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COMPUTER NETWORKS
UNIT - I
INTRODUCTION
 When we communicate, we are sharing information. This sharing can be local or remote. Between
individuals, local communication usually occurs face to face, while remote communication takes place over
distance.
 Before the internet and computer came into existence, we had a limited amount of options for
communication. Some of these devices are the telegraph, dial-up phone and writing a letter and using the post
office to mail it.
 Before the computer and internet system were created, the best way to communicate with others was to speak
face to face. The advantages of talking face-to-face is that you know what they look like, you can meet the
"real them" and the two of you could do something together in real life.
 Some reasons why the internet and computers may be a better choice are they are faster, it easier to talk to
people from far away, and people don't judge you on your looks.
Computer Network
 Computer network is a collection of two or more computers an interconnected by transmission media to share
the resources between communicating systems.
Data Communications system
 The effectiveness of a data communications system depends on four fundamental characteristics: delivery,
accuracy, timeliness, and jitter.
1. Delivery. The system must deliver data to the correct destination. Data must be received by the intended
device or user only.
2. Accuracy. The system must deliver the data accurately. Data that have been altered in transmission are
deleted.
3. Timeliness. The system must deliver data in a timely manner. Data delivered late are useless. In the case of
video and audio, delivering data in the same order that they are produced without any delay. This kind of
delivery is called real-time transmission.
4. Jitter. Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio
or video packets. For example, let us assume that video packets are sent every 30 ms. If some of the packets
arrive with 30-ms delay and others with 40-ms delay, an uneven quality in the video is the result.
Components of Computer Network
1. Message. The message is the information (data) to be communicated. Popular forms of information
include text, numbers, pictures, audio, and video.
2. Sender. The sender is the device that sends the data message. It can be a computer, workstation,
telephone handset, video camera, and so on.
3. Receiver. The receiver is the device that receives the message. It can be a computer, workstation,
telephone handset, television, and so on.
4. Transmission medium. The transmission medium is the physical path by which a message travels from
sender to receiver. Some examples of transmission media include twisted-pair wire, coaxial cable, fiberoptic cable, and radio waves.
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COMPUTER NETWORKS
5. Protocol. A protocol is a set of rules that govern data communications. It represents an agreement
between the communicating devices. Without a protocol, two devices may be connected but not
communicating, just as a person speaking French cannot be understood by a person who speaks only
Japanese.
Objectives of Computer Network
1. Resource sharing is the main objective of the computer network. All the text, image, audio, video and
hardware available to everyone on the network.
2. The second objective is to provide the high Reliability. It is achieved by replicating the files on two or
more machines, so in case of unavailability (due to fail of hardware) the other copies can be used.
3. Increases system performance to complete a given task.
4. Only authorized user can access resource in a computer network. Users are authenticated by their user
name and password. Hence it is not possible to access the data without proper account. This increases
security.
Data Flow
Communication between two devices can be simplex, half-duplex, or full-duplex.
Simplex
In simplex mode, the communication is unidirectional. Only one of the two devices on a link can
transmit; the other can only receive. Keyboards and traditional monitors are examples of simplex devices.
Half-Duplex
In half-duplex mode, each station can both transmit and receive, but not at the same time. When one
device is sending, the other can only receive, and vice versa. The half-duplex mode is used in cases where there
is no need for communication in both directions at the same time. Walkie talkie is the example of half-duplex
mode.
Full-Duplex
In full-duplex mode, the communication is bidirectional. Both stations can transmit and receive
simultaneously. One common example of full-duplex communication is the telephone network.
Data Representation
Information today comes in different forms such as text, numbers, images, audio, and video.
Text
In data communications, text is represented as a bit pattern, a sequence of bits (0s or 1s).
Numbers
Numbers are also represented by bit patterns. The number is converted to a binary number to simplify
mathematical operations.
Images
Images are also represented by bit patterns. In its simplest form, an image is composed of a matrix of pixels
(picture elements), where each pixel is a small dot. The size of the pixel depends on the resolution.
Audio
Audio refers to the recording or broadcasting of sound or music. Audio is by nature different from text, numbers,
or images. It is continuous, not discrete.
Video
Video refers to the recording or broadcasting of a picture or movie. Video can either be produced as a continuous
entity (e.g., by a TV camera), or it can be a combination of images, each a discrete entity, arranged to convey the
idea of motion.
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COMPUTER NETWORKS
Type of Connection
 Point-to-Point A point-to-point connection provides a direct link between two devices. The entire link is
reserved for transmission between those two devices.
 Multipoint A multipoint (also called multidrop) connection is one in which more than two specific devices
share a single link.
Types of Technologies
 Wired: The communication between two devices via cables.
 Wireless: The communication between two devices without cables.
APPLICATIONS OF COMPUTER NETWORKS
Business Applications
 Most companies have a more number of computers, and each provided for workers and use them to design
products, develop products, marketing their products and to maintain database etc.
 Resource sharing: To make all equipment (physical resources like scanners and printers), and especially
data (resources like customer records, product information, financial statements and tax information)
available to anyone on the network without reach the physical location of the resource or the user.
 VPNs (Virtual Private Networks): For large companies the computers and employees may be separated
over branch offices in many countries. Networks called VPNs (Virtual Private Networks) are used to
connect the individual networks at different locations into one extended network.
If a sales person in New York need access to a product database in Singapore. The data are stored on
powerful computers called servers, are centrally housed and maintained by a system administrator. The
employees have computers, called clients, with which they access remote data.
 E-mail (electronic mail): A computer network can provide a powerful communication medium among
employees. Every company that has two or more computers now has email (electronic mail), which
employees generally use for daily communication.
 Voice over IP (VoIP): Telephone calls between employees may be carried by the computer network
instead of by phone. This technology is called IP telephony or Voice over IP (VoIP). Companies use this
way to save their telephone bills.
 Desktop sharing: The employees at distant locations can see and hear each other as they hold a meeting.
When one worker makes a change to an online document, the others can see the change immediately.
 E-commerce (electronic commerce): many companies are doing business electronically, especially with
customers and suppliers. This new model is called e-commerce (electronic commerce). Automobiles,
aircraft, and computers, airlines, bookstores, and other retailers have discovered that many customers like
the convenience of shopping from home.
Home Applications


Remote information access: Home users can connect to the remote computers through the Internet
access. Information available includes arts, business, cooking, government, health, history, hobbies,
recreation, science, sports, travel, and many others.
Person-to-person communication: The second category of network use is person-to-person
communication. E-mail is already used by millions of people all over the world.
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COMPUTER NETWORKS


E-commerce: Many people already pay their bills, manage their bank accounts, and handle their
investments electronically.
Entertainment: Users can find, buy, and download MP3 songs and DVD-quality movies and add them
to their personal collection.
Mobile Users
 Having a wired connection is impossible to carry the devices to different locations.
 People are interested in wireless networks to use their mobile devices such as laptops and cellular phones to
read and send email, tweet, watch movies, download music, play games, or to access the Web for
information.
NETWORK NODES
Network Interface Card (NIC)
A network interface controller (NIC, also known as a network interface card, network
adapter, LAN adapter or physical network interface is a computer hardware component that allows
computers to communicate over a computer network.
Repeater
A network device used to regenerate or replicate a signal. Repeaters are used in transmission systems to
regenerate analog or digital signals distorted by transmission loss, so that the signal can cover longer distances.
Hub
An Ethernet hub, active hub, network hub, repeater hub, multiport repeater or hub is a device for
connecting multiple devices together a network. It has multiple input/output (I/O) ports, in which a signal is
received from any one port, it is send to the output of every port except the original incoming.
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COMPUTER NETWORKS
Switch
A network switch (also called switching hub, bridging hub, officially MAC bridge) is a computer
networking device that connects devices together on a computer network, process and forward data to the
destination device. Unlike network hubs, a network switch forwards data only to one or multiple devices that
need to receive it, rather than broadcasting the same data to all the ports.
Bridge
A network bridge is a network device that connects multiple network segments. A bridge device filters
data traffic at a network boundary. Bridges reduce the amount of traffic on a local area network (LAN) by
dividing it into two segments.
Router
A router is a device that forwards data packets along networks. A router is connected to at least two
networks, commonly two LANs or WANs or a LAN and its ISP's network. Routers are located at gateways, the
places where two or more networks connect.
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COMPUTER NETWORKS
Gateway (Firewall)
A network firewall protects a computer network from unauthorized access. Network firewalls may be
hardware devices, software programs, or a combination of the two. All messages entering or leaving the intranet
pass through the firewall, which examines each message and blocks those that do not meet the
specified security criteria.
Fig 7: Gateway function
The above figure shows the working of a gateway. Consider 2 networks, say in New York, and a network
in London. If data has to be sent from one place to another, we need to ensure that the network technologies that
are being used by both the networks are the same. If not, we need to use a Gateway.
In the more common example, we use a telephone network and internet networks, which works on
different technologies. The telephone network follows the ISDN, and the Internet follows the IP.
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COMPUTER NETWORKS
Here, 2 different technologies are being used. In this case, the router fails to work, since the router cannot
understand the functionalities of both the networks. Hence, we require a Gateway, which acts as a translator in
communicating between the 2 networks.
Device
Hub
Bridge
Switch
Router
Gateway
Layer 1 (Physical) 2 (Data) 2 (Data) or 3 (Network) 4 (Transport)
3 (Network)
Repeater
1 (Physical) ,2 (Data) or
3 (Network)
NETWORK TOPOLOGIES
 Topology represents relationship between all the links and devices to one another. Network topologies are
categorized into the following basic types:
1. Bus
2. Ring
3. Star
4. Tree
5. Mesh
6. Hybrid
BUS Topology
Bus Topology A bus topology is multipoint. One long cable acts as a backbone to link all the devices in a
network. Nodes are connected to the bus cable by drop lines and taps.
Features of Bus Topology
1. It transmits data only in one direction.
2. Every device is connected to a single cable.
Advantages of Bus Topology
1.
2.
3.
4.
It is cost effective.
Cable required is least compared to other network topology.
It is easy to understand.
Easy to expand joining two cables together.
Disadvantages of Bus Topology
1.
2.
3.
4.
Cables fails then whole network fails.
If network traffic is heavy or nodes are more the performance of the network decreases.
Cable has a limited length.
It is slower than the ring topology.
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COMPUTER NETWORKS
RING Topology
Ring Topology In a ring topology, each device has a dedicated point-to-point connection with only the two
devices on either side of it. A signal is passed along the ring in one direction, from device to device, until it
reaches its destination. Each device in the ring acts as a repeater.
Features of Ring Topology
1. A number of repeaters are used and the transmission is unidirectional.
2. Data is transferred in a sequential manner that is bit by bit.
Advantages of Ring Topology
1. Transmitting network is not affected by high traffic or by adding more nodes, as only the nodes having
tokens can transmit data.
2. Cheap to install and expand
Disadvantages of Ring Topology
1. Troubleshooting is difficult in ring topology.
2. Adding or deleting the computers disturbs the network activity.
3. Failure of one computer disturbs the whole network.
STAR Topology
Features of Star Topology
1. Every node has its own dedicated connection to the hub.
2. Hub acts as a repeater for data flow.
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COMPUTER NETWORKS
Advantages of Star Topology
1.
2.
3.
4.
Fast performance with few nodes and low network traffic.
Easy to troubleshoot.
Easy to setup and modify.
Only that node is affected which has failed rest of the nodes can work smoothly.
Disadvantages of Star Topology
1. Cost of installation is high.
2. If the hub is affected then the whole network is stopped because all the nodes depend on the hub.
3. Performance is based on the hub that is it depends on its capacity
MESH Topology
Mesh Topology In a mesh topology, every device has a dedicated point-to-point link to every other device. A
fully connected mesh network with n nodes, each node must be connected to n - 1 nodes. We need n(n - 1)/2
physical links.
Types of Mesh Topology
1. Partial Mesh Topology : In this topology some of the systems are connected in the same fashion as mesh
topology but some devices are only connected to two or three devices.
2. Full Mesh Topology : Each and every nodes or devices are connected to each other.
Features of Mesh Topology
1. Fully connected.
2. Robust.
3. Not flexible.
Advantages of Mesh Topology
1.
2.
3.
4.
Each connection can carry its own data load.
It is robust.
Fault is diagnosed easily.
Provides security and privacy.
Disadvantages of Mesh Topology
1. Installation and configuration is difficult.
2. Cabling cost is more.
3. Bulk wiring is required.
TREE Topology
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COMPUTER NETWORKS
Tree Topology In a tree topology, each device has a dedicated point-to-point connection with one or more
devices. Like ring topology each device in the tree acts as a repeater.
Features of Tree Topology
1. Ideal if workstations are located in groups.
2. Used in Wide Area Network.
Advantages of Tree Topology
1.
2.
3.
4.
Extension of bus and star topologies.
Expansion of nodes is possible and easy.
Easily managed and maintained.
Error detection is easily done.
Disadvantages of Tree Topology
1.
2.
3.
4.
Heavily cabled.
Costly.
If more nodes are added maintenance is difficult.
Central hub fails, network fails.
HYBRID Topology
Hybrid Topology hybrid topology is a interconnection of different topologies.
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COMPUTER NETWORKS
Features of Hybrid Topology
1. It is a combination of two or more topologies
2. Inherits the advantages and disadvantages of the topologies included
Advantages of Hybrid Topology
1.
2.
3.
4.
Reliable as Error detecting and trouble shooting is easy.
Effective.
Scalable as size can be increased easily.
Flexible.
Disadvantages of Hybrid Topology
1. Complex in design.
2. Costly.
TYPES OF NETWORKS
 Different types of (private) networks are distinguished based on their size (in terms of the number of
machines), geographical coverage, and their data transfer speed.
Personal Area Networks:
 Network created over the range of a person. A common example is a network that connects a computer with
its peripherals. The connection must be done by using either wired technology through cables or wireless
technology through Bluetooth.
 It is somewhat hard to find the right cables and plugging them for new users. So the wireless technology is
easier to connect the devices than the wired technology.
 A typical PAN would include one or more computers, telephones, peripheral devices, video game consoles
and other personal entertainment devices.
Bluetooth PAN configuration
Local Area Networks (LAN):
1. A Local area network, generally called LAN, is a private network within a single room or building or
campus of up to a few kilometers in size.
2. LAN network has very high speed mainly due to proximity of computer and network devices.
3. LAN connection speeds can be 10Mbps or 100Mbps or 1000Mpbs also have low delay and make very few
errors.
4. LAN uses Guided Media.
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Ethernet
 Wireless LAN’s are established now a days to eliminate complexity to install the cables. The standard for
wireless LAN’s called IEEE 802.11, known as WiFi.
 Compared to wireless networks, wired LANs can provide high performance. It is just easier to send signals
over a wire or through a fiber than through the air. IEEE 802.3 is the popular standard for wired LAN called
Ethernet.
 In Switched Ethernet, Each computer connected to a box called a switch with a point-to-point link. A
switch has multiple ports, each port is connected to one computer.
 The job of the switch is to forward packets between computers that are attached to it, using the address in
each packet to determine which computer to send it to. To implement large LAN’s switches can be used.
Metropolitan Area Networks (MAN):
1. MAN ((Metropolitan Area Network) is a larger network of computers and other network devices which
are connected together usually spans several buildings or small town or city..
2. MAN network has lower speed compared to LAN.
3. MAN connection speeds can be 10Mbps or 100Mbps.
4. MAN uses Guided Media or Unguided media.
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 The standard for MAN is IEEE 802.16 known as WiMAX (Worldwide interoperability for Microwave
Access).
Wide Area Networks (WAN):
1. A wide area network, or WAN, occupies a very large area, such as an entire country or the entire
world. A WAN can contain multiple smaller networks, such as LANs or MANs. The Internet is the
best-known example of a public WAN.
2. WAN speed varies based on geographical location of the servers. WAN connects several LANs
3. WAN connection speeds can be 10Mbps or 100Mbps.
4. WAN mainly uses Guided Media or Unguided media.
 The subnet is used to carry the messages from host to host between different networks. The subnet consists of
two components: transmission lines and switching elements.
 Transmission lines move bits between machines. Switching elements, such as switches or routers, are
specialized computers that connect two or more transmission lines.
 When data arrive on an incoming line, the switching element must choose an outgoing line to forward them
through that line.
Point-to-Point WAN
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COMPUTER NETWORKS
A point-to-point WAN is a network that connects two communicating devices through a transmission media.
Switched WAN
A switched WAN is a network with more than two ends. A switched WAN is a combination of several point-topoint WANs that are connected by switches.
internetworks:
 Different networks with different hardware and software exist in the world. People connected to one network
want to communicate with the people connected to other networks.
 To fulfill this requirement the networks are interconnected. A collection of interconnected networks called
internetwork or internet.
Switching Networks
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COMPUTER NETWORKS
Circuit-Switched Network
 In a circuit-switched network, a dedicated connection, called a path, is always available between the two
end systems; the switch can make it active or inactive.
 The above figure shows a very simple switched network that connects two computers.
 The computers at each side are connected to a switch. The switches are connected by a high-capacity
communication line that can handle large data communications at the same time; the capacity can be
shared between many computers.
 Let us look at two cases. In the first case, all computers connected to the switch are busy; the capacity of
the thick line is fully used.
 In the second case, only one computer at one side is connected to a computer at the other side; only some
capacity of the thick line is used.
 This means that a circuit-switched network is efficient only when it is working at its full capacity; most of
the time, it is inefficient because it is working at partial capacity.
Packet-Switched Network
 In a computer network, the communication between the two ends is done in blocks of data called packets.
Instead of the continuous communication between two computers, the exchange of individual data
packets can occur between the two computers.
 The switches function for both storing and forwarding because a packet is an independent entity that can
be stored and sent later.
 A router in a packet-switched network has a queue that can store and forward the packet.
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COMPUTER NETWORKS
 If only two computers (one at each site) need to communicate with each other, there is no waiting for the
packets and forwarded in the order they arrived.
 If packets arrive at one router when the thick line is already working at its full capacity, the packets
should be stored and forwarded in the order they arrived.
 A packet-switched network is more efficient than a circuit switched network, but the packets may
encounter some delays.
THE INTERNET
 The figure shows the Internet as several backbones, provider networks, and customer networks.
 The backbones are large networks owned by some communication companies. The backbone networks are
connected through some complex switching systems, called peering points.
 The provider networks, that use the services of the backbones for a fee. The provider networks are connected
to backbones and sometimes to other provider networks.
 The customer networks actually use the services provided by the Internet. They pay fees to provider networks
for receiving services.
 Backbones and provider networks are also called Internet Service Providers (ISPs). The backbones are often
referred to as international ISPs; the provider networks are often referred to as national or regional ISPs.
Accessing the Internet
Using Telephone Networks
The most residences and small businesses have telephone service, through a telephone network. The Internet,
service is also provided by changing the voice line. This can be done in two ways.
Dial-up service. the dial-up service is very slow, and when the line is used for Internet connection, it cannot be
used for telephone (voice) connection.
DSL Service. The DSL service also allows the line to be used simultaneously for voice and data communication.
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COMPUTER NETWORKS
Using Cable Networks
More and more residents using cable TV services to receive TV broadcasting. The cable companies have been
upgrading their cable networks and connecting to the Internet. A residence or a small business can be connected
to the Internet by using this service. It provides a higher speed connection, but the speed varies depending on the
number of neighbors that use the same cable.
Using Wireless Networks
Wireless connectivity has recently become popular. With the growing wireless WAN access, a household or a
small business can be connected to the Internet through a wireless WAN.
Direct Connection to the Internet
A large organization or a large corporation can itself become a local ISP and be connected to the Internet.
For example, a large university with several computers can create an internetwork and then connect the
internetwork to the Internet.
INTERNET HISTORY
Early History
There were some communication networks, such as telegraph and telephone networks, before 1960. These
networks were suitable for constant-rate communication, which means that after a connection was made between
two users, the encoded message (telegraphy) or voice (telephony) could be exchanged. A computer network
should be able to handle bursty data, which means data received at variable rates at different times.
Birth of Packet-Switched Networks
The packet switching for bursty traffic was first presented by Leonard Kleinrock in 1961. At the same time, two
other researchers, Paul Baran at Rand Institute and Donald Davies at National Physical Laboratory in England,
published some papers about packet-switched networks.
ARPANET
In the mid-1960s, mainframe computers were used in research organizations. Computers from different
manufacturers were unable to communicate with one another.
The Advanced Research Projects Agency (ARPA) presented its ideas for the Advanced Research Projects
Agency Network (ARPANET), a small network of connected computers.
The idea was that each host computer would be attached to a specialized computer, called an interface message
processor (IMP). The IMPs would be connected to each other. Each IMP had to be able to communicate with
other IMPs.
Birth of the Internet
In 1972 Vint Cerf and Bob Kahn wanted to link dissimilar networks so that a host on one network could
communicate with a host on another.
There were many problems to overcome for this: diverse packet sizes, diverse interfaces, and diverse
transmission rates, as well as differing reliability requirements.
Cerf and Kahn devised the idea of a device called a gateway to serve as the intermediary hardware to transfer
data from one network to another.
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TCP/IP
Cerf and Kahn in 1973 proposed a paper, the protocols to achieve end-to-end delivery of data. This paper on
transmission control protocol (TCP) included concepts such as encapsulation, the datagram, and the functions
of a gateway.
Shortly thereafter, authorities made a decision to split TCP into two protocols: Transmission Control Protocol
(TCP) and Internet Protocol (IP). IP would handle datagram routing while TCP would be responsible for higher
level functions such as segmentation, reassembly, and error detection. The new combination became known as
TCPIIP.
MILNET
In 1983, ARPANET split into two networks: Military Network (MILNET) for military users and ARPANET for
nonmilitary users.
CSNET
Science Network (CSNET) was a network sponsored by the National Science Foundation (NSF) in 1981.
CSNET was a less expensive network; there were no redundant links and the transmission rate was slower.
By the mid-1980s, most U.S. universities with computer science departments were part of CSNET. Other
institutions and companies were also forming their own networks and using TCP/IP to interconnect.
NSFNET
With the success of CSNET, the NSF in 1986 sponsored the National Science Foundation Network (NSFNET), a
backbone that connected five supercomputer centers located throughout the United States with a 1.544-Mbps
data rate.
ANSNET
In 1991, the U.S. government decided that NSFNET was not capable of supporting the rapidly increasing
Internet traffic. Three companies, IBM, Merit, and Verizon, forming a nonprofit organization called Advanced
Network & Services (ANS) to build a new, high-speed Internet backbone called Advanced Network Services
Network (ANSNET).
Internet Today
The Internet today is a set of peer networks that provide services to the whole world.
World Wide Web
The World Wide Web (WWW) was invented at CERN by Tim Berners-Lee. This invention has added the
commercial applications to the Internet.
Multimedia
Recent developments in the multimedia applications such as voice over IP (telephony), video over IP (Skype),
view sharing (YouTube), and television over IP (PPLive) has increased the number of users and the amount of
time each user spends on the network.
Peer-to-Peer Applications
Peer-to-peer networking is also a new area of communication with a lot of potential.
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COMPUTER NETWORKS
PROTOCOLS AND STANDARDS
Protocols
A protocol is a set of rules that governs communication between two people or two devices. A protocol
defines what is communicated, how it is communicated, and when it is communicated.
For example, in a telephone conversation, there are a set of rules that we need to follow. There is a rule how to
make connection (dialing the telephone number), how to respond to the call (picking up the receiver), how to
start and proceed the communication by each party, and finally how to end the communication (hanging up).
The key elements of a protocol are syntax, semantics, and timing.
❑ Syntax.
Syntax specifies the structure or format of the data, the order in which they are presented. The order is
applied when the data are stored or transmitted. Different computers may store data in different orders.
For example, a simple protocol specifies the first 8 bits of data to be the address of the sender, the second 8 bits
to be the address of the receiver, and the rest of the stream is message.
❑ Semantics.
Semantics refers to the meaning of each section of bits.
❑ Timing.
Timing refers to two characteristics: when data should be sent and how fast it can be sent.
For example, if a sender produces data at 100 megabits per second (100 Mbps) but the receiver can process data
at only 1 Mbps, the transmission will overload the receiver and data will be lost.
Standards
 A standard is a document that provides requirements, specifications, guidelines or characteristics that can be
used to ensure that materials, products and services.
 A de facto (by fact) standard is achieved as a result of market competition. A process that a product becomes
a standard after it is brought to the market and exposed to competition is called the “classical model”.
 Because a winner is determined after products are brought to the market, the companies have a burden on
their capital investment. The consumers also get lost with their purchased products.
 The de jure (by law) standards are developed after complete a certain procedure. Those standards developed
by ISO through a procedure that experts gather from all over the world and discuss on standardization
proposals, which are finally approved through voting by national standards institutes.
 The companies bring their products to the market based on de jure standards; have no burden on corporations
or consumers. However, development of de jure standards takes a long time.
INTERNET STANDARDS
An Internet draft is a initial version of a document with a six-month lifetime. The draft can be published as a
Request for Comment (RFC) after implementation.
Each RFC is edited, assigned a number, and made available to all interested parties. RFCs standardized through
maturity levels.
Maturity Levels
During the lifetime of RFC, it can be implemented into one of six maturity levels: proposed standard, draft
standard, Internet standard, historic, experimental, and informational.
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Proposed Standard
An RFC is changed to proposed standard status after at least one successful implementation. At this level, the
RFC is tested and implemented by several different groups.
Draft Standard
A proposed standard is changed to draft standard status after at least two successful implementations.
Internet Standard
A draft standard is changed to Internet standard status after at least two successful implementations.
Historic
If the same specifications of Internet standards are used for long time gets historic status. If they never passed
through the maturity levels also gets a historic status.
Experimental
An RFC classified as experimental describes work related to an experimental situation that does not affect the
operation of the Internet. Such an RFC should not be implemented in any functional Internet service.
Informational
An RFC classified as informational contains general, and historical information related to the Internet.
INTERNET ADMINISTRATION
Internet Society (ISOC)
The Internet Society (ISOC) is an international organization which provides support for the Internet standards
process. Three organizations under the Internet Society are responsible for the actual work of standards
development and publication
Internet Architecture Board (IAB)
The Internet Architecture Board (IAB) is a group of researchers and professionals that allows to the engineering
and technical development of the Internet. IAB provides guidance and broad direction to the Internet Research
Task Force (IRTF) and the Internet Engineering Task Force (IETF).
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Internet Engineering Task Force (IETF)
Internet Engineering Task Force (IETF) is a forum of working groups managed by the Internet Engineering
Steering Group (IESG). IETF is responsible for identifying problems and proposing solutions to these problems.
The working groups are divided into different areas, and each area working on a specific topic, any interested
party may participate on any area. Currently nine areas have been defined. The areas are:
❑ Applications
❑ Internet protocols
❑ Routing
❑ Operations
❑ User services
❑ Network management
❑ Transport
❑ Internet protocol next generation (IPng)
❑ Security
During the development, a working group will make a draft version of the document called as an Internet Draft,
which is placed in the IETF's "Internet Drafts" online directory.
The document may remain as an Internet Draft for up to six months, and interested parties may review and
comment on the draft. During that time, the IESG may approve publication of the draft as an RFC (Request for
Comment). If the draft has not progressed to the status of an RFC during the six-month period, it is withdrawn
from the directory. The working group may subsequently publish a revised version of the draft.
Internet Research Task Force (IRTF)
The Internet Research Task Force (IRTF) is a forum of working groups managed by the Internet Research
Steering Group (IRSG). IRTF focuses on long-term research topics related to Internet protocols, applications,
architecture, and technology.
Internet Assigned Numbers Authority (IANA) and Internet Corporation for Assigned Names and
Numbers (ICANN)
The Internet Assigned Numbers Authority (IANA), was responsible for the management of Internet domain
names and addresses.
Network Information Center (NIC)
The Network Information Center (NIC) is responsible for collecting and distributing information about
TCP/IP protocols.
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Advantages and disadvantage of having international standards for network protocols
Advantages:
1. Many computers from the entire world can easily connect and communicate together.
2. Maintenance and installation become simplified due to a common standard.
Disadvantages:
1. All companies and manufactures must follow the standards instead of developing new techniques.
2. If new techniques are discovered, it would be difficult to incorporate those techniques into an already
accepted standard.
3. Takes a lot of time for companies to agree on standards.
NETWORK MODELS
PROTOCOL LAYERING
 A network protocol is an agreement between the communicating parties specifies how the communication is
to proceed.
 When communication is simple, we may need only one simple protocol; when the communication is
complex, we may need to divide the task between different layers, in which case we need a protocol at each
layer, or protocol layering.
 To implement protocols most networks are organized as a set of layers or levels. The number of layers, the
name of each layer, the contents of each layer, and the function of each layer differ from network to network.
 The entities called peers such as software processes, hardware devices, or human beings that communicate
by using the same protocol to talk to each other.
 The purpose of each layer is to provide services to the higher layers. In reality, no data are directly transferred
from layer n on one machine to layer n on another machine. Instead, each layer passes data and control
information to the layer immediately below it, until the lowest layer is reached.
 A five-layer network is shown in the below diagram. The actual communication occurs through the physical
medium. The virtual communication is shown by dotted lines and physical communication by solid lines.
 The interface defines which operations and services the lower layer provides to the upper layer. A set of
layers and protocols is called network architecture. A list of the protocols used by a certain system is called
a protocol stack.
Scenarios
First Scenario
 In the first scenario, communication can occur in only one layer. Assume Maria and Ann are neighbors
with a lot of common ideas. Communication between Maria and Ann takes place in one layer, face to
face, in the same language.
 In this simple scenario, a set of rules needs to be followed.
 First, Maria and Ann should greet each other when they meet.
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



Second, they should restrict their vocabulary to the level of their friendship.
Third, each party should be calm when the other party is speaking.
Fourth, each party should have the opportunity to talk about the issue.
Fifth, they should exchange some nice words when they leave.
Second Scenario
 Imagine two philosophers (processes in layer 3), one of whom speaks Urdu and English and one of whom
speaks Chinese and French. Since they have no common language they can use a translators (processes at
layer 2), translators may contacts a secretary (processes in layer 1). Philosopher 1 wishes to convey his
message (in English) ‘‘I like rabbits’’ to Philosopher 2, he send across the 2/3 interface to his translator.
 The translators have agreed on a neutral language known to both of them, Dutch, so the message is
converted to ‘‘Ik vind konijnen leuk’’. The translator then gives the message to a secretary for
transmission, for example, by email (the layer 1 protocol).
 When the message arrives at the other secretary, it is passed to the local translator, who translates it into
French and passes it across the 2/3 interface to the second philosopher.
Encapsulation and Decapsulation
 A message, M, is produced by an application process running in layer 5 and given to layer 4 for
transmission. Layer 4 puts a header in front of the message to identify the message and passes the result
to layer 3. The header includes control information, such as addresses, sequence numbers, sizes, and
times to allow layer 4 on the destination machine to deliver the message.
 In any networks, there is no limit on the size of messages transmitted in the layer 4 protocol but there is a
limit at the layer 3 protocol. So layer 3 must break up the incoming messages into smaller units called
packets, and layer 3 header added to each packet. In this example, M is split into two parts, M1 and M2
that will be transmitted separately.
 Layer 3 decides which of the outgoing lines to use and passes the packets to layer 2. Layer 2 adds not
only a header but also a trailer, and gives the resulting unit to layer 1 for physical transmission. At the
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receiving machine the message moves upward, from layer to layer, with headers being removed at each
corresponding layer as it progresses.
Principles of Protocol Layering
First Principle
 If we want bidirectional communication, we need to make each layer is able to perform two opposite
tasks, one in each direction.
 For example, the third layer task is to listen (in one direction) and talk (in the other direction). The second
layer needs to be able to encrypt and decrypt. The first layer needs to send and receive mail.
Second Principle
 The second principle, the two objects under same layers at both sites should be identical.
 For example, the object under layer 3 at both sites should be a plaintext letter. The object under layer 2 at
both sites should be a ciphertext letter. The object under layer 1 at both sites should be a piece of mail.
Logical Connections (Virtual Connections)
 Logical connection between each layer represents layer-to-layer communication.
 Maria and Ann had a logical (imaginary) connection at each layer through which they can send the object
created from that layer.
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Design Issues for the Layers:
Reliability:
 During the transmission of data there is a chance that some of these bits will be damaged (inverted) due to
electrical noise, random wireless signals, hardware problems, software bugs and so on.
 One mechanism for finding errors in received information uses codes for error detection. Information
that is incorrectly received can then be retransmitted until it is received correctly.
 More powerful codes used for error correction, where the correct message is recovered from the
incorrect bits that were received. Both of these mechanisms work by adding redundant information.
 Another reliability issue is finding a working path through a network. In a large network there are
multiple paths between a source and destination. If some links or routers that are broken, packets can sent
through the different links and routers. The network should automatically make this decision. This is
called routing.
Evolution of the network:
 Since there are many computers on the network, every layer needs a mechanism for identifying the
senders and receivers that are involved in a communication. This mechanism is called addressing or
naming.
 The networks can transmit maximum size of a message that needs mechanisms for disassembling,
transmitting, and then reassembling messages. This is called internetworking.
 When networks get large, new problems arise. Cities can have traffic jams, a shortage of telephone
numbers, and it is easy to get lost. Designs are implemented for network gets large are said to be
scalable.
Resource allocation:
 Networks provide a transmission lines to each host in the network. The mechanisms that share network
bandwidth to prevent interference of one host with another. Communication channels are shared by
multiple signals is called multiplexing.
 An allocation problem that occurs at every level is how to prevent a fast sender from a slow receiver with
data. Feedback from the receiver to the sender is often used. This is called flow control.
 Sometimes the network is oversubscribed because too many computers want to send too much traffic, and
the network cannot deliver it all. This overloading of the network is called congestion. One solution is
that each computer has to reduce its demand when it identifies congestion.
Security:
 Network is protected by preventing different kinds of threats. One of the threats is eavesdropping (secret
listening or monitoring) on communications.
 Mechanisms that provide confidentiality against this threat, and they are used in multiple layers.
Mechanisms for authentication prevent someone from impersonating (acting) as someone else.
Mechanisms for integrity prevent changes on the messages, such as altering ‘‘debit my account $10’’ to
‘‘debit my account $1000.’’
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Connection-Oriented Versus Connectionless Service:
S.No
Characteristic
Connectionless Service
Connection Oriented Service
1
Connection setup
No connection is established.
Connection must be established
prior transmission.
2
Data transmission
Each packet can transmit
individually through the different
paths.
All the packets transmitted
through a single path.
3
Addressing
Source and destination addresses
can include in each packet.
Source and destination addresses
can include in first packet.
4
Reliability and
acknowledgments
Unreliable, data delivery without
acknowledgments
Reliable delivery of messages;
all data is acknowledged.
5
Retransmission
Not performed.
Lost data is retransmitted
automatically.
6
Loss of data
Less.
High.
7
Flow control
None.
Flow control using sliding
window protocols.
8
Transmission speed
Very high.
Low.
9
Example of Protocol
UDP (User Datagram Protocol).
TCP (Transmission control
protocol).
THE OSI REFERENCE MODEL
Physical Layer:
 Physical layer is the lowest layer of all. It is responsible for sending bits from one computer to another. This
layer defines electrical and physical details represented as 0 or a 1.
1. Representation of Bits: Data in this layer consists of stream of bits. The bits must be encoded into signals
for transmission. It defines the type of encoding i.e. how 0’s and 1’s are changed to signal.
2. Data Rate: This layer defines the rate of transmission which is the number of bits per second.
3. Synchronization: It deals with the synchronization of the transmitter and receiver. The sender and receiver
are synchronized at bit level.
4. Line Configuration: This layer connects devices with the medium: Point to Point configuration and
Multipoint configuration.
5. Topologies: Devices must be connected using the following topologies: Mesh, Star, Ring and Bus.
6. Transmission Modes: Physical Layer defines the direction of transmission between two devices: Simplex,
Half Duplex, and Full Duplex.
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Data Link Layer:
 Data link layer is most reliable node to node delivery of data. It forms frames from the packets that are
received from network layer and gives it to physical layer.
1. Framing: The bits received from the network layer is divided into manageable data units called frames.
2. Physical Addressing: The Data Link layer adds a header to the frame in order to define physical address of
the sender or receiver of the frame.
3. Flow Control: A flow control mechanism to avoid a fast transmitter from running a slow receiver. This
prevents traffic jam at the receiver side.
4. Error Control: Error control is achieved by adding a trailer at the end of the frame. Duplication of frames is
also prevented by using this mechanism.
5. Access Control: Protocols of this layer determine which of the devices has control over the link at any given
time, when two or more devices are connected to the same link.
Network Layer:
 The main aim of this layer is to deliver packets from source to destination across multiple links (networks). If
two computers (system) are connected on the same link then there is no need for a network layer.
1. Translation: It translates logical network address into physical address.
2. Routing: Routers and gateways operate in the network layer. Network Layer provides a mechanism to route
the packets to final destination.
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3. Connection services: Network layer provides flow control, network layer error control and packet sequence
control.
4. Fragmentation: Breaks larger packets into small packets called fragments or packets.
Transport Layer:
 The main aim of transport layer is to be delivered the entire message from source to destination. Transport
layer ensures whole message arrives correct and in order, ensuring both error control and flow control at the
source to destination level. Transport layer breaks the message (data) into small units called segments.
1. Service Point Addressing: Transport Layer header includes service point address which is port address.
This layer gets the message to the correct process on the computer.
2. Segmentation and Reassembling: A message is divided into segments; each segment contains sequence
number, which enables this layer in reassembling the message. Message is reassembled correctly upon
arrival at the destination and replaces packets which were lost in transmission.
3. Connection Control: It includes 2 types :
o Connectionless Transport Layer: Each segment is considered as an independent packet and delivered to
the transport layer at the destination machine.
o Connection Oriented Transport Layer: Before delivering packets, connection is made with transport
layer at the destination machine.
4. Flow Control: In this layer, flow control is performed end to end.
5. Error Control: Error Control is performed end to end in this layer to ensure that the complete message
arrives at the receiving transport layer without any error. Error Correction is done through retransmission.
Session Layer:
 Its main aim is to establish, maintain and synchronize the interaction between communicating systems.
1. Dialog Control: This layer allows two systems to start communication with each other in half-duplex or
full-duplex.
2. Synchronization: This layer allows a process to add checkpoints which are considered as synchronization
points into stream of data. Example: If a system is sending a file of 800 pages, adding checkpoints after
every 50 pages is recommended. This ensures that 50 page unit is successfully received and acknowledged.
This is beneficial at the time of crash as if a crash happens at page number 110; there is no need to retransmit
1 to100 pages.
Presentation Layer:
 The primary goal of this layer is to take care of the syntax and semantics of the information exchanged
between two communicating systems. Languages (syntax) can be different of the two communicating
systems.
1. Translation: Before being transmitted, information in the form of characters and numbers should be
changed to bit streams.
2. Encryption: It carries out encryption at the transmitter and decryption at the receiver.
3. Compression: It carries out data compression to reduce the bandwidth of the data to be transmitted.
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Application Layer:
 It is the top most layer of OSI Model. Manipulation of data (information) in various ways is done in this layer
which enables user or software to get access to the network. Some services provided by this layer includes:
E-Mail, transferring of files, distributing the results to user, directory services, network resource etc.
1. Mail Services: This layer provides the basis for E-mail forwarding and storage.
2. Network Virtual Terminal: It allows a user to log on to a remote host. The application creates software
emulation of a terminal at the remote host. User’s computer talks to the software terminal which in turn talks
to the host and vice versa. Then the remote host believes it is communicating with one of its own terminals
and allows user to log on.
3. Directory Services: This layer provides access for global information about various services.
4. File Transfer, Access and Management (FTAM): It is a standard mechanism to access files and manages
it. Users can access files in a remote computer and manage it.
THE TCP/IP REFERENCE MODEL
 TCP/IP means transmission control protocol and internet protocol. Protocols are set of rules which govern
every possible communication over the internet. These protocols describe the movement of data between the
host computers or internet.
Layer 1: Link or Host-to-network Layer
1. Lowest layer of the all.
2. Protocol is used to connect the host, so that the packets can be sent over it.
3. Varies host to host and network to network.
Layer 2: Internet layer
1.
2.
3.
4.
5.
It the layer which holds the whole architecture together.
It allows the host to insert the packets.
It helps the packet to travel independently to the destination.
Packets are received is different from the way they are sent can make order.
IP (internet protocol) is used in this layer.
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Layer 3: Transport Layer
1. It decides if data transmission should be on parallel path or single path.
2. Functions such as multiplexing, segmenting can be done on the data.
3. Transport layer breaks the message (data) into small units so that they are handled more efficiently by the
network layer.
4. Functions of the transport layer are same as the OSI model.
5. Transport layer also arrange the packets sent in sequence.
Layer 4: Application Layer
1. Include any session and presentation functions that are required.
2. Protocols used in this layer are high level protocols such as TELNET, HTTP, RTP, DNS, FTP, and SMTP.
Comparison of OSI Reference Model and TCP/IP Reference Model
OSI(Open System Interconnection)
TCP/IP(Transmission Control Protocol /
Internet Protocol)
1. OSI is a reference model.
1. TCP/IP model is implemented from the OSI.
2. In OSI model the transport layer guarantees the
delivery of packets
2. In TCP/IP model the transport layer does not
guarantees delivery of packets.
3. Model is implemented before the protocols
3. Model was implemented after the protocols
4. OSI model has a separate presentation layer
4. TCP/IP does not have a separate presentation
layer.
5. OSI is a general model. Any application can be
implemented using OSI model.
5. TCP/IP model cannot be used in any other
application.
6. Network layer of OSI model provide both
connection oriented and connectionless service.
6. The Network layer in TCP/IP model provides
connectionless service.
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7. Protocols are hidden in OSI model and are easily
replaced as the technology changes.
7. In TCP/IP replacing protocol is not easy.
8. OSI model defines services, interfaces and
protocols very clearly and makes clear distinction
between them.
8. In TCP/IP it is not clearly separated its services,
interfaces and protocols.
9. It has 7 layers.
9. It has 4 layers.
Write short notes on interface, service and protocol.



Services: Each layer performs some services for the layer above it. The service definition tells what the
layer does, not how to access entities or how the layer works.
(Explain the different services provided by different layers)
Interfaces: The mechanism for communication between adjacent layers in the model is called
an interface. A layer’s interface tells how to access the processes above it. It specifies what the
parameters are and what results to expect.
Protocols: A set of rules that enable two devices to connect and transmit data to one another. Protocols
determine how data are transmitted between computing devices and over networks.
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