded-index
100/125 100
125
Multimode Graded-index
7/125
7
125
Single-mode
• Fiber-optic cables are composed of glass or plastic inner core surrounding by
cladding, all encased in an outside jacket.
• Fiber-optic cables transmit data signal in the form of light. The signal is propagated
along the inner core by reflection
• Fiber-optic transmission is become popular due to its noise resistance, low
attenuation and high bandwidth capabilities.
• In optical fibers signal propagation can be multimode (multiple beams from a light
source) or single-mode (essentially one beam from a light source).
• In multimode step-index propagation, the core density is constant and the light
beam changes direction suddenly at the interface between the core and the cladding.
• In multimode graded-index propagation, the core density decreased with distance
from the center. This causes a curving of the light beams.
• Fiber-optic cable is used in backbone networks, cable TV networks,
Telecommunication, and Fast Ethernet networks.
• Mode of data transmission is half-duplex.
• An optical transmission system has three components: the light source (LED or
laser), the transmission medium (Fiber-optics cable) and the detector (Photodiode).
• Used mainly for digital data. A pulse of light indicates a 1 bit and the absence of
light indicates a 0 bit.
• The detector generates an electrical pulse when light falls on it.
51
•
The repeater convert the incoming light to an electrical signal, regenerated full
strength if it has weakened and retransmitted as light
Advantage of Optical Fiber
1.
2.
3.
4.
5.
6.
7.
8.
Fiber-optic cable carry signals with much less energy loss than twister cable or coaxial
cable and with a much higher bandwidth. Hence the data rate data rate is also higher
than other cables.
Fiber-optic cables are much lighter and thinner than copper cables with the same
bandwidth. This means that much less space is required in underground cabling ducts.
Also they are easier for installation engineers to handle.
Fiber-optic cables suffer less attenuation than other guided media because light beam
traveling in the fiber. This means that fibers can carry more channels of information
over longer distances and with fewer repeaters.
Fiber-optic cables are not affected by electromagnetic interference, power surges or
power failure or corrosive chemicals in the air.
Fiber-optic cables cannot easily be tapped. It has more immunity to tapping than
copper cables.
As fibers are very good dielectric, isolation coating is not required.
No electric connection is required between the sender and the receiver.
Fiber-optic cables are much more reliable than other cables. It can better stand
environment condition, such as pollution, radiation etc. Its life longer in compare to
copper wire.
Disadvantage of Optical Fiber
1. Fiber-optic cables and the interfaces are more expensive than other guided media.
2. Propagation of light is unidirectional. So two fibers are needed if we need
bidirectional communication.
3. It is new technology, therefore only few trained mechanics are available. Optical
fibers cannot be joined together as an easily as copper cable and requires additional
training of personnel and expensive precision splicing and measurement
equipment.
Unguided Media
Unguided media (usually air) transport electromagnetic waves without the use of a physical
conductor. This type of communication is often referred to as wireless communication.
As signals are broadcast through air; signals are available to anyone who has a device
capable of receiving them.
Unguided signals (wireless data) are transmitted from source to destination in different
ways. They are ground propagation, sky propagation and line-of-sight propagation.
52
In ground propagation, radio waves through the lowest portion of the atmosphere. In
skypropagation, higher frequency radio waves radiate upward into ionosphere where they
reflected back to the earth. In line-of-sight propagation, very high-frequency signals are
transmitted in the straight lines directly from antenna to antenna.
Wireless transmission can be classified into three groups:
1. Radio Waves.
2. Microwaves.
3. Infrared waves.
Radio Waves
Normally electromagnetic waves having frequencies between 3 KHz and 1 GHz are called
radio waves. Those radio waves that propagate in sky mode can travel long distance. That
is why for long distance broadcasting radio waves are used such as AM radio.
Radio waves are omni directional. Radio waves use omni directional antennas that are sent
out signals in other directions. Base on the wavelength, strength and the purpose of
transmission there are several antenna.
Low and medium frequency radio waves can pass through building walls.
Radio wave band is narrow, just 1 GHz. Therefore when this band divided into sub-bands
are also narrow, as a result to a low data rate for digital communication.
Radio waves are used for multicast communication such as AM and FM radio, television,
cordless phones and paging
Radio waves include the following types:
1. Short-wave
2. Very-high-frequency (VHF) television and FM radio.
3. Ultra-high-frequency (UHF) radio and television.
Microwaves
Electromagnetic waves having frequencies between 1 to 300 GHz are called microwaves.
Microwaves are unidirectional; propagation is line of sight. Microwaves are used for
cellular phone, satellite and wireless LAN communications. The parabolic dish antenna
and horn antenna are used for transmission of microwaves. When an antenna transmits
53
microwave they can be narrowly focused. That means the sending and receiving antennas
need to be aligned.
Since microwave travels in the straight line, so the towers with the mounted antennas need
to be in direct sight of each other and tall enough or close enough together. If towers are
too far apart then the towers are needed to be very tall. Repeaters are often needed for longdistance communication. The curvature of the earth and other blocking obstacles do not
allow two short towers to communicate using microwaves. Antennas must be directional,
facing each other.
Very high frequency microwaves cannot pass through building walls. As a result receiver
cannot get signal inside buildings.
The microwave band is relatively wide almost 299 GHz. Therefore wider sub-bands can
be assigned and high data rate is possible.
Infrared Waves
Electromagnetic waves having frequencies between 300 GHz and 400 THz are called
infrared waves. Such a wide bandwidth can be used to transmit digital data with very high
data rate.
The wavelengths of infrared signal are from 1mm to 770nm. As the wavelengths of infrared
signal are too small, cannot pass through building walls.
Infrared waves are used for short distance communication such as those between a PC and
a peripheral device such as keyboards, mice and printers. But in short range
communication, these do not interfere with the use of another system in the next room.
Infrared signals are useless for long range communication. We cannot use infrared signals
outside a building because the sun’s rays contain infrared waves that interfere with the
communication. The remote controls used on televisions, VCRs and stereos all use infrared
communication. Infrared waves also can used in wireless LANs.
Band
Range
Propagation
Application
VLF
3-30 KHz
Ground
Long-range radio
LF
30-300 KHz
Ground
Radio MF
300 KHz - 3 MHz Sky
AM radio
HF
3-30 MHz
Sky
Ship/aircraft communication
VHF
30-300 MHz
Sky and Line-of-sight VHF TV, FM radio
Line-of-sight
UHF
300 MHz- 3GHz
UHF TV, cellular phones,
SHF
3 -30GHz
Line-of-sight
Satellite communication
EHF
30-300 GHz
Line-of-sight
Radar, satellite
Guided media
Unguided media
Guided media transmit data in the form of Unguided media transmit data by
electrical current or light.
electromagnetic wave through air.
Here the data is transmitted by metal or glass Here the data is transmitted without used
conductor.
of any physical conductor.
54
Through guided media signal can travel Through unguided media signal can travel
according physical path.
according to their propagation like ground
propagation, sky propagation and line-ofsight propagation.
This type of media is generally used at low This type of media is generally used for
frequency data transmission.
high frequency data transmission.
This type of media is used for generally short This type of media is used for large
distance transmission.
distance transmission such as satellite.
Here the data transmission is private; only that Here any one can receive transmitted data
the receiver can receive the data to which the which have capable receiver to receive it.
cable is connected.
In the case of guided media analog and digital Only analog transmission happens.
transmission possible.
It has lower bandwidth.
It has higher bandwidth than guided
media.
Through this media signal can pass through In the case of unguided media external
least undesired external interference. It is interference can cause serious effect to the
protected from as much as possible from signal.
external interference.
Guided media include twisted-pair cable, Unguided media is wireless such as radio
coaxial cable and fiber-optic cable
and lasers through the air.
It is a lower cost media such as twisted-pair or The media is high media as satellite is used
coaxial cable.
or several high capable transmitter and
receiver are used.
Microwave Communication
Microwave communication widely used for long-distance telephone communication,
cellular telephones, television distribution, etc.
• Microwave is relatively economical as compared to fiber optics systems.
• Microwave systems provide high speed data transmission rates.
There are two types of microwave data communication systems. These are:
1. Terrestrial Communication.
2. Satellite Communication.
Terrestrial Communication
Terrestrial microwave systems usually use directional parabolic antennas to send and
receive signals in the lower range. Most terrestrial microwave systems produce signals in
the low range usually at 4 to 6 GHz and 21 to 23 GHz. The signals are highly focused and
physical path must be line-if sight. Relay towers are used to extend signals.
55
Short distance systems can be relatively inexpensive but long distance systems can be very
expensive.
•
•
•
•
•
•
Most terrestrial systems use low frequency ranges.
Bandwidth capacity depends on the frequency used.
Attenuation depends on frequency, power, antenna size and atmospheric
conditions. Normally, over short distances, attenuation is not significant.
Highfrequency microwaves are more affected by rain and fog.
Short-distance systems can be economical, but long distance systems can be very
expensive.
Installation of terrestrial microwave systems is extremely difficult. Because the
transmission is line-of-sight, antenna must be carefully aligned.
Data rates are from 1 to 10Mbps.
Satellite Communication
A satellite network uses a combination of nodes that provides communication between any
points on the earth. Example of different nodes in the network is a satellite, an earth station,
or an end-user terminal or a telephone. Satellite microwave systems transmit signals
between directional parabolic antennas. Like terrestrial microwave systems, they use low
frequency ranges usually at 4 to 6 GHz and 11 to 14 GHz and must be in line-ofsight. The
main difference with terrestrial microwave systems is that satellite microwave systems can
reach the most remote places on earth and communicate with mobile devices.
A communication satellite is basically a big microwave repeater in the sky. It contains
several transponders, which can receive from the earth station incoming weak signal,
amplifies it into high power signal, and rebroadcasts at another frequency (to avoid
interference with the incoming signal) to the receiving earth station. A typical satellite has
12-20 transponders with a 36-50 MHz bandwidth.
•
•
•
•
•
•
Satellite microwave systems usually use low frequency ranges.
The cost of building and launching a satellite is extremely expensive. Although
satellite communications are expensive, the cost of cable (fiber optics) to cover the
same distance may be even more expensive.
Installation of satellites is extremely technical and difficult. The earth-based
systems may require exact adjustments.
Attenuation depends on frequency, power, antenna size and atmospheric
conditions. High-frequency microwaves are more affected by rain and fog.
Bandwidth capacity depends on the frequency used.
Data rates are from 1 to 10Mbps.
VSATs (Very Small Aperture Terminals) are low-cost microstations used in satellite
communication. These tiny terminals have 1 meter antennas and can put out about 1 watt
power. In many VSAT systems, the microstations do not have enough power to
communicate directly with another via satellite. Hub is a special ground station, with a
large high-gain antenna which needed to relay traffic between VSATs. In this mode of
operation, either the sender or the receiver has a large antenna and a powerful amplifier.
56
There is a time delay of 540 m second between a transmitted and received signal for a
VSAT system with a hub.
An artificial satellite need a path in which it travels around the earth is called orbit. Based
on the location of the orbit, satellite can be divided into three categories:
1. GEO (Geosynchronous Earth Orbit). GEO is at the equatorial plane and resolves in
phase with the earth.
2. LEO (Low-Earth Orbit). LEO satellite provides direct universal voice and data
communications for handheld terminals and also provides universal broadband internet
access.
3. MEO (Medium earth Orbit). MEO satellite provides time and location information
for vehicles and ships.
GEO Satellite
Sending and receiving antennas must be in line-of-sight. For short time communication, a
satellite can move faster and slower than the earth rotation. But for relay (constant)
communication; the satellite must be move at the same speed as the earth so that it seems
to remain fixed above a certain spot. Such satellites are called geosynchronous. The
rotation period of geosynchronous satellite is same as the earth (rotation period is 23 hours
56 minutes 4.09 seconds). Only one orbit can be geosynchronous. This orbit is at equatorial
plane and is 35,786 km above the equator.
To avoid interference geosynchronous satellites are spacing 2 degree in the 360 degree
equatorial plane, as a result there can only be 180 geosynchronous communication
satellites in the sky at once. To provide full global transmission, it takes minimum of three
satellites to cover whole earth. Three satellites are spacing 120 degree from each other in
geosynchronous orbit.
•
•
•
•
It is capable of providing continuous and uninterrupted communication over the
desired area.
There is a time delay of 250 to 300 m second between a transmitted and received
signal.
Small areas near north and south poles are not covered in the communication range
of the satellite.
A costly launch vehicle is required.
MEO Satellite
MEO satellites are positioned between the two Van Allen belts. MEO satellites are located
at altitudes between 5000 and 15,000 km. and a rotation period of 6 hours.
Global Positioning System (GPS) satellites are MEO satellites that provides time and
location information for vehicles and ships. GPS has 24 satellites in six orbits, with each
orbit hosting four satellites. A GPS receiver can tells the current position of a satellite and
sends a signal to four satellites. It calculates your position on the earth.
LEO Satellite
57
LEO satellites are normally at altitude between 500 to 2000 km. LEO satellites have polar
orbits. The satellite has a speed of 20,000 to 25,000 km/h with a rotation period of 90 to
120 min.
Iridium is designed to provide direct world wide voice and data communication using
handheld terminals, a service similar to cellular telephony but on a global scale. The
Iridium System has 66 satellites in six LEO orbits; each at an altitude of 750 km.
Communication between two distant users requires relaying between several satellites.
Globalstar system has 48 satellites in six polar orbits with each orbit hosting eight
satellites. Communication between two distant users requires both satellites and earth
stations, which means that ground stations can create more powerful signals.
Teledesic satellites are LEO satellites that will provide universal broadband Internet
access. Teledesic has 288 satellites in 12 LEO orbits with each orbit hosting 24 satellites.
The orbits are at an altitude of 1350 km.
Two frequencies are designed for each satellite to send and receive. Transmission from the
earth to the satellite is called uplink. Transmission from the satellite to the earth is called
downlink. Frequency bands for satellite communication are given in the following table.
Band
L
S
C
Ku
Ka
Bandwidth MHz
15
70
500
500
3500
Uplink (GHz)
1.6
2.2
6
14
30
Downlink(GHz)
1.5
1.9
4
11
20
Problems
Terrestrial interference
Rain
Rain; Expensive
Transmission Impairments
After signal travels through transmission media, which are not perfect, the received signal
is not same as the transmission signal.
The imperfection causes impairment in the signal. Three types of impairment usually occur
in transmission line:
1. Attenuation
2. Distortion
3. Noise
Attenuation
Attenuation means loss of energy. When a signal travels through a medium, it losses some
of its energy. So that it can overcome the resistance of the medium. Some of the electrical
energy in the signal is converted to head. The loss is expressed in decibels (dB) per
kilometer. The decibel is negative is if a signal is attenuated and positive if a signal is
amplified. The amount of energy loss depends on the frequency. To compensate for
frequency-dependent attenuation, amplifiers are used to amplify the signal. But it can never
restore the signal to its original shape.
58
Distortion
Distortion means that the signal changes it forms or shape. Distortion occurs in a composite
signal, made of different frequencies. It is caused by the fact that different Fourier
components travel at different speeds. For digital data, fast components from one bit may
catch up and overtake slow components from the bit ahead, mixing the two bits and
increasing the probability of incorrect reception.
Noise
Noise is an unwanted energy from sources other than the transmitter. Several types of noise
may corrupt the signal.
Thermal Noise: Thermal noise is caused by random motion of electrons in a wire, which
creates an extra signal not originally sent by the transmitter. The thermal noise also known
as white noise. That means frequencies corresponding to all colors of the light spectrum
are present in equal amount in the thermal noise.
Cross talk: Cross talk is caused by inductive coupling between two wires that are close to
each other. One wire acts as a sending antenna and the other acts as a receiving antenna.
Impulse Noise: Impulse noise caused by spikes (a signal with high energy in a very short
period of time) on the power line, lighting or other causes. For digital data, impulse noise
can wipe out one or more bits.
Induced Noise: Induced noise come from the source such that motor and appliances. These
devices act as a sending antenna and transmission media acts as a receiving antenna.
59
CHAPTER 4
NETWORK STRUCTURES
Network Topology
Topology is the structure of a network including physical arrangement of devices. Two or
more devices connect to a link; two or more links form a topology. The topology of a
network is the geometric representation of the relationship of all the links and linking
devices (typically called as nodes) to one another. There are four basic topologies possible:
bus, ring, star and mesh. Some other possible topologies are tree, intersecting rings and
irregular.
Bus Topology
In bus topology all nodes are connected by a single long bus cable which acts as backbone.
Nodes are connected to the bus cable by drop lines and taps.
•
•
•
•
•
The bus topology is simple, reliable in small networks, easy to use and also easy to
understand.
In bus topology least amount of cable is required. So it is less expensive than other
topology.
We can easily extent a bus by joining two cables with a connector and allow more
computers to join the network.
Many computers within a bus can sl ow down the performan ce.
A cable break or loose connector or any malfunctioning computer anywhere
between two computers can cause them not no able to communicate with each other
and may be break down the whole network.
Ring Topology
In ring topology all nodes are arranged in a circle. Data travels around the ring in one
direction, with each node on the ring acting as a repeater. Each node passes information to
its next node, until it arrives at intended destination. Ring networks typically use a token
passing protocol.
•
In ring topology performance is faster than bus topology.
60
•
•
•
•
Installation and reconfigure is easy because each device is linked to its immediate
neighbors (either physically or logically).
Signal loss problem is not subject in ring topology.
Failure of one node on the ring can affect the whole network.
Adding or removing nodes disrupts the network.
Star Topology
In star topology all nodes are attached to a central controller device usually called as hub.
The all nodes are only point-to-point like only to the central device. The nodes are not
directly liked to one another.
•
•
•
•
•
•
A star topology is less expensive than a mesh topology.
Installation and reconfigure is easy because each device needs only one link and
one I/O port to connect.
We can easily add new computers to a star network by run a new line from the
computer to central position and plug in into the hub.
Single computer failure is not affect the network. The hub can detect a network
fault, isolate the offending computer or network cable and allow the rest of the
network to continue operating.
If the central hub fails then the whole network fails to operate.
In star topology more cable is require than other network topologies (such as ring
or bus). Although a star requires far less cable than a mesh topology.
Mesh topology
In a mesh topology each node has a dedicated point-to-point link to every other device.
The term dedicated means that the link carries traffic only between the two devices it
connects. A fully connected mesh network has n(n-1)/2 physical channels to link n devices;
every device on the network must have n-1 I/O ports.
•
•
•
•
•
•
•
•
Privacy or security is more than other topologies. When data travels along a
dedicated path, only the intended recipient notices it.
The dedicated links eliminating traffic problems. Each dedicated links guarantees
that each connection can carry its own data load.
Single computer failure or one link unusable is not affecting the network.
Point-to-point links make fault identification and fault isolation easy. Traffic can
be routed to avoid links with suspected problems.
In mesh topology more cable is require and more I/O ports are required.
Installation and reconnection are difficult because every device must be connected
to every other device.
The hardware required to connect each link (I/O port and cable) can be expensive.
Wiring can be greater than the available space can accommodate.
Controlled Access
61
In controlled access, the stations consult one another to find which station has the right to
send. A station cannot send unless it has been authorized by other stations. Poll and select
are popular controlled access methods.
Polling
One of the stations is designation as a primary station and other stations are secondary
stations. Data transfer must be made through the primary station even when the final
destination is a secondary station. The primary station controls the links. If the primary
station is neither sending nor receiving data, it knows the link is available.
If the primary station wants to receive data, it asks the secondary stations if they have
anything to send; this method is called polling. If the primary station wants to send data, it
tells the secondary stations to get ready to receive data; this method is called selecting.
Select
The select method is used whenever the primary station has anything to send.
Primary station must alert the secondary station to the upcoming transmission and wait for
an acknowledgment of the secondary station (as ready status). So before sending data, the
primary station creates and transmits a select (SEL) frame, one field of which includes the
address of the intended secondary station. After receiving acknowledgment the primary
station send data to the destination secondary station. When secondary station has received
data it sends acknowledgment to the primary station.
Poll
The poll method is used whenever the primary station has anything to send.
When the primary is ready to receive data, it must ask (poll) each secondary station in turn
if it has anything to send. When the secondary station is received, it responds either with a
NAK frame if it has nothing to send or with data frame if it does. If the response is negative
(a NAK frame), the primary station then polls the next secondary in the manner until it
finds one with data to send. When the response is positive (a data frame), the primary
station reads the frame and returns an acknowledgment (ACK frame) to the secondary
station.
Channel Sharing Techniques
It is better to sharing a communication like among many users rather than dedicating it to
a single user. Channel sharing technique leads a much better utilization of the channel
bandwidth and also reduces the cost of networking.
In computer networking, well-known channel sharing techniques are following:
1. Frequency Division Multiplexing (FDM).
2. Wave Division Multiplexing (WDM).
3. Time Division Multiplexing (TDM).
Switching
62
There are the following approaches for providing reliable and efficient transport of
messages across a point-to-point subnet.
Circuit Switching
A switching technology that establishes an electrical connection between stations using a
dedicated path is called circuit switching. Circuit switching creates a direct physical
connection between two devices such as computer or phones. When a call has been set up,
a call request signal must propagate from caller source host to destination host and call
accept signal must come back from destination host to the caller host. Once a call has been
set up connection between both ends exists and it will continue until the exchange of
messages is over and a disconnect request signal is issued by either host.
A circuit switch is a device with n inputs and m outputs that creates a temporary connection
between an input link and output link. The number of inputs does not have to match the
number of outputs.
Physical connection
Set up when call is
made
Switching office
•
•
•
•
•
•
•
•
•
An end-to-end dedicated physical path need to set up before any data can be sent.
Once a call has been set up, a connection between both ends exists and it will continue
until the exchange of messages is over and a disconnect request signal is issued by
either host.
Circuit switching provides continuous-time end-to-end transport of messages.
The charge is based on distance and time.
Available bandwidth is fixed and unused bandwidth on an allocated circuit is wasted.
Each data follows the same route.
Circuit switching has only suffered circuit set-up delay.
Circuit switching is completely transparent. The sender and receiver can use any bit
rate, format, or framing method. Carrier does not know or care about these basic
parameters.
Telephonic communication is an example of circuit switching.
63
Circuit switching can use in two technologies: the space-division switch and timedivision
switch.
Message Switching
A switching technology that messages to be transmitted between stations are all sent to a
central station, which gathers them and routes them to the appropriate destination. When a
sender has a block of data to be sent, it is stored in the first switching office (i.e. router)
and then forwarded later, one hop at a time. Each block is received entirely, inspected for
error and then retransmitted.
Message switching can use in store-and-forward network.
•
•
The message is transport across the subnet from the source to destination, one hop at a
time.
Message has switching suffered propagation delay and queuing delay.
64
•
•
•
Message switching provides discrete-time hop-by-hop transport of messages.
In message switching there is no upper limit on the block size.
Simultaneously availability of the sender and receiver is not required in the message
switching.
Packet Switching
In this switching technology the messages to be transmitted in the form of packets and
placed into channels. Long massages are broken into smaller units called packets. Packets
are formed by adding information to the beginning (which is called header) and adding
information at the end (which is called trailer) of each group of user data. The packet header
contains source and destination addresses, control bits, packet number and as well as
different types of information such as billing and accounting information. The trailer
contains error checking information, such as CRC (which is used to determine whether the
packet has been corrupted in transmission) and end-of-message indicator.
In packet switching the packets are transmitted by store-and-forward technique but provide
a big improvement in performance. Packet switching network place a tight upper limit on
block size (not more than a few kilo bytes). And also reduce the store-andforward delay
because transmission of each packet as soon as that packet arrives, without waiting for its
successor packets to arrive. The packets are routes using the source and destination
addresses.
Header
User Data
Trailer
Header Contents:
Beginning-of-Message Indicator.
Trailer Contents:
Source Address.
Error-Checking Code.
Destination Address.
End-of-Message Indicator.
Description of Data to follow (e.g.
User Info, Control Info).
Packet Sequence Number.
Routing Information.
Billing Information.
•
•
•
•
In packet switching long massages are broken into smaller units called packets.
Packet switching network place a tight upper limit on block size (not more than a few
kilo bytes).
Packet switching may utilize the bandwidth on an allocated circuit because circuits are
never dedicated.
Carrier determines the basic parameters, such as bit rate, format or framing method.
65
•
•
•
Header is adding at beginning of each user data and trailer adding at the end.
Packets from the same message may travel along several different routes.
An example of packet switching is an airline reservation system.
Item
Use dedicated path
Bandwidth available
Call setup required
Use store-and-forward transmission
Charging based on
Congestion can occur
Data follows the same route
Wasted bandwidth
Carrier determine the basic parameters
( i.e. bit rate, format or frame method ))
Circuit switching
Yes
Static
Yes
No
Time and distance
At setup time
Yes
Yes
No
66
Packet switching
No
Dynamic
No
Yes
packet
On every packet
No
No
Yes
CHAPTER 5
FLOW CONTROL AND ERROR CONTROL
Automatic Repeat Request
The data link controls (DLCs) generally use the concept of Automatic Repeat reQuest
(ARQ). At the receiving end DLC module detect erroneous frames and then send a request
to the transmitting DLC module to retransmission the incorrect frames.
Retransmission of data necessitate due to in three cases: damaged frame, lost frame and
lost acknowledgement.
Damaged Frame
A recognizable frame does arrive, but some of the bits have been altered during
transmission. That means receiving frame has some error.
Lost Frames
A frame fails to arrive at the other side. For example, a noise burst may damage a frame to
the extent that the receiver is not aware that a frame has been transmitted.
Lost acknowledgement
An acknowledgement fails to arrive at the source. The sender is not aware that
acknowledgement has been transmitted from the receiver.
The purpose of ARQ is to turn an unreliable data transmission to reliable one. There are
two categories of ARQ: 1. Stop-and-wait ARQ
2.
Continuous ARQ
Stop-and-wait ARQ
Stop-and-wait ARQ is based on stop-and-wait flow control technique. The stop-and-wait
process allows the transmitter DLC station to transmit a block of data and then wait for the
acknowledgement (ACK) signal from receiver station, which indicates correct reception.
No other data frames can be sent until the receiver’s reply (ACK) arrives at the source
station. There is chance that a frame that arrives at the destination (receiver) is damaged.
The receiver detects this error by using error detection technique. If the receiver detects an
error, it simply discards corrupted frames and it sends a negative acknowledgement
(NAK). The sender waits for acknow, and the message is then retransmitted. This type of
protocol is most often found in simplex and half-duplex communication.
67
The ACK/NAK frame is a small feedback frame from receiver to sender. Little dummy
frames are used to carry ACK/NAK response.
The acknowledgement is attached to the outgoing data frame from receiver to sender (i.e.
full duplex operation) using ack field onto the header of the frames in the opposite
direction. The technique of temporarily delaying outgoing acknowledgements so that they
can be hooked onto the next outgoing data frame is known is piggybacking. In affect, the
acknowledgement gets a free ride on the next outgoing data frame.
The advantages of piggybacking are
• Better use of available channel bandwidth.
• Less traffic due to absence of dummy frames.
• Less frame arrival interrupts to DLC software/hardware.
• Perhaps fewer buffer requirement in the receiver.
Continuous ARQ
In the continuous ARQ frames transmit continuously. The transmitter contains buffer
storage and both transmitter and receiver monitor frame counts. If the receiver detects an
erroneous frame then NAK is sent to the transmitter with defective frame number N. when
the transmitter gets the NAK message then it can retransmission in either of the following
ways:
1. Go-Back-N
2. Selective Repeat
Go-Back-N
In Go-Back-N ARQ transmitter retransmission all frames starting from N. That means
whenever transmitter received a NAK message, it simply goes back to frame N and resume
transmission as before. Every time an NAK is received, the transmitter repositions the
frame pointer back to frame N. The number of frames, which must be retransmitted, is at
least one and often more, as all the frames from the erroneous frame are transmitted by the
sender. The receiver simply discards all subsequent frames, sending no acknowledgements
for the discarded frames. Go-Back-N ARQ is the most widely used type of ARQ protocol.
Selective Repeat
In Selective Repeat ARQ transmitter retransmission only the defective frame N and not the
subsequent frames. The number of frames, which must be retransmitted, is always one, it
being the frame containing error. The receiver buffered all correct frames following the
erroneous frame. When transmitter receive a NAK message, it just retransmission the
defective frame, not all its successors. If the second try succeeds, the receiver will rearrange
the frames in sequence. Selective repeat is more efficient than goback-N, if less number of
errors occur. But in this approach can require large amount of buffer memory. Both require
the use of a full-duplex link.
In comparison with stop-and-wait protocol, link efficiency is improved overall by both
implementations because line idle and line turnaround times eliminated.
68
Go-Back-N strategy
Selective repeat strategy
Sliding Window Protocols
When the channel is not error-free and/or the receiver has limited buffer spaces, flow of
fames from sender to receiver must be controlled to prevent buffer overflow and/or
duplication of frames at the receiver.
A data link between a sender A and a receiver B is said to be window flow controlled, if
there is an upper bound on the number of frames that can be transmitted by A but not yet
been acknowledged by B. this upper bound (a positive integer) is called the window size
or simply the window. The number of outstanding (i.e. unacknowledged) frames at any
instant should not exceed the window size. Acknowledgements are either contained in
special frames or are piggybacked on regular data frames in the reverse direction. The
sliding window technique follow either go-back-N or selective repeat.
69
The semantics of an n-bit sliding window protocol:
• Each transmitting frame contains a sequence number between 0 to (2n-1). For stopand-wait sliding window protocol, n=1, so that only two sequence numbers, (i.e. 0 and
1) are possible. A common value of n is 3, which allow eight frames to be transmitted
without any acknowledgement.
• At any point of time, the sender or receiver maintains a list of consecutive sequence
numbers corresponding to frames it is permitted to send or receiver.
• Initially, no frames are outstanding, so the lower and upper edges of the sender’s
window are equal.
• The frames are said to fall within the opening of the sending window. The receiver
also maintains receiving window corresponding to the set of frames it is permitted to
accept. The sending and receiving window sizes need not be identical.
• The open portion of the sending window represents frames transmitted but as yet
not acknowledged.
• When a new frame arrives, it is assigned the next highest sequence number and
upper edge of the window is advanced by one and provided the window is not fully
closed.
• When an acknowledgement comes in, the lower edge of the window is advanced
by one. In this way the window continuously maintains a list of unacknowledged
frames.
• The open portion of the receiving window corresponds to the frames that the
receiver is waiting to accept. When a frame is received, whose sequence number is
equal to the lower edge of the open portion, its acknowledgement is generated and the
window is rotated by one.
• The sender window always remains at its initial size. Since any frame within the
open portion of sending window may have to be retransmitted, the sender must keep
all these frames in its buffer.
In example window size is 1 with a 2-bit sequence number. The corresponding window
operations are shown in the following figure.
• Initially the lower and upper edges of the sender’s window are equal. The receiving
window is open to accept the frame numbered 0.
• Sending window is open and has transmitted frame 0 to the receiver. The receiving
window still pen to receive frame 0.
• After frame 0 is received, the receiving window is rotated by one to be ready to
receive the next frame. The acknowledgement is issued by receiver before the window
is rotated. Sending window open to receive acknowledgement of frame 0.
• The number of the acknowledgement frame indicates the number of the last frame
received correctly. If this number matches with the number of the outstanding frame in
the sender’s buffer, the frame is taken to be received properly by the receiver and the
sender takes up the next frame for further transmission. If there is a mismatch, the
sender retransmits the frame from the buffer.
70
3
Sender
0
3
0
2
1
1
Window open to
2
Receiver
Window open to
( a)
3
0
Initial Setting
2
1
received frame 0
(b) After frame 0 is sent
send frame 0
Window has send frame
0
Window open
to received
frame 0
(c) After frame 0 is received and ACK 0 is sent
71
to accept
frame
00
3 receive ACK
3
2
0
3
1 as ACK 0 sent
0
1
3
(d)
After
2 Window open
1
Window open to send
frame 1
0
2
1
Window rotated
2
1
ACK0 is received
Window open
to received
frame 1
An example of sliding window protocol. Window size is 1 with a 2-bit sequence.
number.
72
Error detection and Correction
When data units transfer from one device to another device, the data units can become
corrupted. Networks must be able to transfer data with complete accuracy. That is why a
reliable networking system must have a mechanism for detecting and correcting those
errors.
Type of errors
Errors are divided into two types:
1. Single-Bit Error.
2. Burst Error.
Single-Bit Error
Single-bit error means that only one bit of given data unit (for example byte, character,
frame or packet) is changed from one to zero or zero to one.
Suppose that a sender transmit group of 8 bits of ASCII characters. In following example,
01010001 ( ASCII Q) was sent but 01000001 (ASCII A) was received at other end.
Single-bit errors most likely occur in parallel transmission, but rare in serial transmission.
One of the reasons is that one of the 8 wires is may be noisy.
Suppose a sender transfer data at 1 Mbps. That means each bit lasts only 1/1,000,000
second or 1 µ s. If a noise lasts only 1µ s (normally noise lasts much longer than µ s)
then it can affect only one bit.
Sender
Receiver

Burst Error
A burst error means that two or more bits in the data unit have changed from one to zero
or from zero to one.
In the following example, 01011101 01000011 was sent but 01000100 01000011
wasSender Receiver receive. Burst errors may not occur in consecutive bits. The length of
burst error is calculated from first corrupted bit to the last corrupted bit. Some bits between
01010001 01000001
them may
not be corrupted. In this example, three bits has been
changed from one to zero but the

length of burst error is
5.
01011101 01000011 01000100 01000011
Burst errors most likely occur in serial transmission. Because data transfer in serial
transmission is at slow speed. Suppose a sender transfer data at 1 Kbps. That means each
bit lasts only 1/1000 second. If a noise lasts 1/100 second then it can affect 10 bits.
Given any two codewords W1 and W2, the number of bit position in which the codewords
differ is called the Hamming distance, dw1w2 between them. It is possible to determine how
many bits differ, just Exclusive OR the two codeword and count the number of 1 bits in
the result. It is given by
dw1w2 = W1 ⊕ W2
It is significance that if two codewords are a Hamming distance d apart, it will require d
single-bit errors to convert one into the other.
Error Detection
Three common types of error detection methods in data communications are follows:
1.
2.
3.
Parity check.
Cyclic redundancy check.
Checksum.
Parity Check
The most common and least expensive method for error detection is the parity check. A
parity bit is added to each data unit so that total number of 1s in the data unit becomes even
or odd. For example, we want to transmit the data unit 1110111; total number of 1s in the
data unit is 6, an even number.
Before transmission we pass the data unit through a parity generator which counts the
number of 1s and appends a parity bit at the end of the data unit. Most system uses evenparity checking but some system may use odd-parity.
The even-parity checking function counts the number of 1s in the data unit; if the number
is odd then it appends a parity bit 1 at the end of the data unit otherwise appends a parity
bit 0. On other hand in odd-parity checking function counts the number of 1s in the data
74 78
unit; if the number is even then it appends a parity bit 1 at the end of the data unit otherwise
appends a parity bit 0.
Simple parity check can detect all single-bit errors. It can detect burst error only if the total
number of errors in each data unit is odd.
Two-dimensional parity check
• In two-dimensional parity check, a block of bits is arranged in a table. For example,
following data unit divided in four rows and seven columns.
• Calculate the parity bit for each data unit and create a new column. Note that eight
column is calculated based on parity bit of each data unit. This is known as LRC
(longitudinal redundancy check).
• Calculate the parity bit for each column and create a new row. Note that fifth row
is calculated based on parity bit of each column. This is known as VRC (vertical
redundancy check).
Original block of data:
1011011
1
0
1
1
0
1
0
1
0
1
1
1
1
1
0
0
0
1
0
1
Transmitted block of data: 10110111
0101001
0
0
0
1
1
01010011
0111001
1
0
0
1
0
01110010
1100111
1
1
1
1
0
11001111
1
1
0
1
1
01011001
Receiver checks the parity bits, if some of the bits do not follow the even-parity rule and
the whole block is rejected and negative acknowledgement sent to the sender and this block
of data must be retransmitted by the sender. Otherwise the block of data is accepted.
Cyclic Redundancy Check (CRC)
One of the most common and widely used error-detection methods for synchronous data
transmission is Cyclic Redundancy Check (CRC). CRC developed by IBM, uses CRC-16
as specific application of the CRC method.
Mathematically, a bit string of length n is represented in powers of x such as x n-1 + xn-2 +
…+ x2 + x1 + x0. As an example, 1011 has 4 bits and represents a 4-term polynomial with
coefficients 1, 0, 1 and 1; the binary representation of x3 + x + 1 is 1011, where missing
terms in the polynomial are represented by 0’s. Polynomial arithmetic is done modulo-2.
If we represent the data bits as dk-1, dk-2…, d1, d0, the data polynomial is given by d(x)
=dk-1xk-1 + dk-2xk-2 + …+ d1x+d0
Polynomial arithmetic is done module 2, according to the rules of arithmetic field theory.
There are no carries for addition or borrows for subtraction. Both addition and subtraction
are identical to EXCLUSIVE OR (XOR). For example:
11001011
11001011
75
+10011001
- 10011001
01010010
01010010
In Cyclic Redundancy Check, the sender and receiver must be agreeing upon a
generator polynomial G (x). Let the degree of frame M (x) is m, which must be longer
than generator polynomial.
The steps are as follows:
1. Let the degree of G (x) is r. Append r zeros at the right-end of the frame. Now the frame
contains m+r bits and the polynomial becomes xrM(x).
2. Divide xrM(x) by generator polynomial G(x) using modulo 2 division.
3. Subtract the remainder (which is less to equal to r bits) from x rM(x) using modulo 2
subtraction.
4. The resultant frame T (x) is to be transmitted.
Data
00...0
n bits
Data
CRC
Data
CRC
Divisor n+1 bits
n bits
Divisor n+1 bits
Remainder
CRC
If zero accept, Remainder
nonzero discard
Sender
n bits
Receiver
Now T (x) is divisible by G (x). When the receiver gets the transmitted frame and tries to
divide by G (x). If there is a remainder then there has been a transmission error, otherwise
the frame is error-free.
Frame:
-1101011001
Generator: - 1 0 0 1 1
After adding 4 zero bits: - 1 1 0 1 0 1 1 0 0 1 0 0 0 0
1 0011
10011
11010110010000
11000010 0
76
10011
10011
10010
10011
1000
Remainder: - 1 0 0 0
Transmitted frame: - 1 1 0 1 0 1 1 0 0 1 1 0 0 0
If there are no errors, the receiver receives T (x) intact. The received frame is divided by
G(x).
1 0011 11010110011000
11000010 0
10011
10011
10011
10011
10011
000
Since there is no remainder, it is assumed that there have been no errors.
There are different types of generator polynomials, which are used for this purpose. Both
the high and low order bits of G (x) must be one. A polynomial should be selected to have
following properties:
• It should not divisible by x. It guarantees that all burst errors of a length equal to the
degree of polynomial are detected.
• It should be divisible by x + 1. It guarantees that all burst errors affecting an odd number
of bits are detected.
Some standard polynomials are following:
= x8 + x2 + x + 1
=x10 + x9 + x5 + x 4 + x2 + 1
= x12 + x11+ x3 + x2 + x +
1
CRC-16
= x16 + x15+ x2+1
CRC-CCITT / ITU-16 = x16 + x12 + x5 + 1
CRC-8
CRC-10
CRC-12
77
ITU-32
= x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1
Checksum
In sender, the checksum generator divides the data unit into equal segments of n bits
(typically 16). These segments are added using ones complement to get the sum in such a
way that the total is also n bits long. The sum is then complemented and appended to the
end of the original data unit as redundancy bits, called the checksum field. This extended
data unit (data unit with checksum) is transmitted across the network. So, if the sum of the
data segments is S then the checksum will be –S.
Checksum generator follows these steps:
•
The data unit is divided into equal segments of n bits.
•
All the sections are added using ones complement to get the sum.
•
The sum is complemented which called as checksum.
•
The checksum append with data unit and sent with data.
In receiver, the checksum receiver divides the extended data unit into equal segments of n
bits. All the segments are added using ones complement to get the sum and sum is then
complemented. If the extended data unit is intact, the result should be zero. Otherwise the
data unit contains an error and receiver rejects it.
Checksum checker follows these steps:
•
The data unit is divided into equal segments of n bits.
78
•
•
•
All the sections are added using ones complement to get the sum.
The sum is complemented.
If the result is zero, the data are accepted; otherwise the data unit contains an
error so they are rejected.
Error Correction
Error correction can be handles in several methods. The two most common are error
correction by retransmission (which referred as backward error correction) and forward
error correction. Forward error corrections are used in some situations where
retransmission is impractical. Examples are broadcast situations in which there are multiple
receivers for one transmission and space probes, which essentially use simplex
transmission. These find more use in applications other than data communication, such as
computer memory.
Forward Error Correction
Single-bit error can be detected by the addition of a redundancy bit (parity bit). The
additional bit can detect single-bit errors, because it distinguishes between only two states:
error or not error. These two states are sufficient for error detection.
In the case of single-bit error correction in a 7-bit ASCII character, the error correction
code must determine which of the 7 bits has changed. In this case there are eight different
states: no error, error in position 1, error in position 2, and so on, up to error in position 7.
To show all eight states (000 to 111), it requires 3-bit redundancy code. Seven bits of ASCII
character plus 3 bits of redundancy equals to 10 bits. Three bits can identify only eight
possibilities. But what happen if an error occurs in redundancy bits?
To calculate the number of redundancy bits r required to correct a given number of data
bits m, it is need to find a relationship between r and m. With m data bits and r redundancy
bits, the total resulting code is (m + r). If the transmitting unit has (m + r) bits, then r must
be able to indicate at least (m + r + 1) different states. One state is no error and other (m +
r) states mean error in each of the (m + r) positions.
Therefore, r bits must be discover (m + r + 1) states. And r bits can indicates 2r different
states. So, 2r must be equal to or greater than (m + r + 1). 2r>= m + r + 1
For example, if the value of m is 7, the smallest r value that can satisfy this equation is 4:
24>= 7 + 4 + 1
Following table shows possible m values and the corresponding r values.
Number of Data Bits (m)
1
2-4
5-7
Number of Redundancy Bits (r)
2
3
4
Total Bits (m+r)
3
5-7
9-11
Single-bit Error correction by Hamming code
Hamming code can correct single bit error. To correct the single bit error it follows these
steps:
79
• The bits of the codeword are numbered consecutively, starting with bit1 at the right
end. The bits that are power of 2 (1, 2, 4, 8, 16, etc.) are check bits or redundancy
bits(r). The rest bits (3, 5, 6, 7, 9, etc.) are filled up with the m data bits.
• For example, a 7 bit ASCII character requires 4 redundancy bits that can placed in
positions 1, 2, 4 and 8 (that are powers of 2), these bits are refer as r1, r2, r4, r8. The
80
data bits are found in bit positions 3, 5, 6, 7, 9, 10, 11 and 12. Therefore, the original
codeword encoded as 11 bits codeword using a Hamming code.
12
d
11
d
10
d
9
d
8
7
d
r8
6
d
5
D
4
r4
3
d
2
r2
1
r1
Layout of data and redundancy bits (8+4=12):
Bit position
Position number
Redundancy bit
1
0001
r1
2
0010
r2
3
0011
4
0100
r4
5
0101
6
0110
7
0111
8
1000
r8
9
1001
10
1010
11
1011
12
1100
In the Hamming code, each redundancy bit is the parity bit; for one
bits as given below:
Data bit
d1
d2
d3
d4
d5
d6
d7
d8
combination of data
r1: bits 1, 3, 5, 7, 9 and 11 r2:
bits 2, 3, 6, 7, 10 and 11
r4: bits 4, 5, 6, 7 and 12 r8:
bits 8, 9, 10, 11 and 12
• Each redundancy bit operates on each data bit position whose position number contains a
‘1’ in the corresponding column position.
11
1
10
1
9
0
8
r8
7
1
6
0
5
0
4
r4
3
1
2
r2
1
r1
11
1
10
1
9
0
8
r8
7
1
6
0
5
0
4
r4
3
1
2
r2
1
1
11
1
10
1
9
0
8
r8
7
1
6
0
5
0
4
r4
3
1
2
0
1
1
11
1
10
1
9
0
8
r8
11
1
10
1
9
0
8
0
85
Redundancy bits calculation
7
6
5
4
1
0
0
1
7
1
6
0
5
0
4
1
3
1
2
0
1
1
3
1
2
0
1
1
Calculating redundancy bits:
• Placed the data bits in its original position in the 11-bit codeword. Data bit positions
are 3, 5, 6, 7, 9, 10 and 11.
• Calculate the redundancy bits with even parities for the various bit combination.
The parity value for each combination is the value of the corresponding r bit.
• The total 11-bit unit sends to the receiver.
1
0
1
0
10
Error detection using Hamming code
Error Detection and Correction:
• Suppose, by the time the above transmission is received, the number 10 bit has been
changed from 1 to 0.
• The receiver gets the transmission and recalculates four redundancy bits, using
same sets of combination with even parities.
• Assembles the four redundancy bits into a binary number in order of r position (r8,
r4, r2, r1).
• The binary number is the precise position of the bit in error. In example this binary
number is 1010 (10 in decimal).
• After the error detection, the receiver can easily invert its value and corrects the
error. In example 10th bit invert from 0 to 1 and accept the changed data unit.
Burst Error correction by Hamming codeNote: This technique can easily be
implemented in hardware and the code is corrected before the receiver knows about it.
Hamming codes can only correct single bit errors; it cannot correct a burst error. But it is
possible to correct burst error by applying Hamming code.
Error Detection and Correction:
• Sequences of k consecutive codewords are arranged as a matrix, one codeword in
each row.
• The data should be transmitted one column at a time starting with the leftmost
column. When the first column has been sent, the
second column is sent and so on.
•
At receiver when the frame arrives, the matrix is
time.
reconstructed, one column at a
•
1 0 0 1 1 1 0 0 1 0 1
If the burst error of length k or less occurs, at
most 1 bit in each of the k codewords will be
affected. The Hamming code can correct the corrupted bit in each codeword. So the
entire block of data can be automatically corrected.
In example six data units are sent where each data unit is a 7 bit ASCII character
with Hamming code redundant bits. These data units are arranged as a matrix.
87
1 1 0 0 1 0 0 1 1 0 1
1 1 1 1 1 0 0 1 1 0 0
1 1 1 1 1 0 0 0 0 1 1
1 0 1 0 1 0 1 1 1 1 1
0 1 1 1 1 0 0 1 1 1 1
Data before being sent
1
0
0
1
1
1
0
0
1
0
1
1
1
0
0
1
0
0
1
1
0
1
1
1
1
1
1
0
0
1
1
0
0
1
1
1
1
1
0
0
0
0
1
1
1
0
1
0
1
0
1
1
1
1
1
0
1
1
1
1
0
0
1
1
1
1
Data in transition
1 0 0 1 1 1 0 0 1 0 1
The data unit transmitted
one column at a time
starting with the leftmost
column. When the first
column has been sent,
the second column is sent
and so on. The bits are
corrupted by a burst error
are shown in red colors.
1 1 1 1 1 0 0 0 0 1 1
Five consecutive bits
are corrupted during transmission. After
receiving, the data unit reorganized as a
matrix. If the length of burst error is equal to
or less than number of data unit, then at most
1 bit in each of the k data unit will be affected
And apply Hamming code to correct the
corrupted bits. Therefore the whole block of
data can be corrected.
1 0 1 0 1 0 1 1 1 1 1
CHAPTER 6
1 1 0 0 1 0 0 1 1 0 1
1 1 1 1 1 0 0 1 1 0 0
0 1 1 1 1 0 0 1 1 1 1
Received data
LOCAL AREA NETWORK
85
An interconnection of autonomous computers geographically spread within a small area ( a
few kilometers in diameter) is called a Local Area Network or LAN in short.
IEEE 802 family of Standards
The 802 family of Standards was developed by the IEEE (Institute of Electrical and
Electronics Engineers) to enable equipments of different manufactures to communicate
between themselves over LANs and MANs. The IEEE 802 family of standards has several
individual standards already developed and accepted and some more standards are under
developed. The developed standards within the IEEE 802 Standards Family have also
adopted by other standards bodies like ISO and ANSI.
The components parts of the IEEE family of standards are shown below.
IEEE family Standards
Descriptions
802.1
Relationship of 802.X standards to ISO model, higher layer protocols,
internetworking, network management and control, etc.
802.2
LLC architecture and protocol
802.3
CSMA/CD bus architecture and access protocol
802.4
Token passing bus architecture and access protocol
802.5
Token passing ring architecture and access protocol
802.6
MAN architectures and access protocol
802.7
Broadcast transmission
802.8
Optical fiber based LANs
802.9
Integration of LANs with PABX technology
802.10
Interoperable LAN security also known as SILS
802.11
Wireless LAN MAC and physical layer specification
802.12
100 Mbps demand priority access method physical layer and repeater, also
known as 100VG-AnyLAN.
The first six components, which together represent the organization of the IEEE, project
802 as the next three components represent later developments dealing with specialized
topics related to LAN technology. The component part 802.2 is common to component
parts 802.3 through 802.6, so that 802.2 Standards together with one of the latter four
Standards describe a particular type of LAN architecture.
802 Architecture
The IEEE 802 committee recommended three-layer communication architecture for LANs
where the layers are respectively named Physical, Media Access Control (MAC) and
Logical Link Control (LLC). The three layers may be viewed as the functional replacement
of the lowest two layers (Physical and Data Link) of the OSI model. The physical layer in
the IEEE 802 LAN standards performs similar functions as the physical layer in the OSI
model, the functions being encoding/decoding of bits, transmission/reception of electrical
signals, synchronization (generation/removal of the preamble signal), etc.
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The LLC and the MAC layers in 802 together perform the basic function of the OSI data
link layer. The LLC layer, the higher of the two layers, provided a service to its higher layer
for moving frames between two stations on the LAN. The function of the MAC layer is to
allocate the multi-access channel between the randomly accessing stations so that each
station can successfully transmit its frame without undue interference from the other
stations.
Application
Presentation
Session
Transport
Network
Data Link
Physical
Network
LLC
MAC
Physical
Lower layers in the OSI and the 802 models
Ethernet LAN
In 1976, Zerox Corporation built a Local Area Network named Ethernet, which connected
100 personal workstations on a 1 km cable and used a 2.94 Mbps data rate. The LAN
employed an access protocol called Carrier Sense Multiple Access with Collision Detect
(CSMA/CD), which really evolved from ALOHA protocol. Based on Ethernet LAN, Zerox
Corporation, DEC and Intel Corporation collaborated to draw up a standard for a 10 Mbps
Ethernet, submitted it to the IEEE and this really became the basis for the IEEE 802.3
standard on Ethernet. The access protocol in IEEE 802.3 standard is called 1-persistent
CSMA/CD.
The Ethernet frame contains eight fields:
Preamble: The first field of the 802.3 frame contains 7 octets of alternating 0s and 1s that
alert the receiving system to the incoming frame and enable it to synchronize its input
timing. The pattern provides only an alert and a timing pulse. The 56-bit pattern allows the
stations to miss some bits at the beginning of the frame. The preamble is actually added at
the physical layer and is not part of the frame.
Start frame delimiter (SFD): This field (10101011) signals the beginning of the frame.
The SFD tells the stations that they have a last chance for synchronization. Destination
address (DA): This field is 6 octets and contains the physical address of the destination
station or stations to receive the packet. The address consisting of all 1 bits is reserved for
broadcast. The higher order bit of the destination address is a 0 for ordinary addresses and
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1 for group addresses. When a frame is sent to a group address, all the stations in the group
receive it. Sending to a group of stations is called multicast.
Source address (SA): This field is 6 octets and contains the physical address of the sender
of the packet.
Length/type: This field is defined as a length or type field. If the value of the field is less
than 1518, it is a length field and defines the length of the data that follows.
On the other hand, if the value of the field is greater than 1536, it defines the type of the PDU
packets that is encapsulated in the frame.
Data: This field carries data encapsulated from the upper-layer protocols. It is a minimum
of 46 and a maximum of 1500 bytes.
Pad: If the data portion of a frame is less than 46 bytes, the pad field is used to fill out the
frame to the minimum size.
CRC:
The last field contains the error detection information, in this case a CRC-32.
7-octet
Preamble
1-octet
Start
delimiter
6-octet
Destination
address
6-octet
Source
address
Length
of Data,
field
Data
0 to 1500
octet
Pad
0 to 46
octet
CRC
4-octet
The IEEE 802.3 data frame format
1 Persistent CSMA protocol works as follows. When a station has data to send, it first listens
to the channel to see if anyone else is transmitting at that moment. If the channel is busy,
the station continues sense the channel until it becomes unused. When the station detects
an unused channel, it transmits a frame. If a collision occurs, the station waits a random
amount of time and starts all over again. The protocol is called 1-persistant because the
station transmits with probability of 1 whenever it finds the channel unused. The Ethernet
protocol i.e. the 1-persistent CSMA/CD protocol, significantly improves upon the
performance of the 1-persistent by adding the “collision detect” feature. In 1persistent
CSMA/CD protocol the station continues sense the channel even while transmitting. When
a transmitting station detects a collision it stops its transmission immediately, which
minimized the channel capacity wastage.
Five types of cabling are commonly used in Baseband 802.3 LANs.
Name
Cable
Max. segment Nodes/segment Advantage
10Base5 Thick coax 500 m
100
Good for backbones
10Base2 Thin coax
200 m
30
Cheapest system
10Base-T Twisted pair 100m
1024
Easy maintenance
10Base-F Fiber optics 2000 m
1024
Best between buildings
First type of cable is 10Base5; it means that it operates at 100 Mbps; using Baseband
signaling and can support segments of up to 500 meters. Connections are generally made
using vampire taps. Second cable type is 10Base2. BNC connectors are using to form
Tjunction.
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Token Bus LAN
Token Bus LAN was standardized in the IEEE 802.4 specification. Physically, the token
bus is a linear or tree-shaped cable onto which the stations are attached. Logically, the
stations are organized in the ring.
The cable forming the bus in 802.4 is a 75-ohm broadband coaxial cable, which is used for
cable television.
Station address
37
11
13
17
Logical ring
Broadband
coaxial cable
34
7
22
19
Direction of token
movement
14
This station
currently out of
logical ring
Architecture of a token bus LAN
The LAN access by a special control frame called token, which is passed around the logical
ring, formed by connecting the highest numbered station, the successive lower numbered
station and back to the highest numbered stationed itself. Each station knows the address of
its predecessors (upstream neighbour in the logical ring) and the successors (downstream
neighbour). After receiving the token, a station sends its data frames and then passes the
token to its successor. The token bus LAN uses a logical ring based upon a coaxial cable,
which allows bidirectional propagation of electrical signals rather than a physical ring. The
position of a station in the logical ring is determined by its address rather than its physical
position to the cable. The token must contain the successor address and be passed to the
successor. A station is never physically disconnected but is only logically disconnected
during time of power-off, faulty operation, etc. this means that the insertion into or the
deletion from the logical ring of any station can be carried out simply and no physical
switching is necessary. The Token bus frame contains eight fields:
Preamble: The first field of the 802.4 frame contains 1 octet of alternating 0s and 1s that
alert the receiving system to the incoming frame and enable it to synchronize its input
timing. The pattern provides only an alert and a timing pulse.
Start frame delimiter (SFD): This field signals the beginning of the frame. It contains
analog encoding of symbols other than 0s and 1s.
Frame control: The frame control is used to distinguish data frames from control field it
can also carry an indicator requiring the destination station to acknowledge correct or
incorrect receipt of the frame.
Destination address (DA): This field is 6 octets and contains the physical address of the
destination station or stations to receive the packet.
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Source address (SA): This field is 6 octets and contains the physical address of the sender
of the packet.
Data:
The data field has variable length of data. It may be up to 8182 bytes long
when 2 octet addresses are used and up to 8174 bytes long when 6 octet addresses are
used. Checksum: The checksum is used to detect transmission errors.
End frame delimiter (EFD): This field are used to marks the frame boundary.
This field signals the ending of the frame.
1-octet
Preamble
1-octet
Start
delimiter
1-octet
Frame
control
6-octet
Destination
address
6-octet
Source
address
Variable
length of
Data, up to
8182 octet
4-octet
Checksum
1-octet
End
delimiter
The IEEE 802.4 data frame.
Token Ring
Token ring is a network system that uses a ring topology and a token channel access method.
Token-passing LAN developed and supported by IBM. Token ring was standardized in the
IEEE 802.5 specification. Token ring runs at 4 or 16 Mbps over a ring topology.
Station
Unidirectiona
l ring
Ring
interface
Architecture of a token ring
In a token ring a small frame with special format, called token rotate around the ring in one
consistent direction whenever all stations are idle (no data are being sent). If a station needs
to send data, it waits for the token. The station receives the token from its nearest active
upstream neighbour (NAUN) and sends one or more frames (as long as if has frames to
send or the allocated time has not expired). And finally it released the token to its nearest
active downstream neighbour (NADN). Each station is given an equal chance to have the
token and take control in order to pass data. This is called media access.
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1-octet
Start
delimiter
1-octet
Start
delimiter
1-octet
Access
control field
(a)
1- octet
End
delimiter
1-octet 1-octet 6-octet
6-octet Variable
Access Frame Destination Source length
control control address
address of Data,
up to
4099
octet
4-octet
1-octet
1-octet
Checksum Ending
Frame
delimiter status
(b)
The IEEE 802.5 (a) token frame, (b) data frame.
The token frame contains three fields:
Start delimiter:
This field signals the beginning of the frame.
Access control field: Access control field of the token frame contains four fields: Priority
field (3-bit), Token field (1-bit), Monitor field (1-bit) and Request Priority field (3- bit ).
End delimiter:
This field signals the ending of the frame. The
Token ring frame contains nine fields:
Start frame delimiter (SFD):
This field signals the beginning of the frame. Access
control:
The access control field contains the token bit and also the Monitor bits and
Reservation bits.
Frame control: The frame control is used to distinguish data frames from various possible
control frames. This field indicates whether the variable length user data ( FF=00) or a
management message in the form of a MAC vector (FF =01)
Destination address (DA): This field is 6 octets and contains the physical address of the
destination station or stations to receive the packet.
Source address (SA): This field is 6 octets and contains the physical address of the sender
of the packet.
Data: The data field has variable length of data. It may be as long as possible, provided that
the frame can still be transmitted within the token-holding time. Checksum: The checksum
is used to detect transmission errors.
End frame delimiter (EFD): This field are used to marks the frame boundary.
This field signals the ending of the frame.
Frame status: The frame status field which is the acknowledgement field. It contains the
A and C bits. Both the Address Recognized (A) and the Frame Copied (C) bits are reset to
0 at the time of transmission and are set to 1 by the receiver if it is able to recognize the
address and copy the frame respectively. The frame retransmits only if A=1 and C=0. Three
combinations are possible:
1. A=0 and C=0; destination not present or not powered up.
2. A=1 and C=0; destination present but frame not accepted.
3. A=1 and C=1; destination present and frame copied.
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Token Ring has following features:
• Unlike Ethernet, Token Ring continues to operate reliably under heavy loads.
• Build-in diagnostic, repair program and recovery mechanisms, such as beaconing and
auto-reconfiguration, make the protocol more reliable.
• Token Ring makes connecting a LAN to an IBM Mainframe easier.
• Fault-tolerance features are provided through ring reconfiguration, called ringwrap.
• Token Ring can be very difficult to troubleshooting and requires considerable
proficiency.
• Token Ring cards and equipment are very costly than Ethernet or ARCnet systems.
FDDI
FDDI (Fiber Distributed Data Interface) is a high-speed (100 Mbps) fiber optics token ring
LAN over distances up to 200 km with up to 1000 stations connected. Today an FDDI
network is also used as a MAN.
FDDI uses fiber optics cables to implement very fast, reliable networks. FDDI uses
multimode fiber, which is less expensive than single mode fiber. It also used LEDs as a
light source rather than lasers.
The FDDI uses dual ring topology. The cabling consists of two fiber rings, one transmitting
clockwise and other transmitting counterclockwise. If either one break, the other can be
used as a backup. If both break at the same point, the two rings can be joined into a single
ring. Each station contains relays that can be used to join the two rings or bypass the station
in the incident of station problem.
There are two classes of stations, A and B, used in FDDI. Class A stations can be connected
to both types of rings. The class B stations can be connected to only one type of ring and
are cheaper. During installation, either class A or class B is selected depending upon the
importance of fault tolerance.
Class B
node 4
Primary ring Class A
node 1
Writing
Concent
rator
Secondary
ring
Class A
node 2
Class B
node 3
FDDI class A and class B node connections
The basic FDDI protocols are closely modeled by the 802.5 protocols. If a station wants to
transmit data, it must first capture the token. After getting the token, the station transmits a
frame and removes it when it comes around again. There is one difference between FDDI
and 802.5. In the case of 802.5, a station may not generate a new token until its frame has
gone all the way around and comes back. In FDDI having 1000 stations and 200 km of
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fiber, the wastage of time while waiting for the frame is substantial. For this reason, a
decision has been taken to allow a station to put a new token back on the ring as soon as it
has finished transmitting its frames. In this situation, several frames must be present in the
ring at the same time.
(a)
(b)
The X.25 protocol mapped to the OSI model
The FDDI frame contains nine fields:
Preamble: The first field of the 802.5 frame contains 1 octet of alternating 0s and 1s that
alert the receiving system to the incoming frame and enable it to synchronize its input
timing. The pattern provides only an alert and a timing pulse.
Start frame delimiter (SFD): This field signals the beginning of the frame. It contains
analog encoding of symbols other than 0s and 1s.
Frame control: The frame control field tells what kind of frame this is (data, control, etc.)
Destination address (DA): This field is 6 octets and contains the physical address of the
destination station or stations to receive the packet.
Source address (SA): This field is 6 octets and contains the physical address of the sender
of the packet.
Data:
The data field has variable length of data. It may be up to 8182 bytes long
when 2 octet addresses are used and up to 8174 bytes long when 6 octet addresses are
used. Checksum: The checksum is used to detect transmission errors.
End frame delimiter (EFD): This field are used to marks the frame boundary.
This field signals the ending of the frame.
Frame status: The frame status byte holds acknowledgement bits, similar to those of 802.5.
8-octet
Preamble
1-octet
Start
delimiter
1-octet
Frame
control
6-octet
Destination
address
6-octet
Source
address
Wireless LAN
Bluetooth
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Variable
length of
Data, up to
4478 octet
4-octet
Checksum
1-octet
Ending
delimiter
1-octet
Frame
status
Bluetooth is a wireless LAN technology designed to connect devices of different functions
such as telephones, notebooks, personal computers, cameras, printers, and so on. Bluetooth
is a network, which formed spontaneously. The devices on the network sometimes called
gadgets, which connects each other and form a network called piconet
Bluetooth defines two types of networks: piconet and scatternet.
Piconet
A Bluetooth network is called a piconet. A piconet can have up to eight stations, one of
which is called the master. The rest stations are called slaves. All the slaves are
synchronized their clocks and hopping sequence with the master. A piconet can have only
one master station and maximum of seven slaves. But an additional eight slaves can be in
the parked state. A slave station in the parked state is synchronized with the muster but
cannot take part in communication until it is moved from the parked sate. As only eight
stations can active in a piconet, this is why when an active station goes to the parked state
then a station can be active from the parked state.
Scatternet
Piconets can be combined to form a scatternet. A slave station in one piconet can become
the master in another piconet. This station can receive data from the master in the first
piconet (as a slave) and deliver the data to the slaves in the second piconet (as a master). A
station can be member of two piconets.
CHAPTER 7
INTERNET WORKING
Electronic mail (Email)
Email (Electronic mail) is most popular network services. Email is used for ending a single
message that includes text, voice, video or graphics to one or more receivers. SMTP (Simple
Mail Transfer Protocol) has to design to handle electronic mail in Internet.
telnet
The telnet protocol is used to establish an on-line connection to a remote system. It is used
the same way as the telnet program.
The telnet utility used to connect remote UNIX system from Windows system. Before the
use of telnet there must have an account on the remote system. If you enter telnet command
with the IP address you have to enter user name and password to gain the access to the
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remote system. As long as you are logged in, anything you type host terminal is sent to the
remote system. Any files that you access or any commands that you run will always be on
remote system. At the time of log out you can press <Ctrl+d>or type exit to log out and
return to the local host.
FTP (File Transfer Protocol)
TCP/IP has a special command ftp to transfer files between two computers that is widely
used in internet. The command can be used to transfer both binary and text files.
The ftp command works in two steps. First, it makes a connection with the remote system.
This can be done in two ways: either by ftp with hostname (comp) or another ftp and later
open command with hostname. After the connection has been established, it asks for the
username and the password.
Termination of ftp is done in two steps. At first disconnect the remote system by close
command and then quit ftp by typing bye or quit.
World Wide Web (WWW)
The World Wide Web is an architectural framework for accessing linked documents spread
out over thousands of computers all over the internet. The WWW has a unique combination
of flexibility, portability and user-friendly features. The WWW project was initiated in
1989 by CERN, the European center for nuclear research.
The WWW
Internet Telephony
Short Messaging Services (SMS)
Internet Fax
Video Conferencing: VoIP
Voice over IP (VoIP) is a real-time interactive audio/video application. The idea is to use
the Internet as telephone with video and some other capabilities. Instead of communicating
over circuit-switching network, this application provides communication between two
parties over the packet-switched Internet. Two protocols have to design to handle this type
of communication: SIP and H.323.
HTML (HyperText Markup Language)
We pages are written in a language called HTML (HyperText Markup Language). HTML
allows users to produce Web pages that include text, graphics and linkers/pointers to other
Web pages.
A proper Web page consists of a head and a body enclosed by <HTML> and </HTML>
tags (formatting commands).
Head
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The head is the first page of a Web page. The head contains the title of the page and other
parameters that the browser will use. The head is bracketed by the <HEAD> and < /HEAD>
tags. Body
The actual contents of a page are in the body, which includes the text and the tags. Whereas
the text is the actual information contained in a page and the tag define the appearance of
the document. The body is bracketed by the <BODY> and </BODY> tags.
A tag is enclosed in two signs (< and >) and usually comes in pairs. The beginning tag starts
with the name of the tag and the ending tag starts with a slash followed by the name of the
tag. The commands inside the tags are called directives. Tags can be in either lowercase or
uppercase. That is <HEAD> and <head> means the same thing.
A tag can have a list of attributes, each of which can be followed by an equals sign and a
value associated with the attribute.
The format of beginning tag:
< TagName Attribute = Value Attribute = Value Attribute = Value ……>
The format of ending tag:
< /TagName >
URL (Uniform Resource Locators)
DHTML
XML
ASP
Network programming concepts with Java/PHP
Concepts of Web Site Design and Hosting
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CHAPTER 8
NETWORK SECURITY
Security provides four services: privacy, authentication, integrity and non-repudiation.
Privacy
Privacy has to do keeping information out of the hands of unauthorized persons. That means
the sender and the receiver expects confidentiality.
Authentication
Authentication means that the sender needs to be sure of the sender’s identity and that an
imposter has not sent the message.
Integrity Nonrepudiation
Encryption/Decryption
Encryption is a process which converting stored or transmitted data to a coded form in
order to prevent it from being read by unauthorized persons. It is also application of a
specific algorithm to alter the appearance of data, making it incomprehensible to those who
are not authorized to see the information.
Decryption is the reverse application of an encryption algorithm to encrypted data, in that
way restoring the data to its original, unencrypted state.
Digital Signature
An electronic message can be authenticated by a digital signature. Digital signatures are
another way to assume the recipient of an electronic message that the message is coming
from the right party. Also string of bits appended to a message that provides authentication
and data integrity.
97
APPENDIX -I
Hubs
Hub is a central controller device in a star topology that provides a common connection
among the devices. Each device has a dedicated point-to-point link only to a central
controller. If one device wants to send data to another, it sends the data to the hub, which
then relays the data to the other connected device. A hub is actually a multiport repeater
and as such it obeys the same rules as repeater. They operate at the OSI model Physical
Layer.
Hubs can also be used to create multiple levels of hierarchy (tree topology). Connecting
Hubs together through ports creates Cascading Hubs. One master hub (level 1) is connected
to many slave hubs (level 2). The slave hubs are masters to slave hubs (level 3) in a
hierarchy tree. The maximum number of stations in a Cascaded Hub Network is limited to
128.
•
•
•
•
•
•
•
•
Hubs can use as a central unit from which to connect multiple nodes into one
network.
It can permit large numbers of computers to be connected on single or multiple
LANs.
It can reduce network congestion by centralizing network design.
Enable high speed data communication.
Hubs provide connections for several different media types (e.g. twister pair,
coaxial, fiber optics).
Consolidate the network backbone.
Provide multi-protocol services, such as Ethernet-to-FDDI connectivity.
Enable centralized network management.
Switches
Switch is a device connecting multiple communication lines together. Switches have all but
replaced bridges except for small application. Switches constantly monitor the traffic that
comes across them and reroute their internal connections atomically to provide the most
efficient operation for the network.
The switch normally has a buffer for each link to which it is connected. When it receives a
packet, it stores the packet in the buffer of the receiving link and checks the address to find
the outgoing link. If the outgoing link is idle, the switch sends the frame to the particular
link.
Switches are made based on two different strategies: store-and-forward. A store-andforward
switch stores the frame in the input buffer until the whole packet has arrived.
Gateway
Gateway is a device that is used to interface two different incompatible network facilities.
Gateways perform protocol conversion for all seven layers of the OSI model. A common
98
use for a gateway is to connect the Internet to the telephone networks or to connect a LAN
and a larger system, such as a mainframe computer or a large packet-switching network,
whose communications protocols are different. A gateway reformats the data so that it will
be acceptable to the system it is passing into by changing protocols and transmitting packets
between two entirely different systems.
Gateways handle messages, addresses and protocol conversions necessary to deliver a
message from one network to different network. Gateways offer greatest flexibility in
internetworking communications.
A gateway is a combination of hardware and software with its own processor and memory
used to perform protocol conversions. It is usually slower then a bridge or router because
they need to perform such intensive conversion and that they can be expensive. And more
complex design, implementation, maintenance and operation of a gateway.
• The gateway determines where the packet is going and also converts the message from
one packet to another or from one data code system to another.
• A gateway is slower than router.
Routers
Routers are devices that connect two or more logically separate networks. They consist of
a combination of hardware and software. The hardware may be a network server, a separate
computer and the physical interfaces to the various networks in the internetworking. . The
two most important parts of software in router are the operating system and the routing
algorithms.
Routers use logical and physical addressing to interconnect different physically and
logically separate network segments. They organized a large network into logical network
segments. Each sub-network is given a logical address, which allows the networks to be
separate but can communicate each other and transmit data when necessary. Data is grouped
into packets, each packet having a physical device address, has a logical network address.
The router examines the data contained in every packet it receives for detailed information.
Based on information, the router decides whether to block of packet from the rest of the
network or transmit it. Router also attempts to send the packet by the most efficient path
through the network. Routers do this by using various routing protocols. It also uses one or
more metrics to determine the optimal path along which network traffic should be
forwarded.
Routers operate at the network layer of the OSI model. Routers slow down network
communications, so do not use them unnecessarily.
• Routers perform a function very similar to that a bridge, but routers keep the networks
separate.
• Router processing slower than bridge processing, because they have to check both the
device address and network address.
• Routers are more intelligent than bridges because they use algorithms to determine the
best path to send a packet to a network.
• Routers work at the network layer of the OSI model. Whereas bridges operate in both
the physical and the data link layers.
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•
•
Routers do not alter the form of the packet as gateways. They retransmit the packets in
its original form by store-and-forward services.
A router is slower than bridge but faster than gateway.
Bridges
Bridge is a device that supports LAN-to-LAN communications. Bridges handles traffic
between two similar or different LANs.
This device bridges two different network segments regardless of their topology or wiring.
It memorized all the network addresses on the both sides of the segments and manage the
flow of traffic between the LANs by reading the address of the every packet of data that it
receives. The address is contained in the header of each network packet being transmitted.
A bridge operates in both the physical and the data link layers. As a physical layer device,
the bridge regenerates the signal it receives. As the data link layer device, it can check the
physical (MAC) addresses (source and destination) containing in the frame.
A bridge has filtering and forwarding capabilities. It can check the destination address of a
frame and decide if the frame should be forwarded or dropped. If the frame is to be
forwarded, the decision must specify the port. A bridge has a table that maps addressed to
ports.
Segment A
Bridge
Segment B
The bridge connecting two different types of networks
A bridge operates in the following manner:
2. A bridge receives all signals from both segment A and B.
3. The bridge reads the addresses and filters all signals from A that are addresses to
segment A, because they need not to cross the bridge.
4. Signals from segment A addressed to a computer on segment B are retransmitted to
segment B.
5. The signals from segment B are treated in the same way.
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Difference between Router and Bridge:
• Routers perform a function very similar to that a bridge, bur routers keep the networks
separate.
• Router processing slower than bridge processing, because they have to check both the
device address and network address.
• Routers are more intelligent than bridges because they use algorithms to determine the
best path to send a packet to a network.
• Routers work at the network layer of the OSI model. Whereas bridges operate in both
the physical and the data link layers.
Transceivers
The transceiver is a transmitter and a receiver. It transmits signals over the medium; it
receives signals over the medium; it also detects collisions.
A transceiver can be external or internal. An external transceiver is installed close to the
media and is connected via an AUI to the station. An internal transceiver is installed inside
the station (on the interface card) and does not need an AUI cable.
The transceiver or medium attachment unit (MAU) is medium-dependent. It creates the
appropriate signal for each particular medium. There is a MAU for each type of medium
used in 10-Mbps Ethernet. The coaxial cable need its own type of MAU, the twisted-pair
medium needs a twisted-pair MAU and fiber-optic cable need a fiber-optic MAU.
Repeaters
A repeater is a device that extends the distance a signal can travel by regenerating the signal.
It operates only in the Physical Layer. Signals carry information within a network can travel
a fixed distance before attenuation. A repeater receives a signal before it becomes too weak
or corrupted, regenerates the original bit patterns. The repeater then transmits the
regenerated signal. A repeater can extend the physical length of the network. The portions
of the network separated by the repeater are called segments. Repeater cannot connect two
LANs; it connects two segments of the same LAN. Repeater acts as a two-port node; when
it receives a frame from any of the ports, it regenerates and forwards it to the other port. A
repeater forwards every frame; it has no filtering capability. A repeater does not amplify
the signal; it regenerates the signal. When it receives a weakened or corrupted signal, it
creates a copy, bit for bit, at the original strength. A repeater can retransmit signals in both
directions.
X.25 Protocol
X.25 is an ITU-T (International Telecommunications Union) standard that defines how a
connection between a DTE and DCE is maintained for remote terminal access and computer
communications in packet-switching networks.
X.25 allows a variety of devices that are designated as data terminal equipment (DTE) to
talk to the public data network (PDN). The PDN is designated as data communications
equipment (DCE), as are devices as modems, packet switches and other ports.
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Hardware/software devices, such as terminals, hosts and routers that deliver data to or from
a network I/O port are DTE. The X.25 protocol is a DTE-to-DCE synchronous interface.
To begin communication, one DTE device calls another DTE to request a data exchange
session. The DTE called can accept or refuse the connection. If the called DTE accepts the
connection, the two systems begin full-duplex data transfer. Either side can terminate the
connection at any time.
X.25 is connection-oriented and supports both switched virtual circuits and permanent
virtual circuit. A switched virtual circuit is created when a DTE sends a packet to the
network requesting to make a call to a remote DTE. A permanent virtual circuit is used
the same way but it is set up in advanced by agreement between the customer and carrier.
It is always present and no call setup is required to use it.
X.25 protocol is commonly used in wide area communications with multiple
communicating devices. It is much simpler for interfacing equipment or networks. X.25
network is typically has very low 64 kbps data rate. This is rarely sufficient to support
modern networking. X.25 is usually far less expensive than –Frame Relay and ATM. X.25
functions
at the network layer. It normally interfaces with the protocol called
Application
LAPB
(Link
Presentation Access Procedures Balanced) at the data link layer, which in turn
runs over
X.21 or another physical layer CCITT protocol, such as X.21bis or
Session
V.32.
Transport
X.25 is a
packet-switching protocol that defines the interface between a
Network
synchronous packet-switching host computer and analog
dedicated Data Link circuits or dial-up switched circuits in the voice-grade public data
Physical
network.
It dominant features are:
• Virtual circuit switching and dynamic virtual
X.25
self contained, self-addressed message
LAPB
• Ability to use any available network channels
• Ability to use redundancy error checking at X.21 and others
routing to transport
packets.
or links.
every node.
(a)
The X.25 protocol defines several levels of interface. At the physical level there is the
electrical connection between DCE and DTE. This level uses the X.21 standard for
fullduplex synchronous transmission. X.21 specifies the physical, electrical and procedural
interface between the host and the network.
The second level of X.25 defines the link access procedure. The link access procedure
manages the link between the DTE and the DCE. It designed to deal with transmission
errors on the telephone line between DTE and the DCE.
The third level of the X.25 describes a packet-level procedure, which controls virtual call
through a public data network. This permits the establishment of end-to-end virtual circuits
analogous to vice telephone call. And it deals with addressing, flow control, delivery
confirmation, interrupt etc.
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Finally, X.25 illuminates the function of the packet assembler/ disassembler (PAD). PAD
or black box is installed to which terminals can connect. The PAD provides a connection
to the network and translates the data to and from the terminal into packets that the X.25
network’s DCE can accept.
MODEM
Modem is a device consisting of a modulator and a demodulator. It converts a digital signal
into an analog signal and also converts an analog signal into a digital signal.
For analog channel, mapping is required at the transmitter to convert digital signals into a
suitable waveform by modulation and back-mapping is required at the receiver to reconvert
the received waveform into digital data by demodulation. The respective modules are
known as modulation and demodulation and it perform by MOdulator and DEModulator
respectively which collectively called MODEM. Modem is the device responsible for
allowing a digital signal to be carried over an analog channel. The communication can be
bidirectional. It performs modulation at the sending end and demodulation at the receiving
end.
Modem used to connect the (digital) computer with the (analog) telephone line. Modem
speed range from 300 bps to 56 kbps.
Today, many of the most popular modems available are based on the V-series standards
published by the ITU-T.
ITU Recommendations
Bit rate (bps)
V.21
300
V.22
1200
V.22 bis (bis means second)
2400
V.27 terbo (terbo means third) 4800
V.29
9600
V.32
9600
V.32 bis
14400
V.34
28800
Analog modulation can occur in three ways:
1. Amplitude Modulation (AM).
2. Frequency Modulation (FM).
3. Phase Modulation (PM).
Modulation
FSK
PSK
ASK/PSK
PSK
ASK/PSK
ASK/PSK
ASK/PSK
ASK/PSK
Amplitude modulation
Amplitude modulation varies the strength of the signal to determine whether a zero or a one
bit is being transmitted. A signal with low amplitude represents a zero and signal that has
high amplitude represents a one. Frequency modulation
Frequency modulation uses a change in the number of times per second the sine wave
repeats to indicate a zero or a one. The particular frequency of a sine wave being used to
represent the zero and a frequency that is twice the original to represent the one. Phase
modulation
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Phase modulation uses a change in the phase of the wave to indicate a zero or a one. The
phase of a sine wave being used to create the zero and a phase shift being used to indicate
the one.
Routing Algorithms
The main function of the network layer is routing packets from the source host to destination
host. When the source and destination are not on the same network, the routing algorithms
decide which output line an incoming packet should be transmitted on. Routing algorithms
can be grouped into two major classes: nonadaptive and adaptive.
Nonadaptive algorithms do not base their routing decisions on measurements or current
traffic and topology. When the network is booted the choice of route for each destination is
computed in advance and created a static routing table. This table is not update
automatically when there is a change in the network. The table must be manually altered
by the administrator. This procedure is sometimes called static routing. Static routing is a
packet switching technique, which is more likely to malfunction, and congestion but
reduces s a network’s overhead by relieving the node of many computational
responsibilities. Two types of static routing are available. These are fixed routing and
flooding.
Adaptive algorithms change their routing decisions on current traffic and changes in the
network topology. When the network is booted the choice of route for each destination is
stored in dynamic routing table. The dynamic routing table is updated periodically
whenever there is a change in the network topology or in the current traffic.
Congestion
Congestion in a network may occur if the load on the network (the number of packets sent
to the network) is greater than the capacity of the network (the number of packets a network
can handle). Congestion is an important issue in packet-switching network. When too many
packets are present in the subnet, the performance degrades. This situation is called
congestion.
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Maximum carrying
capacity of subnet
Congested
Packets sent
When too many packets are sent, the performance degrades
When the number of packets transmitted into the subnet by the hosts is within capacity of
the network, they are all delivered. The number of packets are delivered is proportional to
the number of packets are sent. When the number of packets increases too far then the
network no longer able to manage and they begin losing packets. At very high traffic
performance collapses completely and almost no packets are delivered.
Congestion control refers to the techniques to the control the congestion and keeps the load
below the capacity.
Congestion control involves two factors that measure the performance of a network: delay
and throughput.
Congestion control mechanisms can divide into two categories: open-loop congestion
control (prevention) and closed-loop congestion control (removal).
Open-Loop Congestion Control
Open-loop congestion controls are applied to prevent congestion before it occurs. This type
of congestion control is handled by either the source or the destination. Open-loop
congestion controls are follows:
Retransmission Strategy
A good retransmission strategy can prevent congestion. This strategy must be designed to
optimize efficient and at the same time prevent congestion.
Windows Strategy
The size of window at the sender may also affect congestion. The selective repeat window
is better than Go-Back-N window for congestion window.
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Acknowledgement Strategy
The acknowledgement strategy imposed by the receiver may also affect congestion. If the
receiver does not acknowledge every packet it receives, it may slow down the sender and
help prevent congestion.
Discarding Strategy
A good discarding strategy by the router can prevent congestion and at the same time may
not harm the integrity of the transmission. If the routers discard less sensitive packets when
congestion is likely to happen, the integrity of transmission is still preserved and congestion
is prevented.
Closed-Loop Congestion Control
Closed-loop congestion controls are applied to alleviate congestion after it occurred.
Several type of congestion controls have been used by different protocols. Open-loop
congestion controls are follows:
Choke Point
A choke point is a packet sent by a router to the source to inform it of congestion.
Back Pressure
When a router is congested, it can inform the upstream router to reduce the rate of outgoing
packets. The action can be recursive all the way to the router before the source. This process
is called back pressure.
Implicit Signaling
The source can detect an implicit signal concerning congestion and slow down its sending
rate. For example, the delay in receiving an acknowledgement can be a signal that the
network is congested.
Explicit Signaling
The routers that experience can send an explicit signal to inform the sender or the receiver
of congestion. Explicit signaling can occur in either forward or the backward direction.
Backward signaling set in a packet moving in the direction opposite to the congestion. This
signaling can warn source that there is congestion and that it needs to slow down to avoid
the discarding packets. Forward signaling set in a packet moving in the direction of the
congestion. This signaling can warn the destination that there is congestion and that it needs
to slow down the acknowledgements to improve the congestion.
APPENDIX -II
An important concept in communications field is the use of decibel (dB) unit as the basis
for a number of measurements. It is the primary measurement unit in telephony to compare
signal powers or voltage ratios.
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To define, dB let us consider the circuit element shown in figure. This could be an amplifier
or an attenuator or a filter or a transmission line. The input voltage V1 delivers a power P1
to the element and the output power and the output voltage are P2 and V2 respectively. The
power gain G in dB unit for this element is defined as the ratio of two power levels, P1 and
P2:
G = 10 log10 (P2/P1) dB,
where (P2/P1) is referred to as the absolute power gain. When P2>P1 the gain is positive that
means the signal passes through the circuit element is amplified. Whereas if P2<P1, the gain
is negative that means the signal is attenuated (power loss in the circuit element).
V
P1
P2
G
V
12
A circuit element of gain G
If the input and output impedances are constant R, decibels can also be used to represent the
ratio of two voltages:
G = 10 log10 ((V2/R) / (V1/R)) dB
= 20 log10 (V2/V1) dB
The decibel unit is not absolute unit; rather it is a ratio (dimensionless quantity).
The use of decibels is particularly important when computing the overall gain of a cascaded
series of amplification and attenuation elements. Thus using the formula log (AB) = log A
+ log B
If G1, G2… Gn are the gain expressed in dB of cascaded circuit elements, then the overall
gain of those elements is
n
Gtotal = G1 + G2 +… + Gn =∑Gi
i=1
Example
A signal travels a long distance from point 1 to point 4. The signal is attenuated -3 dB by
the time it reached point 2. Between points 2 and 3, the signal is amplified 7 dB. Again,
between points 3 and 4, the signal is attenuated -3dB. We can calculate decibel for the signal
just by adding the decibel measurements between each set of points. In the case, the dB can
be calculated as dB = -3 +7 -3 = +1
This means that the signal has gained power.
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TCP/IP (Transmission Control Protocol / Internet Protocol)
Different features of TCP/IP
•
•
•
•
•
TCP/IP is independent of the network hardware.
TCP/IP can recover failure; it is able to divert data immediately through other routers if
one or more parts of network failed.
TCP/IP provides the facility to connect new subnetworks without significant interference
of services.
TCP/IP is reliable to handling high error rate with facilities for full error control.
TCP/IP is also reliable of transmission of files, remote login, and remote execution of
commands.
Every host in a network has two addresses: a hardwired MAC address and a logical IP
address. TCP/IP uses both these addresses.
MAC Address
Every Ethernet network card has a 48-bit physical address hard-coded into the board by the
hardware manufacturer. This address is unique all over the world and it is known as the
MAC (Media Access Control) address or Ethernet address. The MAC address consists of
a set of six colon-delimited hexadecimal numbers. The IEEE and other standard
organizations have ensured the uniqueness of this address all over the world. This address
is used by one of the layers of the TCP/IP protocol stack.
The MAC address can be known by administrator’s ifconfig (Interface Configuration)
command. A typical line from the command output could read like as:
Eth0 Link
encap: 10Mbps Ethernet
HWaddr
00:00:E8:2E:47:0 c
The ifconfig command (in UNIX) sets the IP address, the subnet mask and the broadcast
address for each interface. Its most basic function is assigning the IP address.
IP Address
An IP address is the logical software address, which is also known as the Internet address
because the numbering scheme of IP address also followed on the Internet. The IP address
is a sequence of four dot-delimited decimal numbers. A typical address looks like as:
192.0.0.101
To use the IP configuration (WINIPCFG) utility (for Windows 98/ME)
1. Click Start, and then click Run.
2. In the Open box, type:
winipcfg
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3. To see address information for your network adapter(s), select an adapter from the
list in Ethernet Adapter Information.
Notes: The IP Configuration utility allows users or administrators to see the current IP
address and other useful information about your network configuration.
TCP/IP has only four layers and these four layers are:
1. Application Layer, representing the application
2. Transport Layer, which controls the reliability of transmission
3. Internet Layer, which takes care of addressing of the data packets
4. Network access Layer, which makes sure that IP addresses are finally converted to
MAC address.
Application Layer
Transport Layer
Internet Layer
Network access Layer
Protocol Stack Layer
TELNET FTP SMTP HTTP DNS SNMP NFS
TCP
UDP
SCTP
IP
ARP RARP
ICMP IGMP
Application
Transport
Internet
Network access
FTP: File Transfer Protocol
IP: Internetworking Protocol
TCP: Transmission Control Protocol
UDP: User Datagram Protocol
ARP: Address Resolution Protocol
RARP: Reverse Address Resolution Protocol
ICMP: Internet Control Message Protocol
IGMP: Internet Group Message Protocol
SMTP: Simple Mail Transfer Protocol
SCTP: Stream Control Transmission Protocol
The Application Layer
On top of the TCP/IP protocol architecture is the application layer. Data generates from the
application layer and it transferred to the transport layer in the form of stream. The
application layer does not add any header to this stream. This layer contains all higherlevel
protocols like FTP, TELNET, SMTP, DNS and so onward. The virtual terminal protocol
(TELNET) allows a user on one machine to log into a remote system and work there. The
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file transfer protocol (FTP) provides a technique to transfer data from one machine to
another machine. SMTP is a specialized protocol that was developed for Electronic mail.
Domain Name Service (DNS) is use for mapping host names onto their network addresses.
HTTP is use for delivering web pages over the network.
Some protocols (such as FTP, TELNET etc.) can be used only if the user has some
knowledge of the network. Other protocols (like OSPF-Open Shortest Path First) run
without the user even knowing that they exist.
The Transport Layer
The top of the internet layer is the Host-to-Host Transport Layer, usually called as transport
layer. The transport layer defines two end-to-end protocols-TCP (Transfer Control
Protocol) and UDP (User Datagram Protocol).
The first one TCP is a reliable connection-oriented protocol that allows a byte stream
generating on one machine to be transferred without error on any other machine in the
network. At the source it divides the incoming stream from application layer into segments,
encapsulating each a header and passes each one onto the internet layer. The header mainly
contains a checksum and a sequence number to facilitate reassembly in the right order at
the other end. At the destination it reassembles the received message into the output stream.
TCP provides error detection and recovery facilities. TCP also handles flow control to make
sure a fast sender cannot swamp a slow receiver with more messages than it can handle.
TCP is used by most applications like ftp, telnet, rlogin, etc. TCP is a connection-oriented
protocol. That means it exchanges control information with the remote system to verify that
it is ready to receive data before any data is sent. When the handshake is successful, the
systems are said to have established a connection and TCP then proceeds with data transfer.
TCP provides reliability with a mechanism called Positive Acknowledgment with
Retransmission (PAR). When the data segment is received errorless, the receiver sends a
positive acknowledgement back to the sender. If the data segment is erroneous, the receiver
discards it. The sending TCP module re-transmits any segment for which no positive
acknowledgement has been received within the timeout period. TCP is quite efficient to retransmission simply the segment which caused the problem, rather than the entire data.
These features of acknowledgements, timeout and retransmission make TCP reliable
protocol. These facilities are not available with UDP.
The second protocol in this layer is UDP, which provides low-overhead, connectionless
datagram delivery service. UDP is an unreliable connectionless protocol. The term
“unreliable” means that there are no techniques in this protocol for verifying that data
reached other end of the network correctly. UDP widely used for client-server requestreply
queries and such applications in which prompt delivery is more important than accurate
delivery. UDP is used some applications like DNS, NFS, etc.
The Internet Layer
The internet layer also known as network layer. The network layer is control to transfer of
IP packets to their proper destination. The internet layer defines two protocols-IP (Internet
110
Protocol) and ICMP (Internet Control Message Protocol). The Internet has several control
protocols used in the network layer, including ARP (Address Resolution Protocol), RARP
(Reverse Address Resolution Protocol) and BOOTP (Bootstrap Protocol).
IP is an unreliable connectionless protocol that receives a segment from transport layer; it
first compares it with the maximum size (MTU, the Maximum Transfer Unit) that the next
layer can handle. Although IP itself can handle a datagram consisting of 65,535 bytes, the
network access layer datagrams hardly ever exceed 1500 bytes. After knowing the MTU of
the data, IP may have to segments into packets or datagrams. The IP datagram header
includes both the source and destination address and sequence number of the fragmented
segments. At the receiving end IP checks the header information and when the header fails
the integrity test it use the ICMP protocol to relay the error message to the sender. IP also
has a role to play in routing the datagrams to their proper destination.
IP is a connectionless protocol. That means it does not exchange control information ( called
a handshake) to establish an end-to-end connection before transmitting data.
IP is called unreliable protocol because it contains no error detection and recovery code. It
does not check whether the data was correctly received to the connected network or not.
ICMP protocol is use to relay an error message to the sender and echoing requests. ICMP
is also used by the ping command, which echoes a request to a host to test the
connectivity of the network.
The Network Access Layer
The network access layer is the lower layer in the TCP/IP protocol hierarchy. The network
layer is also known as link layer. The layer comprises the network interface card, the
protocols and the details of the physical media. It accepts datagrams from internet layer and
encapsulates them after converting all IP addresses to MAC addresses. It finally sends outs
a frame to the wire. ARP (Address Resolution Protocol) is the main protocol used by the
layer.
In IP header contains source and destination address. These are logical 32-bit addresses
consisting of a sequence of four dot-delimited decimal numbers. But the network access
layer can understand only the MAC address, the 48 bits physical address consisting of a
sequence of six colon-delimited hexadecimal numbers. The network access layer has a
translation facility, which converts all IP addresses to MAC addresses and vice versa. ARP
handles this translation. ARP maintains a temporarily storage memory that contains latest
MAC addresses; these are use for transmission without making a broadcasting with host
each time.
Internet Protocol
The functions of Internet Protocol:
• Defining the datagram, this is the basic unit of transmission in the Internet.
• Defining the Internet addressing scheme.
• Moving data between the Network Access Layer and the Transport Layer.
111
•
•
Routing datagrams to remote hosts.
Performing fragmentation and re-assembly of datagrams.
32 bits
Source Port
Destination Port
Sequence n umber
Acknowledgem ent number
TCP
header
length
Reserved
U A P R S F
R C S S Y I
G K H T N N
Checksum
Window size
Urgent pointer
Options (0 or 32 bit words)
Data (optional)
User Datagram Protocol (UDP)
32 bits
Source port
UDP length
Destination port
UDP checksum
IP Addressing
Every host and router on the Internet has an Internet address or IP address. An IP address
is a 32 bit dot-delimited decimal numbers that uniquely and universally defines the
connection of a host or a router in the Internet: no two machines have the same IP address.
Range of Host
addresses
1st octet
2nd octet
3rd octet
4 th octet
Class
0
A
1.0.0.0127.255.255.255 to
112
128.0.0.0
Class
to
10
B
191.255.255.255
192.0.0.0
Class C
to
110
223.255.255.255
Class
D224.0.0.0 to
1110
239.255.255.255
Class E 11110 240.0.0.0
to
247.255.255.255
Classes with IP addressing
There are five classes of IP addresses: Class A, B, C, D and E. Classes A, B and C differ in
the number of hosts allowed per network. Class D is for multicasting and class E is reserved.
IP addresses are designed with two levels of hierarchy. Every IP address consists of two
portions- a network address/number (also known as netid) and a host address/number (also
known as hostid). All nodes in the network have same network address, while the host
address is unique to the host only. The type of network can easily understand by looking at
the first octet of the address. The lowest IP address is 0.0.0.0 and the highest is
255.255.255.255. Network addresses are assigned by InterNIC (Internet Network
Information Center) to avoid conflicts. Local system administrator can set host addresses.
Class A address
This address takes the form N.H.H.H where N is the network address and H is the host
address. The value of N takes in between 1 and 128. The 24 bits are available for H: a class
A network theoretically can handle up to 16,777,216 hosts (but actually can not handle).
Class A address are allotted to very large corporations and universities but are no longer
assigned now. Class A addresses range from 1.0.0.0 to 127.255.255.255
• The first bit of a Class A address is always 0.
• Class A is divided into 128 blocks.
• Total number of addresses in each block is 16,777,216. But many addresses are wasted.
• Class A addresses range from 1.0.0.0 to 127.255.255.255
• Class A was designed for big-size organization.
Class B address
This address takes the form N.N.H.H where first two octets are network address and has
the values in between 128 to 191 in the first octet and any value for the second octet.
Class B network theoretically can handle up to 64,516 hosts. Class B addresses range
from 128.0.0.0 to 191.255.255.255
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•
•
•
•
The first two bits of Class B address are 10.
Class B is divided into 16,384 blocks. 16 blocks are reserved for private address. The
remaining 16,368 blocks are assigned for mid-size organizations.
Each block in Class B contains 65,536 addresses. But many addresses are wasted.
Class B addresses range from 128.0.0.0 to 191.255.255.255
Class C address
This address takes the form N.N.N.H where network address uses first three octets. The
first octet varies from 192 to 223 and the second and third octets can take any values.
This network can support 256 hosts, but in reality, it cal support up to 254 (the 0 and 255
are reserved). Class C addresses range from 192.0.0.0 to 223.255.255.255
•
•
•
•
•
The first three bits of a Class C address are 110.
Class C is divided into 2,097,152 blocks. 256 blocks are reserved for private addresses.
The remaining 2,096,896 blocks are assigned for small organization.
Class C addresses range from 192.0.0.0 to 223.255.255.255
Each block in Class C contains 256 addresses. But many addresses are wasted.
The number of address in class C is smaller than the needs of most organizations.
Class D address
There is one block of class D addresses. These addresses are used for multicasting but have
seen only limited usage. Multicasting addresses are used to address groups of computers all
at one time. They identify a group of computers that share a common application, such as
videoconference, as opposed to a group of computers that share a common network. A
multicast address is a unique network address that directs packets with that destination
address to predefined groups of IP addresses. ). Class D addresses range from 224.0.0.0 to
239.255.255.255
•
The first four bits of Class D address are 1110.
Class E address
There is one block of class E addresses which are reserved by the InterNIC for its own
research and for future use. It is designed for use as reserved addresses. Therefore, no Class
E addresses have been released for use on the Internet. ). Class E addresses range from
240.0.0.0 to 247.255.255.255
•
The first five bits of Class E address are 11110.
Range of Host
addresses
1st octet
2nd octet
3rd octet
114
4 th octet
Class
Network
Host
Network
Host
Network
Host
Multicast address
Reserved for future use
A
0.0.0.0127.255.255.255 to
128.0.0.0 to
Class B
191.255.255.255
192.0.0.0 to
Class C223.255.255.255
Class D224.0.0.0 to
239.255.255.255
Class E240.0.0.0 to
247.255.255.255
IP address format
Reserved Addresses
•
•
Every network itself has an address. Since all hosts on a network have a common
network number of the IP address. This is the first address in the block is used to identify
the organization which is known as network address. This address is obtained by setting
the host number of any node’s address to zeroes. So the organization is not allowed to
use the first address of the block for any host in the network.
Every network needs a separate broadcast address. Directed broadcast; this is the last
address in the block is used to addresses all hosts on another network. This address is
obtained by setting the host number of any node’s address to ones. Obviously this
address can not be assigned to any host in the network. This allows to be sent broadcast
packets to distant LANs anywhere in the network.
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•
Addresses the current host on the current network, such as during a DHCP (Dynamic
Host Configuration Protocol) transaction before a workstation is assigned an IP address.
This address is obtained by setting the all bits of the IP address to zeroes.
• Limited broadcast; addresses all the hosts on the current network. This address is
obtained by setting the all bits of the IP address to ones. This allows broadcast packets
on the local network.
• Addresses a specific host on the current network. This address is obtained by setting the
network number of the IP address to zeroes.
• Internal host loop-back address. This address is obtained by setting first quad to 127.
Packets sent to that address are not put onto network; they are processed locally and
treated as incoming packets. This allows packets to be sent to the local network without
the sender knowing its address.
Subnet
A subnet is simply a subdivision of the network address by taking some of the host number
bits and using them as a subnet number.
Sometimes, an organization needs to divide a network into several smaller groups: each
group is a collection the hosts. For, example, a university may want to group its hosts
according to department. However, the university has one network address, but needs
several subnetwork addresses. The outsides the organization knows the network by its
network address. Inside the organization each subnetwork is known by its subnetwork
address.
Subnetting divides one large network into several smaller networks. It adds an intermediate
level of hierarchy in IP addressing and become three levels of hierarchy. To reach a host on
the Internet, we first reach the network by using network address (netid). Then we reach
the subnetwork by using subnetwork address (subnetid). Finally we can reach the host by
using host address (hostid).
Mask
Since a network administrator knows the network address and the subnetwork addresses,
but router does not know, the router outside the organization has a routing table with one
column based on the network addresses. The router inside the organization has a routing
table based on the subnetwork addresses. The 32-bit number mask is the key. The routers
outside the organization use a default mask and the routers inside the organization use a
subnet mask.
Default Mask
Default masking is a process that extracts the network address from an IP address. A default
mask is a binary number that gives the network address when ANDed with an address in
the block.
Class
11111111
A
00000000
Binary
00000000
Decimal
00000000 255.0.0.0
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Slash
/8
11111111
00000000
11111111
C
00000000
Subnet Mask
B
11111111
00000000 255.255.0.0
/16
11111111
11111111 255.255.255.
0
/24
Subnet masking is a process that extracts the subnetwork address from an IP address. A
subnet mask is a 32-bit binary numbering which the bits correspond to those of the IP
address.
255.255.0.0
Default Mask
11111111
11111111
00000000
00000000
16
255.255.252.0
Subnet Mask
11111111
11111111
111111
00
00000000
10
6
3
Default mask and Subnet mask of Class B.
The number of subnets is determined by the number of extra 1s in subnet mask than the
number of 1s in the corresponding default mask. If the number of extra 1s is n, the number
of subnets is2n. If the number of subnets in N then the number of extra 1s in the subnet mask
is log2N.
In a network each router has a table listing some number of several network addresses (IP
addresses) and some number of host addresses (IP addresses) on the current network. The
first category tells how to locate to distant networks. The second category tells how to locate
to local hosts.
When an IP packet arrives at a router then its destination address is looked up in the routing
table. If the packet is for distant network, it forwarded to the next router on the interface
given in the table. If the packet is for local host on the current network, it sent directly to
the destination. If the network is not present, the packet is forwarded to a default router with
more extensive tables. However subnetting reduces router table space by creating a threelevel hierarchy.
For example, an organization uses a Class C address and it could split the 16-bit host number
into a 6-bit subnet number and a 10-bit host number. This will allow 62 subnetwork (LANs
where 0 and 63 are reserved), each with up to 1022 hosts (0 and 1023 are reserved).
In this example, the first subnet might use IP addresses starting at 130.50.4.1 (in binary
10000010.00110010.00000100.00000001), the second subnet might start at 130.50.8.1 ( in
binary 10000010.00110010.00001000.00000001) and so on.
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If a packet addresses to 130.50.15.6 and arriving at a router is been AND with the subnet
mask of figure to give the address 130.50.12.0. This address is looked up in the routing
tables to find out how to get to hosts on subnet 3.
ISDN (Integrated Services Digital Network)
ISDN (Integrated Services Digital Network) was developed by ITU-T. ISDN data services
integrate voice, data and video information on to a single channel. There are a number of
types ISDN data channels used for this purpose.
ISDN is fully digital circuit-switching telephone system. The primary goal of ISDN system
is the integration of voice and non-voice services.
ISDN is a digital service that can provide a high bandwidth than standard telephone service,
but unlike a leased line, it is not permanent.
The main features if ISDN are as follows:
1) ISDN supports various services related to voice communications (e.g. telephone calls)
and non-voice communications (e.g. digital data transfer).
2) ISDN supports both circuit switching and packet switching.
3) ISDN provides sophisticated service features, maintenance and network management
functions.
4) ISDN has variety of configurations. ISDN can be implemented in a variety of
configurations according to specific national situations.
ISDN System Architecture
All the ISDN installations must have a device called an NT1 (Network Termination 1)
connecting to the ISDN exchange using the twister pair. The network terminating device
has a connector into which a bus cable can be inserted. Up to eight ISDN telephones, ISDN
fax machines, ISDN terminal, ISDN alarm and other devices can be connected to an NT1
by the cable.
Network Termination 1 (NT1)
Non-intelligent devices concerned with physical and electrical characteristics of the signals.
They primarily perform OSI layer 1 functions such as synchronizing and timing. NT1
devices typically form the boundary between a user’s site and ISDN central office. The
central office, in turn functions such as the telephone system’s central office, providing
access to other sites.
Network Termination 2 (NT2)
Intelligent devices capable of performing functions specified in OSI layer 2 and 3 such as
switching, concentration and multiplexing. A common NT2 device is a ISDN PBX (Private
Branch eXchange). It can be used to connect a user’s equipment together or an NT1 to
provide access to the ISDN central office.
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Network Termination 12 (NT12)
NT12 is a combination of NT1 and NT2 in a single device.
Terminal Equipment 1 (TE1)
Terminal Equipment 1 are ISDN devices such as an ISDN terminal, digital telephone or
computer with an ISDN compatible interface.
Terminal Equipment 2 (TE2)
Non-ISDN devices including printers, PCs, analog telephones or anything that has a
nonISDN interfaces such as RS-232 or X.21.
Terminal Adapter (TA)
These devices are designed to be used with TE2 equipment to convert their signals to an
ISDN compatible format. The purpose to integrated non-ISDN devices into a ISDN
network.
Digital
Bit pipe
ISDN
Terminal
ISDN
Telephone
T
U
NT1
U
ISDN
Exchange
ISDN system for domestic or small business purpose
The NT1 performs various operations like network administration, local and remote
loopback testing, maintenance and performance monitoring. To connect a new device to
the bus, the device must be assigned a unique address. When a device is installed in the bus,
it sends a request to NT1 for a unique address. NT1 checks its list of addresses currently in
use and then sends a new address to the new device. NT1 also helps towards contention
resolution. If several devices try to access the bus at the same time, NT1 will determine
which one will get the bus.
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TE1
ISDN
Terminal
S
U
TE1
ISDN
Telephone
NT2
ISDN
PBX
R
U
Non-ISDN
Terminal
LAN
Digital
Bit pipe
T
U
NT1
U
ISDN
Exchange
TA


ISDN system with a PBX for large commercial complex
When we have required more devices to working simultaneously, then NT2 (Network
Termination 2), which is nothing but ISDN PBX (Private Branch eXchange), need
connected to NT1. This provides the actual interface for telephones, terminals and other
equipment. An ISDN PBX is more or less similar to an ISDN exchange; although an ISDN
PBX is smaller and can not handle as many conversions at the same time as an ISDN
exchange can. An ISDN PBX can be directly connected to any ISDN terminal and
telephone. Non-ISDN terminals can be connected to ISDN PBX by using a Terminal
Adapter (TA).
Devices that connect to the S/T interface, such as ISDN telephones, ISDN fax machines,
ISDN terminal, ISDN alarm, are referred as terminal equipment 1 (TE1). Devices that are
not ISDN capable, such as standard analog telephones, fax machines, as well as computers
are called terminal equipment 2 (TE2). To connect a TE2 device to the S/T interface, there
must have an intervening Terminal Adapter.
CCITT defined four reference points; these are R, S, T and U, connecting the various
devices. The R reference point is the connection between the terminal adapter and nonISDN
terminals. The S reference point is connecting the ISDN PBX and the ISDN terminals. The
T reference point is the connection between NT1 and ISDN PBX. The U reference point is
connecting the ISDN exchange and NT1. Nowadays it is a two-wire copper twisted pair,
but in future it may be replaced by optical fibers.
ISDN Communications
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The connection procedure is as follows:
1. The caller transmits a SETUP message to the switch.
2. If the SETUP message is acceptable, the switch returns a CALL PROC (call
proceeding) message to the caller and forwards the SETUP message to the receiver.
3. If the receiver accepts the SETUP message, it rings the phone and sends an
ALERTING message back to the switch, which forward to the caller.
4. When the receiver answers the call, it sends a CONNECT message to the switch,
which forwards it to the caller.
5. The caller then sends a CONNNECT ACK (connection acknowledgment) message
to the switch, which forwards it to the receiver. The connection is now established.
ISDN supports multiple channels by time division sharing. Several channels have been
standardized:
A
B (bearer
channel)
C
D (delta channel)
E
4 kHz analog telephone channel
64 kbps digital PCM channel for voice or data
8 or 16 kbps digital channel
16 kbps digital channel for out-of-band signaling
64 kbps digital channel for internal ISDN
signaling
H
384. 1536 or 1920 kbps digital channel
There are three main types of ISDN interfaces. The services types are as follows:
BRI (Basic Rate Interface): Also called 2B+D, because it consists of two 64-Kbps B
channel and one 16 Kbps D channel. Adding those rates, one avails 144 Kbps. However,
the bit rate of 192 Kbps is achieved including framing, synchronization and other overhead
bits. The basic rate is use for home user or small business or the Internet. It simultaneously
allows voice and data applications like packet-switched access, facsimile and teletex
services. A single multifunctional terminal or several separate terminals can be access those
services. BRI provides 128 kbps bandwidth.
PRI (Primary Rate Interface): Primary rate consists of 23 64-Kbps B channels and one 16
Kbps D channel. It provides 1.5442 Mbps bit rate that is used by the United States, Canada
and Japan. Primary rate may also consist of 30 64-Kbps B channels and one 16 Kbps D
channel. It provides 2.048 Mbps bit rate that is second-hand in Europe.
Hybrid Interface: Hybrid interface consists of one 4 kHz analog telephone A channel and
one 8 or 16 kbps digital channel.
Broadband ISDN (B-ISDN)
Broadband ISDN is based on ATM technology.
Properties of B-ISDN
1. B-ISDN is based on ATM technology.
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2.
3.
4.
5.
B-ISDN can run at 155.52 Mbps and 622 Mbps or more.
In B-ISDN optical fiber will be used.
B-ISDN use packet (cell) switching technology.
It provides different services like video on demand, live television, full motion
multimedia e-mail, CD-quality music, LAN interconnection, high speed data transfer and
many other services.
Properties of N-ISDN
1. Bandwidth of B-ISDN is 2500 times than N-ISDN.
2. N-ISDN can run at 144 Kbps.
3. N-ISDN can be support with twisted pair 4. N-ISDN
technology.
use
circuit
switching
Asynchronous Transfer Mode (ATM)
The basic idea behind ATM is to transmit all data in small, fixed-sized packets called cells.
The cells are 53 octets long, among the cell 5 octets are header and 48 octets are used for
user data.
5 octet
Header
48 octet
User data
The header is composed of six fields:
• Reserved (1 bit)
• Header Error Control (HEC: 8 bits)
• Generic Flow Control (0 or 4 bits)
• Maintenance Payload Type (2 bits)
• Priority Type Identifier (PTI: 1 bit)
• Virtual Path/Circuit Identifier (VPI/VCI: 8 or 12 bits)
Reserved
(1 bit)
Header Error Generic Flow
Control (HEC: Control (0 or
8 bits)
4 bits)
Maintenance
Payload
Type (2 bits)
Priority
Type
Identifier
(PTI: 1 bit)
Virtual
Path/
Circuit Identifier
(VPI/VCI : 8 or
12 bits )
There are two important advantages of using small, fixed-size cells. Firstly, the use of small
cells may reduce queuing delay for a high-priority cell, since it waits less if it arrives slightly
behind a lower-priority cell that has gained access to a resource (e.g. transmitter). Secondly,
the fixed-sized cells can be switched more efficiently for achieving a very high data rate.
In ISDN, evolution begins from old tradition circuit switching into a cell switching in
telephone system.
Advantages of cell-switching over circuit-switching
Item
Circuit switching
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Cell switching
Constant/variable
rate transmission
It can handle only with It can handle with both
variable rate transmission constant rate transmission
(data).
(audio, video) and variable
rate transmission (data).
Broadcasting for television It cannot be use in It provides broadcasting for
distribution
broadcasting for television television distribution.
distribution.
Data rate
It runs with very slow rate It runs at very high rate (
(kilobits per second)
gigabits per second ).
The intended speeds for ATM networks are 155.52 Mbps and 622.08 Mbps with the
possibility of gigabit speeds in future. ATM network was based on AT&T SONET
(Synchronous Optical Network) transmission system as the physical layer carrier.
SONET is preferred for two reasons. First, the high data rate of SONET. Second, in using
SONET, the boundaries of cells can be clearly defined. The transmission rates of SONET
are in range from 51.84 Mbps to 2.48 Gbps.
The B-ISDN ATM Reference Model
The B-ISDN ATM reference model consists of three layers: Physical Layer, ATM Layer
and ATM adaptation Layer. On the top of ATM Adaptation Layer users can put whatever
they want. Unlike ISO OSI model or TCP/IP model, the ATM model is defined as being
three-dimensional.
ATM provides for two types of connections: permanent virtual connection (PVC) and
switched virtual connection (SVC)
PVC: A permanent virtual circuit connection is established between two endpoints by
network provider. The VPI and VCI (Virtual Path/Circuit
Identifier: 8 or 12 bits) are defined for the permanent connections.
SVC: In a switched virtual connection, each time an endpoint wants to make a connection
with another endpoint, a new virtual circuit must be established.
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Plane management
Layer management
Control plane
User plane
Upper layers
Upper layers
CS
SAR
ATM adaptation layer
ATM layer
TC
PMD
Physical layer
The B-ISDN ATM reference model
Physical Layer
The physical layer handle with the physical medium, voltages, bit timing and various other
services. The physical layer is divided into two sublayers: PDM (Physical Medium
Dependent) and TC (Transmission Convergence).
The PDM sublayer interfaces to the actual cable. PDM is medium dependent. It deals with
bit timing and physical network access. For data reception, PDM accepts inbound cell,
verifies the checksum and then forwards to the ATM layer.
The other sublayer of the physical layer is the TC sublayer. It responsible for numerous
functions, including the following:
• Cell rate decoupling.
• Maintaining cell boundaries.
• Generation of Hardware Error Control (HEC) sequence; that is nothing but header
checksum generation and verification
• Cell generation
• Packing or unpacking cells from the enclosing envelope
• Transmission frame adaptation, generation and recovery functions.
ATM Layer
The ATM layer handles with cells and cell transport. It defines the layout of a cell and cell
header. It is responsible for establishment and release virtual connections. The cells are
received from the AAL through those connections. Congestion control is also done in this
layer. The ATM layer is responsible for numerous functions, including the following:
• Flow control
• Cell header generation/extraction
• Virtual circuit/path management
• Cell multiplexing and demultiplexing.
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ATM Adaptation Layer
The ATM Adaptation Layer (AAL) is divided into two sublayers: SAR (Segmentation And
Reassembly) and CS (Convergence Sublayer).
The lower sublayer SAR splits packets into cells on the sender side and puts them back
together again at the destination side.
The upper sublayer CS makes it possible to have ATM systems offers different types of
services to different applications like file transfer and video on demand have different
requirements concerning error handling, timing, etc.
The ATM defines four different AAL: AAL1, AAL2, AAL3/4 and AAL5. The AAL offers
four categories of services: Class A, Class B, Class C and Class D. an example of a Class
A service is circuit emulation. An Example of Class B service is variable bit-rate video.
Class C and Class D correspond to data transfer applications.
CCITT has defined four AAL protocols, one to support each of four classes of service. The
type 1 protocol supports Class A, type 2 supports Class B and so on.
The protocol reference model makes reference to three separate planes:
User Plane
The User Plane provides for user information transfer along with associated controls. It
handles with data transfer, flow control, error correction and other user functions.
Control Plane
Control Plane responsible for cell-control and connection-connection control functions.
Management Plane
Management Plane performs management functions related to a system as a whole and
provides coordination between all the planes and layer management, which provides
management functions relating to resources and parameters residing in its protocol entities.
Bibliography
1. Computer Networks- Andrew S. Tanenbaum.
2. Data Communications and Networking- Behrouz A. Forouzan.
3. Data Communications and Networking- Dr. Madhulika Jain and
Satish Jain.
4. Peter Norton’s Complete Guide to Networking-Peter Norton,
Dave Kearns.
5. Networking: The Complete Reference-Craig Zacker
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VSAT
VSATs (Very Small Aperture Terminals) are low-cost microstations used in satellite
communication. These tiny terminals have 1 meter antennas and can put out about 1 watt
power. In many VSAT systems, the microstations do not have enough power to
communicate directly with another via satellite. Hub is a special ground station, with a large
high-gain antenna which needed to relay traffic between VSATs. In this mode of operation,
either the sender or the receiver has a large antenna and a powerful amplifier. Depending
on the distance between the user and the ground station and the elevation of the satellite,
the end-to-end transit time is 250 to 300 msec; typically 270 msec. There is a time delay of
540 m second between a transmitted and received signal for a VSAT system with a hub.
VSATs can be describes technically as an intelligent earth station connecting to the
geosynchronous satellite suitable for supporting a variety of two-way telecommunication
and information services such as voice, data and video.
The major benefits of VSAT network are:
i. Very simple and easy to install and greater reliability.
ii. High throughput and low bit error rate (BER) for data applications. iii.
Integration of data and voice in one communication medium.
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