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were built and were placed in user work spaces within a
building. This approach allowed users to submit their job from
each office.
As organizations, grew and the need for the computer grew it
became necessary to share the computer with other users in
different buildings. The solution was to utilize the widely used
telephone system to transport these traffic. Even though the
telephone system was designed for voice traffic, various
techniques were employed to send data through the telephone
system. This became known as remote time sharing and is still a
prevalent form of data communications.
CHP: 1 INTRODUCTION
Data Communication:
Data communication is a vital part of the information
society because it provides the infrastructure allowing the
computers to communicate with one another. An airline data
communication system uses data communications to link
reservation offices to the computer. The space flight use data
communications systems to send data to and from the rockets
and command centers on Earth. The purpose of data
communication system is transport of user data between and
among user machines.
Analog and Digital Communication:
Analog data take on continuous values in some intervals.
For example voice and video are continuously varying patterns
of intensity. Similarly, digital data can be text or character
strings. Character form cannot be easily stored or transmitted
by data processing and communication system. Hence, such
systems are designed for binary data.
In a communication system, data are propagated from
one point to another by means of electromagnetic signals. An
analog signal is a continuously varying electromagnetic wave
that may be propagated over a variety of media. A digital signal
is a sequence of voltage pulses that may be transmitted over a
wired medium. For example, a constant +ve voltage level may
represent binary zero and a constant negative voltage level may
represent binary 1. Analog transmission is a means of
transmitting analog signals without regard to their content. The
signals may represent analog data (eg. voice) or digital data( eg.
binary data that pass through a modem). The analog signal will
become weaker after a certain distance. Amplifiers are used to
Evolution of Data Communication:
It came to existence shortly after the computer were
widely used in the organizations. The 1970s and 1980s saw a
merger of the fields of computer science and data
communications that profoundly changed the technology,
products, and companies of the now-combined computercommunications industry. Although the consequences of this
revolutionary merger are still being worked out, it is safe to say
that the revolution has occurred, and any investigation of the
field of data communications must be made within this new
context. In order to obtain the services of the computer user's
simply walk to the room where the computer is located and
submitted a request for the computer to perform a service. This
request was called a job. The computer accepted the user’s job,
performed its operations, and returns the results in hard copy
formats. As the computers grew, it became inefficient for all
users to walk to the computer room, submit their job and return
to get the results. Consequently, computer based terminals
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boost the energy in the signal. However, due to amplifiers the
signal become more distorted in long distance transmission.
Digital transmission is concerned with a content of the
signal. To achieve, signal at greater distances, repeaters are
used. A repeater receives the digital signal, recovers the pattern
of 1s and0s and retransmits a new signal. Thus attenuation is
overcome.
Digital transmission is more preferred due to following
reasons:
1. Digital Technology:
2. Data Integrity
3. Capacity Utilization
4. Security and Privacy
5. Integration
Transmitter: Usually, the data generated by a source system are
not transmitted directly in the form in which they were
generated. Rather, a transmitter transforms and encodes the
information in such a way as to produce electromagnetic signals
that can be transmitted across some sort of transmission
system. For example, a modem takes a digital bit stream from
an attached device such as a personal computer and transforms
that bit stream into an analog signal that can be handled by the
telephone network.
Transmission System: This can be a single transmission line or a
complex network connecting source and destination.
Receiver: The receiver accepts the signal from the transmission
system and converts it into a form that can be handled by the
destination device. For example, a modem will accept an analog
signal coming from a network or transmission line and convert it
into a digital bit stream.
Communication Model:
SOURCE>>TRANSMITTER>>TRANSMISSION
SYSTEM>>RECIEVER>>DESTINATION
Destination: Takes the incoming data from the receiver.
The fundamental purpose of a communications system is
the exchange of data between two parties. Figure presents one
particular example, which is the communication between a
workstation and a server over a public telephone network.
Another example is the exchange of voice signals between two
telephones over the same network. The key elements of the
model are:
Advantages and Disadvantages of Digital Communication:
Advantages:
- It increases immunity to channel noise and external
interference.
- Privacy is preserved by using data encryption.
- Data from voice, video and data sources may be merged and
transmitted over a common digital transmission system.
- Flexible operation of the system.
- Easy to error detection and correction by the use of coding.
Source: This device generates the data to be transmitted;
examples are telephones and personal computers.
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- Long distance communication is possible due to the use of
separator where noise doesn't accumulate.
- Relatively inexpensive digital circuit may be used.
of one or more bytes at a time. As a result, there is a speedup in
parallel transmission bit rate over serial transmission bit rate.
However, this speedup is a tradeoff versus cost since multiple
wires cost more than a single wire, and as a parallel cable gets
longer, the synchronization timing between multiple channels
becomes more sensitive to distance. The timing for parallel
transmission is provided by a constant clocking signal sent over
a separate wire within the parallel cable; thus parallel
transmission is considered synchronous.
Disadvantages:
- Generally, more bandwidth is required than that of analog
communication system.
- Synchronization is required.
- High complexity due to use of analog to digital and digital to
analog.
Serial Transmission: In serial transmission, bits are
sent sequentially on the same channel (wire) which reduces
costs for wire but also slows the speed of transmission. Also, for
serial transmission, some overhead time is needed since bits
must be assembled and sent as a unit and then disassembled at
the receiver. Serial transmission can be either synchronous
or asynchronous. In synchronous transmission, groups of bits
are combined into frames and frames are sent continuously with
or without data to be transmitted. In asynchronous
transmission, groups of bits are sent as independent units with
start/stop flags and no data link synchronization, to allow for
arbitrary size gaps between frames. However, start/stop bits
maintain physical bit level synchronization once detected.
CHP: 2 DATA TRANSMISSION
Serial and Parallel Transmission
Digital data transmission can occur in two basic modes: serial or
parallel. Data within a computer system is transmitted via
parallel mode on buses with the width of the parallel bus
matched to the word size of the computer system. Data
between computer systems is usually transmitted in bit serial
mode. Consequently, it is necessary to make a parallel-to-serial
conversion at a computer interface when sending data from a
computer system into a network and a serial-to-parallel
conversion at a computer interface when receiving information
from a network. The type of transmission mode used may also
depend upon distance and required data rate.
Synchronous and Asynchronous Communication:
Parallel
Transmission:
In
parallel
transmission,
multiple bits (usually 8 bits or a byte/character) are sent
simultaneously on different channels (wires, frequency
channels) within the same cable, or radio path,
and synchronized to a clock. Parallel devices have a wider data
bus than serial devices and can therefore transfer data in words
Asynchronous communication utilizes a transmitter, a
receiver and a wire without coordination about the timing of
individual bits. There is no coordination between the two end
points on just how long the transmitter leaves the signal at a
certain level to represent a single digital bit. Each device uses a
clock to measure out the 'length' of a bit. The transmitting
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device simply transmits. The receiving device has to look at the
incoming signal and figure out what it is receiving and
coordinate and retime its clock to match the incoming signal.
Sending data encoded into your signal requires that the sender
and receiver are both using the same encoding/decoding
method, and know where to look in the signal to find data.
Asynchronous systems do not send separate information to
indicate the encoding or clocking information. The receiver must
decide the clocking of the signal on it's own. This means that the
receiver must decide where to look in the signal stream to find
ones and zeroes, and decide for itself where each individual bit
stops and starts. This information is not in the data in
the signal sent from transmitting unit.
When the receiver of a signal carrying information has to
derive how that signal is organized without consulting the
transmitting device, it is called asynchronous communication. In
short, the two ends do not always negotiate or work out the
connection parameters before communicating. Asynchronous
communication is more efficient when there is low loss and low
error rates over the transmission medium because data is not
retransmitted and no time is spent setting negotiating the
connection parameters at the beginning of transmission.
Asynchronous systems just transmit and let the far end station
figure it out. Asynchronous is sometimes called "best effort"
transmission because one side simply transmits, and the other
does it's best to receive.
EXAMPLES:
Asynchronous communication is used on RS-232 based serial
devices such as on an IBM-compatible computer's COM 1, 2, 3, 4
ports. Asynchronous Transfer Mode (ATM) also uses this means
of communication. Your PS2 ports on your computer also use
serial communication. This is the method is also used to
communicate with an external modem. Asynchronous
communication is also used for things like your computer's
keyboard and mouse.
Think of asynchronous as a faster means of connecting, but less
reliable.
SYNCHRONOUS
Synchronous systems negotiate the communication
parameters at the data link layer before communication begins.
Basic synchronous systems will synchronize both clocks before
transmission begins, and reset their numeric counters for errors
etc. More advanced systems may negotiate things like error
correction and compression. It is possible to have both sides try
to synchronize the connection at the same time. Usually, there
is a process to decide which end should be in control. Both sides
can go through a lengthy negotiation cycle where they exchange
communications parameters and status information. Once a
connection is established, the transmitter sends out asignal, and
the receiver sends back data regarding that transmission, and
what it received. This connection negotiation process takes
longer on low error-rate lines, but is highly efficient in systems
where the transmission medium itself (an electric wire,
radio signal or laser beam) is not particularly reliable.
Transmission Channel:
- In telecommunications and computer networking, a
communication channel, or channel, refers either to a physical
transmission medium such as a wire, or to a logical connection
over a multiplexed medium such as a radio channel.
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- A channel is used to convey an information signal, for example
a digital bit stream, from one or several senders (or
transmitters) to one or several receivers. A channel has a certain
capacity for transmitting information, often measured by its
bandwidth in Hz or its data rate in bits per second
- In information theory, a channel refers to a theoretical channel
model with certain error characteristics.
- In this more general view, a storage device is also a kind of
channel, which can be sent to (written) and received from
(read).
- As your PC's DTE agent, it also communicates with the modem
or other serial device, which, in accordance with the RS-232C
standard, has a complementary interface called the Data
Communications Equipment (DCE) interface.
Interface Standards:
- In telecommunications, an interface standard is a standard that
describes one or more functional characteristics (such as code
conversion, line assignments, or protocol compliance) or
physical characteristics (such as electrical, mechanical, or optical
characteristics) necessary to allow the exchange of information
between two or more (usually different) systems or pieces of
equipment.
RS 232C:
- RS-232C is a long-established standard ("C" is the current
version) that describes the physical interface and protocol for
relatively low-speed serial data communication between
computers and related devices.
- It was defined by an industry trade group, the Electronic
Industries Association (EIA), originally for teletypewriter devices.
- An interface standard may include operational characteristics
and acceptable levels of performance.
- In the military community, interface standards permit
command and control functions to be performed using
communication and computer systems.
- RS-232C is the interface that your computer uses to talk to and
exchange data with your modem and other serial devices.
- Somewhere in your PC, typically on a Universal Asynchronous Receiver/Transmitter (UART) chip on your motherboard, the
data from your computer is transmitted to an internal or
external modem (or other serial device) from its Data Terminal
Equipment (DTE) interface.
- Since data in your computer flows along parallel circuits and
serial devices can handle only one bit at a time, the UART chip
converts the groups of bits in parallel to a serial stream of bits.
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- eg. Periodic signal: Pulse Train
CHP: 3 SIGNALS AND SYSTEM
System:
Signal:
- is a combination of elements, components which perform
some task.
- is a set of element which produces o/p in response to i/p
- Mathematically, y(n) = f[ x(n) ]
- function of independent variables which carry certain
information.
- may be function of time, temperature, pressure, distance
- can be voltage or current in electrical sense.
Discrete System Classification:
Types of Signals:
1. Causal and Non-Causal:
- o/p depends on the present and past value
1. Continuous and Discrete
2. Deterministic and non-Deterministic
3. Periodic and aperiodic
4. Even and Odd signal
5. Energy and Power signal
2. Linear and Non-linear:
- linear if it satisfies principle of superposition
- sum of weighted i/p is same as the sum of weighted o/p
3. Time variant and invariant system:
- Time invariant if the i/p o/p relationship doesnt vary with time
- Shift Invariance
Energy and Power Signals:
- Energy signal has finite energy and zero average power.
0 < E < inf and P = 0
- almost all practical non- periodic signal
- time limited
- rectangular pulse
- E = intg frm - inf to inf | x(t) |2 dt for continuous
- E = sum frm n = -inf to inf | x(n) |2 dn for discrete
4. Static and Dynamic:
- static or memory less if the o/p at any time depends only on
the value of i/p at same time.
- static if its impulse response h(n) is 0 for n != 0
x(n) = del(n) and y(n) = h(n)
5. Stable and unstable:
- stable if it produces bounded o/p from every bounded i/p
- Power signal has finite average power and infinite energy
- 0 < P < inf and E = inf
- almost all practical periodic signal
- can exist over infinte time
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- The minimum sampling rate or minimum sampling frequency,
fs=2fm for complete specification of the continuous time signal is
referred as Nyquist Rate or Nyquist Frequency.
- Sampling of a continuous analog signal is the first step of
transmission of analog signal over digital communication system.
- The sampling theorem states that analog signal can be reproduced
from an appropriate set of its sample taken at some fixed point
interval of time.
Stability of LTI system:
- Consider an input x(n) i.e. is bounded in magnitude |x(n)| < m for
allvaluse of n.
- The o/p of the discrete time LTI system is found by convolution sum
and is given by,
- magnitude of y(n) is given by
- substituting the values of |x(n-k)| <m for all values of k and n,
- From above equation, we can conclude that if the impulse response
h(n) is absolutely summable, then the o/p of the discrete time LTI
system is bounded in magnitude and therefore the system is called
Bounded Input Bounded Output (BIBO) stable.
- A sufficient and necessary condition of stability of a discrete LTI
system is expressed as
S = sum k = - ∞ to ∞ |h(k)| < ∞
Elementary signals:
1. Exponential signal:
- x(t) = Aebt
- Growing exponential and Decaying exponential
2. Sinusoidal signal:
3. Unit step function:
- u(t) = 1 for t >= 0
0 for otherwise
Channel Capacity Theorem:
- This gives the relationship between the channel bandwidth and
signal to noise ratio and the limitation that they impose on
communication.
- Let B be the channel bandwidth and SNR be the recived signal to
Noise Ratio.
- Then, the channel capacity theorem states that,
B = log2(1+SNR) bit/sec
C is defined as the maximum rate at which information may be
transmitted without error through the channel.
4. Unit Impulse Function:
- del(t) = 1 for t = 0
0 otherwise
5. Unit Ramp function:
- r(t) = 0 for t < 0
t for t > 0
6. Sinc Function:
- Sinc(t) = 1 for t = 0
sint/t for t != 0
Nyquist Sampling Theorem:
- Nyquist Sampling theorem states that if x(t) is band limited with no
components at frequencies greater than fm Hz then it is completely
specified by samples, taken at the uniform rate fx>2fm Hz
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- The world's most popular WAN is the Internet. Some segments
of the Internet, like VPN-based extranets, are also WANs in
themselves.
- Finally, many WANs are corporate or research networks that
utilize leased lines.
- WANs generally utilize different and much more expensive
networking equipment than do LANs. Key technologies often
found in WANs include SONET, Frame Relay, and ATM.
CHP: 5 OVERVIEW OF DATA COMMUNICATION
NETWORKING
Types of Network:
1. LOCAL AREA NETWORK
- A local area network (LAN) supplies networking capability to a
group of computers in close proximity to each other such as in
an office building, a school, or a home.
- A LAN is useful for sharing resources like files, printers, games
or other applications.
- A LAN in turn often connects to other LANs, and to the Internet
or other WAN.
Most local area networks are built with relatively inexpensive
hardware such as Ethernet cables, network adapters, and hubs.
Wireless LAN and other more advanced LAN hardware options
also exist.
- Specialized operating system software may be used to
configure a local area network.
- For example, most flavors of Microsoft Windows provide a
software package called Internet Connection Sharing (ICS) that
supports controlled access to LAN resources.
3. METROPOLITAN AREA NETWORK
- A Metropolitan Area Network (MAN) is one of a number of
types of networks.
- A MAN is a relatively new class of network, it serves a role
similar to an ISP, but for corporate users with large LANs. There
are three important features which discriminate MANs from
LANs or WANS:
A. The network size falls intermediate between LANs and
WANs. A MAN typically covers an area of between 5 and 50 km
diameter. Many MANs cover an area the size of a city.
B. A MAN (like a WAN) is not generally owned by a single
organization. The MAN, its communications links and equipment
are generally owned by either a consortium of users or by a
single network provider who sells the service to the users. This
level of service provided to each user must therefore be
negotiated with the MAN operator, and some performance
guarantees are normally specified.
C. A MAN often acts as a high speed network to allow sharing of
regional resources (similar to a large LAN). It is also frequently
used to provide a shared connection to other networks using a
link to a WAN.
2. WIDE AREA NETWORK
- A WAN spans a large geographic area, such as a state, province
or country. WANs often connect multiple smaller networks, such
as local area networks (LANs) or metro area networks (MANs).
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- The dual ring topology varies in having two connections
between each of the network nodes.
- The data flow along two directions in the two rings formed
thereby.
- The ring topology does not require a central server to manage
connectivity between the nodes and facilitates an orderly
network operation.
- But, the failure of a single station in the network can render
the entire network inoperable.
- Changes and moves in the stations forming the network affect
the network operation.
Network Topologies:
1. Bus Topology:
- In this type of network topology, all the nodes of a network are
connected to a common transmission medium having two
endpoints.
- All the data that travels over the network is transmitted
through a common transmission medium known as the bus or
the backbone of the network.
- When the transmission medium has exactly two endpoints, the
network topology is known by the name, ‘linear bus topology'. In case the transmission medium, also called as the network
backbone, has more than two endpoints, the network is said to
have a distributed bus topology.
- Bus topology is easy to handle and implement and is best
suited for small networks.
- But the downside of this topology is that the limited cable
length limits the number of stations, thus limiting the
performance to a less number of nodes.
3. Mesh Topology:
- In a full mesh network, each network node is connected to
every other node in the network.
- Due to this arrangement of nodes, it becomes possible for a
simultaneous transmission of signals from one node to several
other nodes.
- In a partially connected mesh network, only some of the
network nodes are connected to more than one node.
- This is beneficial over a fully connected mesh in terms of
redundancy caused by the point-to-point links between all the
nodes.
- The nodes of a mesh network require possessing some kind of
routing logic so that the signals and the data traveling over the
network take the shortest path during each of the
transmissions.
2. Ring Topology:
- In a ring topology, every node in the network is connected to
two other nodes and the first and the last nodes are connected
to each other.
- The data that are transmitted over the network pass through
each of the nodes in the ring until they reach the destination
node.
- In a ring network, the data and the signals that pass over the
network travel in a single direction.
4. Star Topology:
- In this type of network topology, each node of the network is
connected to a central node, which is known as a hub.
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- The data that is transmitted between the network nodes
passes across the central hub.
- A distributed star is formed by the interconnection of two or
more individual star networks.
- The centralized nature of a star network provides a certain
amount of simplicity while also achieving isolation of each
device in the network.
- However, the disadvantage of a star topology is that the
network transmission is largely dependent on the central hub. The failure of the central hub results in total network
inoperability.
- is the layered structure of hardware and software that
supports the exchange of data between systems and supports
applications such a as electronic mail and file transfer.
- The key features of protocol are":
> syntax: concerns the format of the data blocks
> semantics: Includes control information for coordination and
error handling
>Timing: Includes speed matching and sequencing
OSI :
5. Tree Topology:
- OSI stands for Open System Interconnection.
- The communications concern are partitioned into hierarchical
set of layers.
- Each layer performs a related subset of the functions with
another system.
- It is also known as a hierarchical topology and has a central
root node that is connected to one or more nodes of a lower
hierarchy.
- In a symmetrical hierarchy, each node in the network has a
specific fixed number of nodes connected to those at a lower
level.
1. Physical Layer:
- A physical layers covers the physical interface between devices
and the rules by which bits passed from one to another.
- It relates to the physical properties of the interface to a
transmission medium.
- For example, connector that joins one or more circuits.
- Electrical part of physical layer relates to the representation of
bits.
- Functional parts of physical layer specifies the function
performed by individual circuits between a system and the
transmission medium.
>>> Apart from these basic types of network topologies, there
are hybrid network topologies, which are composed of a
combination of two or more basic topologies.
- These network mappings aim at harnessing the advantages of
each of the basic topologies used in them.
- Network topologies are the physical arrangements of network
nodes and wires. What is interesting is that the inanimate nodes
and wires turn 'live' for the transmission of information!
Protocol Architecture:
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- Similarly, procedural part of physical layer species the
sequence of events by which bit streams are exchanged across
the physical medium.
- It defines the format of the date to be exchanged between
applications.
- It defines the syntax used between application and provides
for the selection and subsequent modification of the
presentation used.
2. Data Link Layer:
- Data link layer attempts to make the physical link reliable and
provides the means to activate, maintain, and deactivate the
link.
- It provides for the reliable transfer of information across
physical link.
- It sends blocks with the necessary synchronization, error
control and flow control.
7. Application Layer:
- Application Layer provides a means for application programs to
access the OSI environment.
- It contains management functions and general purpose
applications such as file transfer, electronic mail and terminal
access to remote computers.
3. Network Layer:
Frame Relay:
- A computer system engage in the dialog with the network to
specify the destination address and to request network facilities.
- More efficient transmission scheme than X.25
- call control signaling is carried on separate logical connection
from user data.
- Intermediate nodes need not to maintain state tables or
process messages relating to call control
- Multiplexing and switching of logical connections takes place at
layer 2 instead of layer 3, eliminating one entire layer of
processing.
- There is no hop-by hop flow control and error control. End to
end flow control and error control are the responsibility of a
higher layer, if they are employed at all.
- Frame relay used access speed up to 2Mbps Frame relay
service at even higher data rates are now available
4. Transport Layer:
- It provides the mechanism for the exchange of data between
and system.
- The connection oriented transport service ensures that data
are delivered error free, in sequence with no loss or duplication.
5.Session Layer:
- It provides the mechanism for controlling the dialog between
application in and systems.
6. Presentation Layer:
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- Frame Relay is designed to provide efficient transmission than
X.25.
- The X.25 approach results in considerable overhead at each
hop through the network.
- The data link control protocol involves the exchange of a data
frame and acknowledgement frame.
- At each intermediate node, state tables must be maintained
for each virtual circuit to deal with cost management and
flow/error, control aspects of X.25 protocol.
- All these overhead may be justified when there is significant
probability of error in any of the links in the network.
- Today's network employee reliable digital transmission
technology over high quality reliable digital transmission
technology over high quality reliable transmission links such as
optical fiber.
- In this environment, the overhead of X.25 is not only
unnecessary but degrades the effective utilization of the
available high data rates.
- Frame Relay is designed to eliminate much of the overhead
that X.25 imposes on end user systems.
- The LLC sub-layer acts as an interface between the Media
Access Control (MAC) sub layer and the network layer.
- As the Ether type in an Ethernet II framing formatted frame is
used to multiplex different protocols on top of the Ethernet
MAC header it can be seen as LLC identifier.
- The LLC sub layer is primarily concerned with:
> Multiplexing protocols transmitted over the MAC layer (when
transmitting) and decoding them (when receiving).
> Providing flow and error control
- The Media Access Control (MAC) data communication protocol
sub-layer, also known as the Medium Access Control, is a sub
layer of the Data Link Layer specified in the seven-layer OSI
model (layer 2).
- It provides addressing and channel access control mechanisms
that make it possible for several terminals or network nodes to
communicate within a multi-point network, typically a local area
network (LAN) or metropolitan area network (MAN). - The
hardware that implements the MAC is referred to as a Medium
Access Controller.
The MAC sub-layer acts as an interface between the Logical Link
Control (LLC) sub layer and the network's physical layer. - The
MAC layer emulates a full-duplex logical communication channel
in a multi-point network. This channel may provide unicast,
multicast or broadcast communication service.
LLC/MAC:
- The Logical Link Control (LLC) data communication protocol
layer is the upper sub-layer of the Data Link Layer (which is itself
layer 2, just above the Physical Layer) in the seven-layer OSI
reference model.
- It provides multiplexing mechanisms that make it possible for
several network protocols (IP, IPX) to coexist within a multipoint
network and to be transported over the same network media,
and can also provide flow control mechanisms.
Routing:
- Routing is the process of selecting paths in a network along
which to send network traffic.
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- Routing is performed for many kinds of networks, including the
telephone network (Circuit switching) , electronic data networks
(such as the Internet), and transportation networks.
- This article is concerned primarily with routing in electronic
data networks using packet switching technology.
- In packet switching networks, routing directs packet
forwarding, the transit of logically addressed packets from their
source toward their ultimate destination through intermediate
nodes, typically hardware devices called routers, bridges,
gateways, firewalls, or switches. - General-purpose computers
can also forward packets and perform routing, though they are
not specialized hardware and may suffer from limited
performance. - The routing process usually directs forwarding
on the basis of routing tables which maintain a record of the
routes to various network destinations.
- Thus, constructing routing tables, which are held in the router's
memory, is very important for efficient routing.
- Most routing algorithms use only one network path at a time,
but multipath routing techniques enable the use of multiple
alternative paths.
- Routing, in a more narrow sense of the term, is often
contrasted with bridging in its assumption that network
addresses are structured and that similar addresses imply
proximity within the network. - Because structured addresses
allow a single routing table entry to represent the route to a
group of devices, structured addressing (routing, in the narrow
sense) outperforms unstructured addressing (bridging) in large
networks, and has become the dominant form of addressing on
the Internet, though bridging is still widely used within localized
environments.
- This architecture was developed by the IEEE 802 committee
and has been adopted by all organizations working on the
specification of LAN standards.
- It is generally referred to as IEEE 802 Reference Model working
from the bottom of the lowest layer of IEEE 802 corresponds to
the physical layer of the OSI model and includes functions as
encoding, decoding of signals, preamble generation/removal
and bit transmission/reception.
1. Physical Layer:
- encoding and decoding of signals
- pre able generation/removal (for synchronization)
- bit transmission/reception
2.MAC(Medium Access Control)
- on transmission assemble data into a frame with error
detection and address fields
- On reception, dissemble frame and perform address
recognition and error detection
- Goren access to LAN transmission medium
3.LLC(Logical Link Control)
- provide an interface to higher layers and perform flow and
error control
4. LLC Services:
- Three services are provided under LLC services:
IEE 802 Reference Model
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- CSMA/CD (Carrier Sense Multiple Access / Collision Detection)
is the protocol used in Ethernet networks to ensure that only
one network node is transmitting on the network wire at any
one time.
- Carrier Sense means that every Ethernet device listens to the
Ethernet wire before it attempts to transmit.
- If the Ethernet device senses that another device is
transmitting, it will wait to transmit.
- Multiple Access means that more than one Ethernet device can
be sensing (listening and waiting to transmit) at a time.
- Collision Detection means that when multiple Ethernet devices
accidentally transmit at the same time, they are able to detect
this error.
> Unacknowledged Connectionless Service:
- It is a very simple service that does not involve any of the flow
and error control mechanisms.
- Delivery of data is not guaranteed.
- There will be some higher layer of software that deals with
reliability issues.
> Connection Mode services:
- A logical connection is set up between two users exchanging
data and flow and error control are provided.
>Acknowledged Connectionless Services:
- It provides acknowledgement but no logical connection is set
up.
How Collisions Occur under CSMA/CD:
- Imagine a very simple Ethernet network with only two nodes.
- Each node, independently, decides to send an Ethernet frame
to the other node.
- Both nodes listen to the Ethernet wire and sense that no
carrier is present.
- Both nodes transmit simultaneously, causing a collision.
- Both nodes detect the collision and each node waits a random
amount of time before transmitting again.
- Collisions are normal on an Ethernet network.
- A small amount of collisions are expected in the protocol
design.
- If too many nodes are transmitting on an Ethernet network the
number of collisions can rise to an unacceptable level.
- This can reduce the amount of available bandwidth on an
Ethernet network because so much bandwidth is lost in
retransmission.
Ethernet CSMA-CD:
- Ethernet is a family of frame-based computer networking
technologies for local area networks (LANs).
- The name came from the physical concept of the ether.
- It defines a number of wiring and signaling standards for the
Physical Layer of the OSI networking model as well as a common
addressing format and Media Access Control at the Data Link
Layer.
- Ethernet is standardized as IEEE 802.3. - The combination of
the twisted pair versions of Ethernet for connecting end systems
to the network, along with the fiber optic versions for site
backbones, is the most widespread wired LAN technology.
CSMA/CD:
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- Ethernet switches greatly reduce the already minor difficulties
experienced with the CSMA/CD protocol.
- call control packets, used for setting up and cleaning virtual
circuits are carried on same channel and same virtual circuit as
data packets
- multiplexing of virtual circuits takes place at layer 3
- Both layer 2 and layer 3 include flow control and error control
mechanisms
X.25:
- The X.25 protocol, adopted as a standard by the Consultative
Committee for International Telegraph and Telephone (CCITT), is
a commonly-used network protocol.
- The X.25 protocol allows computers on different public
networks (such as CompuServe, Tymnet, or a TCP/IP network) to
communicate through an intermediary computer at the network
layer level. - X.25's protocols correspond closely to the data-link
and physical-layer protocols defined in the Open Systems Interconnection (OSI) communication model.
ATM:
- Asynchronous Transfer Mode
- It is a streamlined packet transfer interface.
- ATM makes use of a fixed size packets called cells.
- The use of fixed size and fixed formats results an efficient
scheme for transmission over high speed networks.
- data rate range from 25.6 Mbps to 622.08 Mbps
- Physical layer specifies transmission medium and signal
encoding scheme
- ATM layer defines transmission of data in fixed size cells and
defines the use of logical connection.
- ATM adaptation layer maps higher layer information into ATM
cells to be transported over an ATM network.
- User plane provides user information into ATM cells to be
transported over an ATM network
- user plane provides user information transfer(eg. flow control,
error control)
- Control plane provides call control and connection control
functions.
- Management plane performs coordination between all the
planes and layers management.
- Three levels:
> Physical Layer:
- physical interface between and attached station(computer
terminal and Packet Switching mode.
> Link Level:
- provides reliable transfer of data across physical link
- It is referred as Link Access protocol - Balanced(LABP)
> Packet Level:
- provides virtual circuit service
- enables any subscriber to the network to setup logical
conditions
Following are the key features of X:25:
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- For unguided media, the bandwidth of the signal produced by
the transmitting antenna is more important than the medium in
determining transmission characteristics.
- One key property of signals transmitted by antenna is
directionality.
- In general, signals at lower frequencies are omnidirectional;
that is, the signal propagates
in all directions from the antenna.
- At higher frequencies, it is possible to focus
the signal into a directional beam.
- In considering the design of data transmission systems, a key
concern, generally, is data rate and distance: the greater the
data rate and distance, the better.
- A number of design factors relating to the transmission
medium and to the signal determine the data rate and distance:
CHP: 6 TRANSMISSION MEDIA
Guided and Unguided Media:
- Guided media are those that provides physical conduction
from one device to another which includes twisted pairs, coaxial cables and fiber-optic cables.
- Unguided Media transports electromagnetic waves without
using a physical conductor.
- This type of communication is often referred to as wireless
communication.
Transmission Media:
- Transmission medium is the physical path between transmitter
and in a data transmission system. - Transmission media can be
classified or unguided.
In both cases, communication is in the form of
electromagnetic waves.
- With guided media, the waves are guided along a solid
medium, such as copper twisted pair, copper coaxial cable, and
optical fiber.
- The atmosphere and outer space are examples of unguided
media that provide a means of transmitting
Electromagnetic signals but do not guide them; this form of
transmission is usually referred to as wireless transmission.
- The characteristics and quality of a data transmission are
determined both by the characteristics of the medium and the
characteristics of the signal.
- In the case of guided media, the medium itself is more
important in determining the limitations of
transmission.
>>Bandwidth:
- All other factors remaining constant, the greater the
bandwidth of a signal, the higher the data rate that can be
achieved.
>>Transmission impairments:
- Impairments, such as attenuation, limit the distance.
- For guided media, twisted pair generally suffer more
impairment than coaxial cable, which in turn suffers more than
optical fiber.
>>Interference:
- Interference from competing signals in overlapping frequency
bands can distort or wipe out a signal. Interference is of
particular concern for unguided media, but it is also a problem
with guided media.
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- For guided media, interference can be caused by emanations
from nearby cables.
- For example, twisted pair are often bundled together, and
conduits often carry multiple cables. - Interference can also be
experienced from unguided transmissions.
- Proper shielding of a guided medium can minimize this
problem.
- The wires are paired and twisted around each other to
decrease certain electromagnetic problems.
- The most common twisted pair cable used in communications
is referred to as unshielded twisted pair(UTP) cable.
- STP cable(shielded Twisted Pair) has a metal foil or braidedmesh covering each pair of insulated conductors.
- Although metal casing improves the quality of cable by
preventing the penetration of noise or cross-talk, it is bulkier
and more expensive.
>> Number of receivers:
- A guided medium can be used to construct a point-to point link
or a shared link with multiple attachments.
- In the latter case, each attachment introduces some
attenuation and distortion on the line, limiting distance and/or
data rate.
>>Applications:
- Twisted pair cables are used in telephone lines to provide voice
and data channels.
- The line that connects subscribers to the central telephone
office is most commonly unshielded twisted pair cables.
- Local area networks such as 10 Base-T and 100 Base-T also use
twisted pair cables.
Wired Pairs:
- Wires are described by their size.
- Higher gauge number indicates thinner wire size.
- The smaller the diameter of the wire, the greater is resistance
to the propagation of a signal.
- Increased resistance results in a decreased bit rate across the
communication path.
- At higher transmission frequencies, the signal tends to travel
on the outside surface of the wire.
- A small wire provides less total surface for the radiating signal,
resulting in increased signal loss.
- The local subscriber loops ( of the telephone system) and
usually to 22-26 gauge wire.
- Trunk and toll lines typically employ 19-gauge wires.
- Several Hundred of these wires are packaged into one cables.
Micro-waves:
- Microwave is a directed line of sight(LOS) radio transmission.
- It is used for wide band communication systems and is quite
common in the telephone system.
- Television transmission also utilizes microwave transmission
because microwave transmission is above the 1 GHz and
provides the capacity required for video transmission.
- The high bandwidth gives small wavelength and the smaller
the wavelength, the smaller one can design the microwave
antenna.
- The antenna size has significant implications for distributed
processing systems.
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- The transmitting towers are spaced 20-30 m apart.
- Transmitted radio bean is focused to the receiving antenna.
- As the above figure, if the angle of incidence is less than the
critical angle, the ray diffracts and move to closer to the surface.
- If angle of incidence is equal to the critical angle, the light
bends along the interface and refraction occurs.
- If the angle is greater than the critical angle, the ray reflects
and travels again in the denser substances.
COAXIAL Cables:
- Co-axial cables carries signals of higher frequency ranges than
twisted pair cable.
- Instead of having two wires, co-axial cable has a central core
conductor of solid or standard wire(usually copper) enclosed in
an insulating sheath which in turn is encased in an outer
conductor of metal foil or combination of two.
- The outer metallic wrapping serves both as a shield against
noise and as a second conductor.
- The whole cable is protected by a plastic cover.
- Optical fibres use reflection to guide light through a channel.
- A glass or plastic core is surrounded by cladding of less dense
glass or plastic.
- The difference in density of the two materials must be such
that a beam of light moving through the core is reflected off the
cladding instead of being reflected into it.
Propagation Modes:
a. Multimode step index:
- in multimode step-index fiber, the density of the core remains
constant from the center of the edges,
- A beam of light moves through these constant densities in a
straight line until it reaches the interface of the core and the
cladding.
- As the interface, there is an abrupt change to a lower density
that alters the angle of the beam motion.
- The term step index refers to the suddenness of this change.
Applications:
- The use of co-axial cable is diverse but nowadays it is shrinking
due to fibre optic cable.
- Co-axial cables are used in analog telephone networks and
cable Tv networks.
Fiber optic Cables:
- A fiber optic cable is made of glass or plastic and transmits
signals in the form of lights.
- Lights travels in a straight line as long as it is moving through a
single uniform substance.
- If a ray of light travelling through one substance suddenly
enters another(more or less dense the ray changes direction).
b. Multimode graded index:
- A second type of fiber called multimode graded index fiber is
one with varying densities.
- Density is highest at center of the core and decreases gradually
to its lowest at the edge.
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c. Single mode:
- Single mode uses step index fiber and a highly focused source
of light that limits beams to a small range of angles, all close to
the horizontal.
- The single mode fiber is manufactured with a much smaller
diameter that that of multimode fiber.
- Since, electromagnetic waves doesn't need any medium to
transmit the signal specially in wireless communication system
we often use the atmosphere for transmission of channel.
- Here, interference and propagation condition are strongly
dependent upon the frequency.
Types of Electromagnetic waves:
Applications:
a. Ground Wave Propagation:
- Optical fiber cable is found in backbone networks because of
its wide bandwidth and is cost effective.
- cable TV companies use a combination of optical fibers and coaxial cable, thus creating a hybrid network.
- Dominant mode of propagation for frequency below 2 mHz.
- Electromagnetic waves are guided by the conducting surface
of the earth, along which they are propagated.
- Diffraction of the wave causes it to propagate where this
propagation mode is used in AM broadcasting.
- For efficient radiation, the antenna needs to be longer than
1/10 th of the wave length.
Advantages:
- Higher bandwidth
- Less signal attenuation
- Immunity to electromagnetic interference
- Light weight
- Resistance to corrosive materials
b. Sky Wave Propagation:
- Dominant mode of propagation for frequencies in between 2
to 30 Mhz.
- Long distance coverage is obtained by the reflecting the wave
at the ionosphere and at the earth boundaries.
- This is caused due to reflection.
Disadvantages:
- Installation/maintenance
- Unidirectional
- Cost
c. Line of sight(LOS) or Space Wave propagation:
Electromagnetic waves:
- Dominant mode of propagation for frequencies above 30 Mhz.
- Here, electromagnetic wave propagates in a straight line.
- The electromagnetic waves used e propagation characteristics
of wireless channels are highly dependent on frequency.
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- Very little reflection by the ionosphere,
- This is used for satellite communication.
- Its maximum range is limited to line of sight due to nature of
propagation.
The Satellite
- The satellite itself is also known as the space segment, and is
composed of three separate units, namely the fuel system, the
satellite and telemetry controls, and the transponder.
- The transponder includes the receiving antenna to pick-up
signals from the ground station, a broad band receiver, an input
multiplexer, and a frequency converter which is used to reroute
the received signals through a high powered amplifier for
downlink.
- The primary role of a satellite is to reflect electronic signals.
- In the case of a telecom satellite, the primary task is to receive
signals from a ground station and send them down to another
ground station located a considerable distance away from the
first.
- This relay action can be two-way, as in the case of a long
distance phone call.
- Another use of the satellite is when, as is the case with
television broadcasts, the ground station's uplink is then down
linked over a wide region, so that it may be received by many
different customers possessing compatible equipment.
- Still another use for satellites is observation, wherein the
satellite is equipped with cameras or various sensors, and it
merely downlinks any information it picks up from its vantage
point.
Disadvantages:
- For communication between two each stations, the signal path
has to be above the horizon otherwise they will block the LOS
path.
- Thus antennas need to be placed on tall towers so that
receiver antenna can see the transmitting antenna.
Radio waves and Microwaves:
1. 3 KHz to 1 GHz
- 1 GHz to 300 GHz
2. for the most part are omnidirectional
- unidirectional
3. Omnidirectional antennas are generally used
- line of sight propagation, unidirectional antennas are used
4.AM,FM radio
- cellular phones, satellite networks
Satellite Communications:
- Satellite communications are
components:
The Ground Station:
comprised
of
2
main
- This is the earth segment.
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- The ground station's job is two-fold. In the case of an uplink, or
transmitting station, terrestrial data in the form of baseband
signals, is passed through a baseband processor, an up
converter, a high powered amplifier, and through a parabolic
dish antenna up to an orbiting satellite.
- In the case of a downlink, or receiving station, works in the
reverse fashion as the uplink, ultimately
converting signals received through the parabolic antenna to
base band signal.
- The mobile switching center is sometimes called a mobile
telephone switching office (MTSO), since it is responsible for
connecting all mobiles to the PSTN via central office(CO).
- Each user communicates via radio from a cellular telephone set
to the cell site base station.
- This base station is connected via telephone lines or
microwave link to the mobile switching center.
- The MSC connects the user to the called party if the called
party is land based, the connection is via the central office (CO)
is the terrestrial telephone network.
- If the called party is mobile, the connection is made to the
cellular site that covers the area in which the third party is
located, using an available radio channel in the cell associated
with the called party.
- If more channels are needed, the existing cell sizes are
decreased, and additional small cells are inserted, so that
existing channels can be reused more efficiently.
- The critical consideration is to design the cells for acceptable
levels of a Co channel interference.
- As the mobile user travels from one cell to another, the MSC
automatically switches the user to an available channel in the
new cell and the telephone continues un-interrupted.
Cellular Telephony System:
- A cellular telephone system provides a wireless connection to
the terrestrial telephone network( PSTN: Public switch
Telephone Network) for any user location within the radio range
of the system.
- Cellular systems accommodate a large number of users over a
large geographic area, within a limited frequency spectrum.
- Cellular radio system provide high quality service that is often
comparable to that of the landline telephone systems.
- High capacity is achieved by limiting the coverage of each base
station transmitter to a small geographic area called a cell so
that the same radio channels may be reused by another base
station located some distance away.
- A sophisticated switching technique called a handoff enables a
call to proceed uninterrupted when the user moves from one
cell to another.
- The cellular concept has following advantages:
> large subscriber capacity
> Efficient use of the radio spectrum
> Service to hand held portables, as well as vehicles.
> High Quality telephone and data service to the mobile user at
relatively low cost.
- The basic structure of cellular system is as below:
- The basic cellular system consists mobile stations, base
stations and a mobile switching center(MSC).
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To remedy this problem, each character is preceded by some
information indicating the start of character transmission (the
transmission start information is called a START bit) and ends by
sending end-of-transmission information (called STOP bit, there
may even be several STOP bits).
In a synchronous connection, the transmitter and receiver are
paced by the same clock. The receiver continuously receives
(even when no bits are transmitted) the information at the same
rate the transmitter send it. This is why the transmitter and
receiver are paced at the same speed. In addition,
supplementary information is inserted to guarantee that there
are no errors during transmission.
During synchronous transmission, the bits are sent successively
with no separation between each character, so it is necessary to
insert synchronization elements; this is called character-level
synchronization.
Transmission Modes:
Parallel Transmission:
- Parallel Transmission is a method of data transmission in which
the bits of a data character are transmitted simultaneously over
a number of channels/ports.
- In parallel transmission, coded information are transmitted via
a system with multiple ports/channels. - The port 1 is used to
transport the first MSB (Most Significant Bit) and the second
port carries the second MSBs, so on and so forth.
Serial Transmission:
Synchronous and asynchronous transmission
Given the problems that arise with a parallel-type connection,
serial connections are normally used. However, since a single
wire transports the information, the problem is how to
synchronies the transmitter and receiver, in other words, the
receiver can not necessarily distinguish the characters (or more
generally the bit sequences) because the bits are sent one after
the other. There are two types of transmission that address this
problem:
The main disadvantage of synchronous transmission is
recognizing the data at the receiver, as there may be differences
between the transmitter and receiver clocks. That is why each
data transmission must be sustained long enough for the
receiver to distinguish it. As a result, the transmission speed can
not be very high in a synchronous link
An asynchronous connection, in which each character is sent at
irregular intervals in time (for example a user sending characters
entered at the keyboard in real time). So, for example, imagine
that a single bit is transmitted during a long period of silence...
the receiver will not be able to know if this is 00010000,
10000000 or 00000100...
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Transmission Impairments:
3. Noise:
- The signal received may differ from that is transmitted due to
various transmission impairments.
- for analog signals, these impairments can degrade the signal
quality
- for digital, bit errors may be introduced.
- A binary 1 is transformed into binary 0 and vice versa.
- unwanted man made or natural random signal that adds to
the received signal and degrades the performances.
- Such type of unwanted random signal is noise.
> Thermal noise:
- due to the thermal agitation of electrons in the conductor.
1. Attenuation:
> Shot Noise:
- due to the flow of current at the junction of semi conductor.
- strength of a signal falls off with distance over any transmission
medium.
- Hence, a received signal must have sufficient strength so that
the electronic circuitry in the receiver can detect the signal.
- The signal must maintain a level sufficiently higher than noise
to be received without error.
- Attenuation is an increasing concern of frequency.
- Hence, amplifiers must be used that amplify high frequencies
more than lower frequencies.
> Burst Noise:
-due to the sudden high amplitude making the signal change its
value such as lightning, electrical ignition system.
> Intermodulation Noise:
- due to non linearity’s in the transmitter, receiver and
interviewing transmission media.
> Cross talk Noise:
- due to coupling of nearby line.
2. Delay Distortion:
- occurs because of the velocity of propagation of a signal
through guided medium varies with frequency.
- For a band limited signal, the velocity tends to be highest near
the central frequency and falls off towards the two edges of the
band.
- Thus, various frequency components of a signal will arrive at
the receiver at different times resulting in phase shifts between
the different frequencies.
- Intersymbol Interference occurs due to delay distortion
> Flicker Noise:
- low frequency Noise
> Transit time Noise:
- High frequency Noise
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indeed, can hardly be improved upon when a message is sent in
a few large frames.
- However, it is often the case that a source will break up a large
block of data into smaller blocks and transmit the data in many
frames.
- This is done for the following
reasons:
> The buffer size of the receiver may be limited.
> The longer the transmission, the more likely that there will be
an error, necessitating retransmission of the entire frame. With
smaller frames, errors are
detected sooner, and a smaller amount of data needs to be
retransmitted.
> On a shared medium, such as a LAN, it is usually desirable not
to permit one
station to occupy the medium for an extended period, as this
causes long
delays at the other sending stations.
DATA LINK CONTROL
Flow control:
- Flow control is a technique for assuring that a transmitting
entity does not overwhelm a receiving entity with data. The
receiving entity typically allocates a data buffer of some
maximum length for a transfer.
- When data are received, the receiver
must do a certain amount of processing before passing the data
to the higher-level software.
- In the absence of flow control, the receiver's buffer may fill up
and overflow
while it is processing old data.
Stop-and-Wait Flow Control:
- The simplest form of flow control, known as stop-and-wait flow
control, works as
follows.
- A source entity transmits a frame. After reception, the
destination entity
indicates its willingness to accept another frame by sending back
an acknowledgment to the frame just received. - The source
must wait until it receives the acknowledgment before sending
the next frame.
- The destination can thus stop the flow of data by simply
withholding acknowledgment.
- This procedure works fine and,
Sliding Window Flow control:
- Allows multiple frames to be in transit
- Receiver sends acknowledgement with sequence number of
anticipated frame
- Sender maintains list of sequence number of anticipated frame
- Sender maintains list of sequence numbers it can send,
receiver maintains list of sequence numbers it can receive.
- ACk (acknowledgement) supplemented with RNR (receiver not
ready)
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- The essence of the problem described so far is that only one
frame at a time can be in transit.
- In situations where the bit length of the link is greater than the
frame
length (a > I), serious inefficiencies result.
- Efficiency can be greatly improved by
allowing multiple frames to be in transit at the same time.
> Error detection. As discussed in the preceding section.
> Positive acknowledgment. The destination returns a positive
acknowledgment to successfully received, error-free frames.
> a Retransmission after timeout. The source retransmits a
frame that has not
been acknowledged after a predetermined amount of time.
Error Control:
> Negative acknowledgment and retransmission. The
destination returns a negative acknowledgment to frames in
which an error is detected.
Error control refers to mechanisms to detect and correct errors
that occur in the transmission of frames.
- The model that we will use, which covers the typical case.
- As before, data are sent as a sequence of frames; frames arrive
in the same order in which they are sent; and each transmitted
frame suffers an arbitrary and variable amount of delay before
reception. In addition, we admit the possibility of two types of
errors:
- The source retransmits such frames.
Collectively, these mechanisms are all referred to as automatic
repeat request
(ARQ); the effect of ARQ is to turn an unreliable data link into a
reliable one.
- Three versions of ARQ have been standardized:
> Stop-and-wait ARQ
> Go-back-N ARQ
> Selective-reject ARQ
> Lost frame.
- 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.
Stop or Wait ARQ:
- Stop-and-wait ARQ is based on the stop-and-wait flow-control
technique
- The source station transmits a single frame and then must
await an acknowledgment (ACK).
- No other data frames can be sent
until the destination station's reply arrives at the source station.
> Damaged frame: A recognizable frame does arrive, but some
of the bits are in error (have been altered during transmission).
- The most common techniques for error control are based on
some or all of the
following ingredients:
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- Two sorts of errors could occur. First, the frame that arrives at
the destination
could be damaged; the receiver detects this by using the error
detection technique referred to earlier and simply discards the
frame.
- To account for this possibility, the
source station is equipped with a timer. - After a frame is
transmitted, the source station waits for an acknowledgment.
- If no acknowledgment is received by the time the timer
expires, then the same frame is sent again.
- Note that this method requires
that the transmitter maintain a copy of a transmitted frame
until an acknowledgment
is received for that frame.
- The second sort of error is a damaged acknowledgment.
- If the destination station detects an error in a frame, it sends a
negative acknowledgment (REJ = reject) for that frame.
- The destination station will discard that frame and all future
incoming frames until the frame in error is correctly received.
- Thus, the source station, when it receives an REJ, must
retransmit the frame in error plus all succeeding frames that
were transmitted in the interim.
Go back N ARQ
- The form of error control based on sliding-window flow control
that is most commonly used is called go-back-N ARQ.
- In go-back-N ARQ, a station may send a series of frames
sequentially numbered
modulo some maximum value.
- The number of unacknowledged frames outstanding is
determined by window size, using the sliding-window flow
control
technique.
- While no errors occur, the destination will acknowledge (RR =
receive ready)
incoming frames as usual.
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- provides high availability of the network to all users.
MULTIPLEXING AND SWITCHING
Multiplexing:
Switching:
- There are n inputs to a multiplexer. The multiplexer is
connected by a single data link to a demultiplexer.
- The link is able to carry n separate channels of data.
- The multiplexer combines (multiplexes) data from the n input
lines and transmits over a higher capacity data link.
- The demultiplexer accepts the multiplexed data stream,
separates (demultiplexer) the data according to channel, and
delivers them to the appropriate output lines.
1. Circuit Switching:
- Real time data transmission is provided by the direct
connection
- Dial up delay can be eliminated by using leased lines
- Blockage can occur in which case busy signal is returned to
sender
- Transmissions are point to point
- Once connection is established, any subsequent overload of
the switch is invisible to the connected components
FDM(Frequency Division Multiplexing):
- FDM is possible when the useful bandwidth of the transmission
medium exceeds the required bandwidth of signals to be
transmitted.
- A number of signals can be carried
simultaneously if each signal is modulated onto a different
carrier frequency
and the carrier frequencies are sufficiently separated that the
bandwidths of the signals do not overlap.
- Six signal sources are fed into a multiplexer, which modulates
each signal onto a different frequency (fi, . . . , f6).
- Each modulated signal requires a certain bandwidth centered
around its carrier frequency, referred to as a channel.
- To prevent interference, the
channels are separated by guard bands, which are unused
portions of the spectrum.
2. Message Switching:
- Connection is not a direct physical interface as in circuit
switching
- Data connections use variable slots if TDM is employed
- Messages are stored onto disk, tape before transmission, real
time processing is usually not feasible
- messages can be broadcast to all nodes in the network or
subset of nodes
- priorities are allowed in the message traffic
3. Packet Switching:
- combination of circuit and message switching
- packet contains user and control data
- provides stastical multiplexing
- provides fast response to all users
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> Telephone trunk (multiple phone) lines that terminate at the
PBX
> A computer with memory that manages the switching of the
calls within the PBX and in and out of it
> The network of lines within the PBX
> Usually a console or switchboard for a human operator
Synchronous TDM:
- Synchronous time-division multiplexing is possible when the
achievable data rate
(sometimes, unfortunately, called bandwidth) of the medium
exceeds the data rate of digital signals to be transmitted. Multiple digital signals (or analog signals carrying digital data)
can be carried on a single transmission path by interleaving
portions of each signal in time.
- The interleaving can be at the bit level or in blocks of bytes or
larger quantities.
- For example, the multiplexer has six inputs which might each
be, say, 9.6 kbps. - A single line with a capacity of at least 57.6
kbps (plus overhead capacity) could accommodate all six
sources.
- In some situations, alternatives to a PBX include centrex
service (in which a pool of lines are rented at the phone
company's central office), key telephone systems, and, for very
small enterprises, primary rate Integrated Services Digital
Network.
Switched 56/ Service:
- digital version of analog switched line
- data rates up to 56 Kbps
- both parties must subscribe
- subscribes do not need modem
- digital service unit (DSU) is needed to change the rate to 56
Kbps and encode them in the format of service providers.
- supports video conferencing, multimedia etc
Private Branch Exchange:
- A PBX (private branch exchange) is a telephone system within
an enterprise that switches calls between enterprise users on
local lines while allowing all users to share a certain number of
external phone lines.
- The main purpose of a PBX is to save the cost of requiring a line
for each user to the telephone company's central office.
- The PBX is owned and operated by the enterprise rather than
the telephone company (which may be a supplier or service
provider, however).
- Private branch exchanges used analog technology originally.
- Today, PBXs use digital technology (digital signals are
converted to analog for outside calls on the local loop using
plain old telephone service).
A PBX includes:
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-
DATA ENCODING AND MODULATION
Same as bipolar AMI, except that any string of eight zeros
is replaced by a string with two code violations
Definition of digital signal encoding formats:
HDB3:
- Same as bipolar AMI, except that any string of four zeros
is replaced by a string with one code violation
Nonreturn-to-Zero-Level (NRZ-L):
0 = high level
1 = low level
Encoding:
- In combining the processes of sampling and quantizing, the
specification of a continuous base-band signal becomes limited
to a discrete set of values but not in the form best suited for
transmission over a line or a radio path or optical fiber.
- To exploit the advantages of sampling and quantizing, we
require the use of an encoding process to translate the discrete
set of sample values to a more appropriate form of signal.
- Any plan for representing each member of this discrete values
as a particular arrangement of discrete elements is called
encoding.
- Suppose, in a binary code each code word consists of n bits.
- Then using such a a code we may represent a total of 2^n
distinct numbers
L=2^n
Nonreturn to Zero Inverted (NRZI):
0 = no transition at beginning of interval (one bit time)
1 = transition at beginning of interval
Bipolar- AM1:
0 = no line signal
1 = positive or negative level, alternating for successive ones
Pseudo ternary
0 = positive or negative level, alternating for successive zeros
1 = no line signal
Manchester:
0 = transition from high to low in middle of interval
1 = transition from low to high in middle of interval
Amplitude shift Keying
Differential Manchester:
Always a transition in middle of interval
0 = transition at beginning of interval
1 = no transition at beginning of interval
- The most basic form of ASK involves the process of switching
the carrier on or off, in correspondence to a sequence of digital
pulses that constitute the information signal.
BIZS:
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- binary digit 1 represented by the presence of a carrier and the
binary digit 0 represented by the absence of a carrier.
- Frequency remains fixed.
s(t) = Ac cos2πfct for binary 1
0 for binary 0
Delta Modulation:
- Delta Modulation is 1 bit ( or two level) version of DPCM.
- In delta Modulation the difference between the original
sample and its approximation is quantized in one of the two
possible levels + /\ or -/\ and each level is converted in to 1 bit
codeword.
- Thus, the delta modulation uses only one bit to represent each
sampled value.
Frequency Shift Keying:
- The basic form of FSK involves the process of varying the
frequency of a carrier wave by choosing one of two frequencies
in correspondence of digital pulses that constitute the
information signal.
- Two binary digits 0 and 1 are represented by two frequencies
around the carrier frequency.
- Amplitude remains fixed.
sampled i/p m(nTs)---> Sum---> 1 bit quantizer-------> DM wave
sum
Mq(nTs-Ts)--------------Delay Ts < ------- Mq(nTs)
s(t) = Ac cos2πf1t for 1
Ac cos2πf2t for 0
Quantization Noise in DM:
- Delta Modulation systems are subjected to two types of
quantizing error:
Phase Shift Keying:
Slope over load distortion:
- The most basic form of PSK involves the process of shifting the
phase of a carrier wave in correspondence to a sequence of
digital pulses that constitute the information signal.
- The two binary digits 0 and 1 are represented in which the
carrier phase for each symbol is differ by 180 frequency and
Amplitude remains fixed.
- If the slope of the signal is so high then the step-size may not
be sufficient to follow the rate of change of the signal.
- In this case, the condition is called slope overload distortion
and the resulting quantizing error is called slope overload
distortion or noise.
- Slope Overload distortion can be reduced by filtering the
signal to limit its maximum rate of changes or by increasing the
step noise.
- The condition for no-slope over distortion is:
del/Ts >= max(d(m(t)/dt)
s(t) = Ac cos2π fc t for symbol 1
Ac cos(2π fc t+ π) for symbol 0
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2. Bandwidth:
AM >> Bandwidth requirement for AM is less i.e. BW Am = 2 *
fm
FM >> Bandwidth requirement for FM is greater than AM i.e BW
Fm = 2(beta+1)
Granular Noise:
- When the slope of the signal is low that is signal is almost
constant w.r.t time and /\ step size is relatively high, the
approximation starts to swing from - /\ to + /\ causing high noise
level called the granular noies.
- This noise can be minimized by reducing the step size /\.
3. Propagation:
AM>> Ground Wave propagation
FM >> Ground Vertical Propagation
Pulse code Modulation:
- PCM is an method of converting an analog signal to digital
signal.
- In this method, the analog message signal is sampled
(sampling) and the amplitude of a each sampled signal is
rounded off (quantizing) to the nearest one of the finites set of
discrete levels.
- This discrete signal is then converted into binary digital signalor
digital codeword (encoding)
- PCM is not modulation in conventional sense.
- The term modulation usually refers to the variation of some
characteristics of carrier waves accordance with the information
bearing signal.
4. SNR :
AM >> Required high SNR
FM >> low SNR is sufficient for transmission
6. Distance:
AM>> used in long distance transmission
process amplification is easier
FM>> Low coverage area and LOS communication is required
power amplification is difficult
AM and FM
1. Quality
AM >> low quality than FM and highly susceptible to noise
FM >> High Quality (or good) because noise highly effect the
amplitude rather than frequency so FM has high noise immunity
than AM.
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- In its simplest form, the carrier signal can be switched on or off
to represent the binary state.
- AM modulation is not often used by itself due to transmission
power problems and sensitivity due to distortion.
- However it is commonly used with phase modulation to yield a
method superior to either FM or AM.
MODEMS
Modems:
- The digitally oriented computers and terminals often
communicate with one another through the analog telephone
facilities.
- Therefore the digital messages must be translated into a form
suitable for transmission across the analog network.
-The term modem is derived from the process of accepting
digital bits and changing them into a form suitable for analog
transmission and receiving the signal at other station and
transforming it back to original digital representation.
- i.e. first modulation and then demodulation
- Modems are derived from these two words.
- Modems are designed around the use of carrier frequency.
Frequency Modulation:
- This method changes the frequency of the carrier in
accordance with the digital bit stream.
- The amplitude and phase are held constant.
- In its simplest form, a binary 1 is represented by a certain
frequency and a binary 0 by another.
Phase Modulation:
- Phase modulation Modems interrupts the continuous wave
form and alter the phase of the signal to represent a 1 0r 0.
- the common approach today is to compare the phase of the
cycle in a current time period to the phase of in a previous time
period.
- This approach is called differential phase shift keying(DPSK)
Digital Modulation Methods:
- Three basic modulation methods exists.
- Some modems use more than one of the methods.
- Each method impresses the digital data signal onto the analog
carrier signal.
Amplitude Modulation:
- Amplitude Modulation alter the carrier signal amplitude in
accordance with the modulating digital bit stream.
- The frequency and phase of the carrier are held constant and
the amplitude is raised or lowered to represent a 0 or 1.
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