Department of Computer Science & Engineering

COMPUTER NETWORKS[BCS502]

COMPUTER NETWORKS – BCS502

Dr. M S Sunitha Patel
Associate Professor
Dept. of Computer Science & Engineering
ATMECE, Mysuru
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Contents

➢ Course Information

➢ Introduction to Computer Networks

➢ Course Objectives

➢ Course Outcome

➢ Overview of syllabus
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Course Information

▪ Course Type: IPCC [Integrated Professional Core Course]

▪ No of credits:04
▪ Teaching Hours
Theory: 03
Lab:02
▪ CIE Marks:50

▪ SEE:Marks:50

▪ Total Marks :100

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Course Information contd..

▪ CIE for the theory component of the IPCC (maximum marks 50)

▪ IPCC means practical portion integrated with the theory of the course.

▪ CIE marks for the theory component are 25 marks and that for the
practical component is 25 marks.

▪ 25 marks for the theory component are split into 15 marks for two Internal
Assessment Tests (Two Tests each of 15 Marks with 01-hour duration, are
to be conducted) and 10 marks for other assessment methods
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Course Information contd..

▪ CIE (Theory)
Components Marks
Test-1 40 Scaled down to
Test-2 40 15 marks
Test-3 40
Assignments 10 Scaled down
Activities Assessment 10 to 10 marks
MOOC course 10
Total 25
▪ The student has to secure 40% of 25 marks to qualify in the CIE of the
theory component of IPCC.
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Course Information contd..

▪ CIE (Practical)

Components Marks
Conduction of Experiments 15 Average marks
- Observation book of all
- Record experiments
- Execution of experiment
- Viva
Test 50 Scaled down to
10 marks
Total 25
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Course Information contd..

SEE
▪ The question paper will have ten questions. Each question is set for 20 marks.
▪ There will be 2 questions from each module. Each of the two questions under a
module (with a maximum of 3 sub-questions), should have a mix of topics under
that module.
▪ The students have to answer 5 full questions, selecting one full question from each
module.
▪ Marks scored by the student shall be proportionally scaled down to 50 Marks
▪ The theory portion of the IPCC shall be for both CIE and SEE, whereas the
practical portion will have a CIE component only. Questions mentioned in the
SEE paper may include questions from the practical component.
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Course objectives
This course will enable students to,

▪ Study the TCP/IP protocol suite, switching criteria and Medium Access Control
protocols for reliable and noisy channels.

▪ Learn network layer services and IP versions.

▪ Discuss transport layer services and understand UDP and TCP protocols.

▪ Demonstrate the working of different concepts of networking layers and protocols.

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Overview of syllabus
Modules
Module-1
Introduction: Data Communications, Networks, Network Types, Networks Models:
Protocol Layering, TCP/IP Protocol suite, The OSI model, Introduction to Physical
Layer: Transmission media, Guided Media, Unguided Media: Wireless. Switching:
Packet Switching and its types.
Module-2
Data Link Layer: Error Detection and Correction: Introduction, Block Coding, Cyclic
Codes. Data link control: DLC Services: Framing, Flow Control, Error Control,
Connectionless and Connection Oriented, Data link layer protocols, High Level Data
Link Control. Media Access Control: Random Access, Controlled Access. Check
Sum and Point to Point Protocol
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Overview of syllabus contd..

Modules
Module-3
Network Layer: Network layer Services, Packet Switching, IPv4 Address, IPv4
Datagram, IPv6 Datagram, Introduction to Routing Algorithms, Unicast Routing
Protocols: DVR, LSR, PVR, Unicast Routing protocols: RIP, OSPF, BGP,
Multicasting Routing-MOSPF
Module-4
Introduction to Transport Layer: Introduction, Transport-Layer Protocols:
Introduction, User Datagram Protocol, Transmission Control Protocol: services,
features, segments, TCP connections, flow control, Error control, Congestion
control.
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Overview of syllabus contd..

Modules
Module-5
Introduction to Application Layer: Introduction, Client-Server Programming,
Standard Client- Server Protocols: World Wide Web and HTTP, FTP, Electronic
Mail, Domain Name System (DNS), TELNET, Secure Shell (SSH)
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Course outcomes
At the end of the course, the student will be able to:

1. Explain the fundamentals of computer networks.

2. Apply the concepts of computer networks to demonstrate the working of

various layers and protocols in communication network.

3. Analyze the principles of protocol layering in modern communication systems.

4. Demonstrate various Routing protocols and their services using tools such as
Cisco packet tracer.
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Textbooks
1. Behrouz A. Forouzan, Data Communications and Networking, 5th
Edition, Tata McGraw-Hill,2013.
Reference Books

1. Larry L. Peterson and Bruce S. Davie: Computer Networks – A Systems

Approach, 4th Edition, Elsevier, 2019.

2. Nader F. Mir: Computer and Communication Networks, 2nd Edition, Pearson

Education, 2015.

3. William Stallings, Data and Computer Communication 10th Edition, Pearson

Education, Inc., 2014.
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PRACTICAL COMPONENT OF IPCC(May cover all / major modules)

1. Implement three nodes point – to – point network with duplex links between
them. Set the queue size, vary the bandwidth, and find the number of packets
dropped.

2. Implement transmission of ping messages/trace route over a network topology

consisting of 6 nodes and find the number of packets dropped due to congestion.

3. Implement an Ethernet LAN using n nodes and set multiple traffic nodes and plot
congestion window for different source / destination.

4. Develop a program for error detecting code using CRC-CCITT (16- bits).

5. Develop a program to implement a sliding window protocol in the data link layer.
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PRACTICAL COMPONENT OF IPCC contd…

6. Develop a program to find the shortest path between vertices using the Bellman-
Ford and path vector routing algorithm.

7. Using TCP/IP sockets, write a client – server program to make the client send the
file name and to make the server send back the contents of the requested file if
present.

8. Develop a program on a datagram socket for client/server to display the messages

on client side, typed at the server side.

9. Develop a program for a simple RSA algorithm to encrypt and decrypt the data.

10. Develop a program for congestion control using a leaky bucket algorithm.
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Introduction
What is a Computer Network

A network is a collection or set of computing devices connected to one another

to establish communication and also share available resources.

These networked devices use a system of rules, called communication

protocols, to transmit information over physical or wireless technologies.

A network is a set of devices (nodes) connected by communication links.

A node can be a computer, printer, or any other device capable of sending

and/or receiving data generated by other nodes on the network.

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Why Networking?

Do you prefer these?

Or this?

Sharing information — i.e. data communication

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• Sharing hardware or software

E.g. print document

• Centralize administration and support

E.g. Internet-based, so everyone can access the same administrative or
support application from their PCs
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How many kinds of Networks?

Depending on one’s perspective, we can classify networks in

different ways

• Based on transmission media: Wired (UTP, coaxial cables, fiber-

optic cables) and Wireless

• Based on network size: LAN and WAN (and MAN)

• Based on management method: Peer-to-peer and Client/Server

• Based on topology (connectivity): Bus, Star, Ring

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MODULE-1

Introduction
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Contents
➢ Data Communications
▪ Networks
▪ Network Types
➢ Networks Models
▪ Protocol Layering
▪ TCP/IP Protocol suite
▪ The OSI model
➢ Introduction to Physical Layer: Transmission media
▪ Guided Media
▪ Unguided Media: Wireless.
▪ Switching: Packet Switching and its types.
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Data Communications

▪ The term telecommunication means communication at a distance.

▪ The word data refers to information presented in whatever form is

agreed upon by the parties creating and using the data.

▪ Data communications are the exchange of data between two

devices via some form of transmission medium such as a wire

cable.
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Characteristics of data communication system

▪ Delivery : The system must deliver data to the correct destination .
▪ Accuracy :The system must deliver the data accurately.

▪ Timeliness: The system must deliver data in a timely manner. Data

delivered late are useless.

▪ Jitter: Jitter refers to variation in the packet arrival time. For

example, 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.
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Components of a data communication system

Fig: Five components of data communication

▪ Message : The message is the information (data) to be communicated .

▪ Sender: The sender is the device that sends the data message.

▪ Receiver :The receiver is the device that receives the message.

▪ Transmission medium :The transmission medium is the physical path by

which a message travels from sender to receiver.

▪ Protocol : A protocol is a set of rules that govern data communications

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Data Representation
▪ Text
‣ Represented as bit pattern (sequence of bits 0s or 1s)
‣ Different set of bit pattern used to represent symbols or characters.
‣ Each set is called code
‣ Process of representing symbols is called encoding
‣ Ex: ASCII,Unicode
▪ Numbers
‣ Represented as bit pattern
‣ Directly converted to binary form
▪ Audio
‣ Recording or broadcasting of sound or music.
‣ Continuous not discrete
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▪ Video
‣ Recording or broadcasting of picture or a movie
‣ Produced as :
‣ Continuous entity [TV camera]
‣ Combination of images-discrete entity
▪ Images
‣ Represented as bit pattern
‣ Image is divided into matrix of pixels(smallest element of an image)
‣ Each pixel is assigned a bit pattern (size and value of pattern depend on image)
‣ Ex: black and white dots (chessboard) -1 bit pattern is enough to represent a
pixel, gray scale- 2 bit pattern.
‣ Several methods to represent colour images : RGB,YCM
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Data Flow
Communication between two devices can be simplex, half-duplex, duplex as shown in figure.

1. Simplex

• Communication is unidirectional.

• Only one of the two devices on a link can transmit; the other can only receive.

• E.g. : One way street, Keyboard , Monitor.

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2. Half duplex
▪ 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.
▪ E.g.: Walkie Talkie.

3. Full duplex
▪ Both stations can transmit and receive simultaneously.
▪ It is like a two way street with the traffic flowing in both the directions at the same time.
▪ E.g .: Telephone network
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Networks

▪ A network is a set of devices (often referred to as nodes)

connected by communication links.
▪ A node can be a computer, printer, or any other device capable
of sending and/or receiving data generated by other nodes on the
network.
▪ A link can be a cable, air, optical fiber, or any medium which
can transport a signal carrying information.
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Network Criteria
1. Performance
▪ Measured using:
‣ Transit time: time taken to travel a message from one device to another.
‣ Response time: time elapsed between enquiry and response.
▪ Depends on following factors:
‣ Number of users
‣ Type of transmission medium
‣ Efficiency of software
▪ Evaluated by 2 networking metrics:
‣ Throughput (high)
‣ Delay (small)
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2. Reliability
▪ Measured by
‣ Frequency of failure.
‣ Time taken to recover from a network failure.
‣ Network robustness in a disaster.
3. Security
▪ Protecting data from unauthorized access, damage and
development.
▪ Implementing policies and procedures for recovery from breaches
and data losses.
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Physical Structures
Type of Connection

1. Point to Point :

▪ It provides a dedicated link between two devices .

▪ The entire capacity of the link is reserved for transmission between those
two devices.

▪ It uses an actual length of wire or cable to connect the two ends.

▪ When we change TV channels by infrared remote control, we are establishing a

point-to-point connection between remote control and TV's control system
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2. Multipoint

▪ It is the one in which more than two specific devices share a single
link.

▪ Capacity of the channel is either spatially or temporally shared.

‣ Spatially shared : Several devices can use the link simultaneously.
‣ Temporally shared : Users take turns.
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Physical Topology
▪ Topology of network is the geometric representation of all links and linking
devices to one another
▪ Basic topologies:
1.Mesh
2.Star
3.Bus and
4.Ring
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1. Mesh Topology
▪ Point to Point connection
▪ Every device has a dedicated
point-to point link to every device.
▪ The term dedicated means that the link
carries traffic only between the two
devices it connects.

▪ For n nodes

‣ n(n-1) physical links

‣ n(n-1)/2 duplex mode links

▪ Every device have (n-1) I/O ports to be

connected to other (n-1) devices.
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▪ Advantages:
‣ A mesh topology is robust.
‣ If one link becomes unusable, it does not incapacitate the entire system.
‣ Point-to-point links make fault identification and fault isolation easy.
‣ Privacy or security : When every message travels along a dedicated line, only the
intended recipient sees it.

▪ Disadvantages:

‣ Difficult installation and reconfiguration.

‣ Bulk of wiring occupies more space than available space.

‣ Hardware required to connect each link is expensive.

▪ Practical example: connection of telephone regional offices in which each regional office
needs to be connected to every other regional office.
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2. Star Topology
▪ Point to Point connection
▪ All the devices are connected to a central
controller called a hub
▪ Dedicated point-to-point link between a
device & a hub.
▪ The devices are not directly linked to one
another. Thus, there is no direct traffic
between devices.
▪ The hub acts as a junction:
‣ If device-1 wants to send data to device-2,
‣ the device-1 sends the data to the hub,
then the hub relays the data to the device-
2.
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▪ Advantages:
‣ A star topology is less expensive than a mesh topology.
‣ Each device needs only one link and one I/O port to connect it to any number
of others.

‣ Easy to install and reconfigure.

‣ Requires less cabling, less expensive than mesh topology.

‣ Robustness: If one link fails, only that link is affected.

‣ All other links remain active. As a result fault identification and fault
isolation becomes easy.

▪ Disadvantages :
‣ Dependency of whole topology on one single point, the hub.
▪ Example : Local area network
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3. Bus Topology

▪ Multipoint connection

▪ All the devices are connected to the single cable called bus (backbone)

▪ Devices are connected to the bus by drop-lines and taps.

▪ A drop-line is a connection running between the device and the bus (main
cable).

▪ A tap is a connector that links to the bus.

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▪ As a signal travels along the backbone, some of its energy is transformed into
heat.

▪ As a result there is a limit on the number of taps a bus can support and on the
distance between those taps.
▪ Advantages:
‣ Ease of installation : Backbone cable can be laid along the most path, then
connected to the nodes and drop lines.
‣ Cable required is the least compared to mesh/star topologies.
‣ Redundancy is eliminated : Only the backbone cable stretches through the
entire facility.
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▪ Disadvantages:
‣ Signal reflection at the taps can cause degradation in quality
‣ A fault/break in the cable stops all transmission.
‣ There is a limit on
- Cable length
- Number of nodes that can be connected.
‣ Security is very low because all the devices receive the data sent from the
source.
▪ Example
▪ It is used to implement the basic Ethernet network.
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4. 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 incorporates a repeater.
▪ When a device receives a signal intended for another device, its repeater
regenerates the bits and passes them along
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▪ Advantages

‣ Easy installation and reconfiguration. To add/delete a device, requires changing only 2 connections

‣ Fault isolation is simplified.

‣ If one device does not receive a signal within a specified period, it can issue an alarm. The alarm
alerts the network-operator to the problem and its location

‣ : Because all the traffic flows in only one direction.

▪ Disadvantages
▪ Example: Used in industrial control systems,
‣ Congestion reduced
metropolitan area networks, and office networks
‣ Unidirectional traffic

‣ A fault in the ring/device stops all transmission.

- The above 2 drawbacks can be overcome by using dual ring.

‣ There is a limit on - Cable length - Number of nodes that can be connected.

‣ Slower: Each data must pass through all the devices between source and destination.
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Hybrid Topology
▪ Example: having a main star topology with each branch connecting several stations in a bus
topology
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Categories of Networks
▪ Network Category depends on its size

1.Local Area Networks (LANs)

2.Wide Area Networks (WANs)

3.Metropolitan Area Networks (MANs)

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1. Local Area Networks (LANs)

▪ It is privately owned and links the devices in a single office, building,
or campus.

▪ LAN can be as simple as two PCs and a printer in someone's home office. Its
size is limited to a few kilometers.

▪ LANs are designed to allow resources to be shared between personal

computers or workstations.

▪ The resources to be shared can include hardware (e.g., a printer),software

(e.g., an application program).
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Fig: An isolated LAN connecting 12 computers to a hub in a closet

▪ Advantages:
‣ Resource Sharing: Computer resources like printers and hard disks can be shared by all
devices on the network.
‣ Expansion: Nowadays, LANs are connected to WANs to create communication at a wider
level.
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▪ Disadvantages
‣ Limited distance: Local area networks are used only in buildings or apartment
complexes it cannot be occupied in bigger areas.
‣ Installing LAN is expensive: It is expensive to establish a LAN. Here specialized
software is essential to install a server. Communication hardware such as hubs,
switches, routers, and cables are expensive to buy.
‣ Limited scalability: LANs are limited in terms of the number of devices that can be
connected to them. As the number of devices increases, the network can become
slow and congested.
‣ Single point of failure: LANs typically have a single point of failure, such as a
central server. If this server fails, the entire network can go down.
‣ Maintenance and management: LANs require regular maintenance and
management to ensure optimal performance. This can be time-consuming and costly.
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2. Wide Area Networks (WAN)

▪ It provides long-distance transmission of data, image, audio, and video

information over large geographic areas that comprise a country, a continent or
even the whole world.

▪ Two types of WAN:

‣ Point to Point WAN : A point-to-point WAN is a network that connects two

communicating devices through a transmission media (cable or air).
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‣ The switched WAN : A switched WAN is a network with more than two ends.
switched WAN is a combination of several point-to-point WANs that are
connected by switches.
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3. Metropolitan Area Networks (MAN)

▪ A metropolitan area network (MAN) is a computer network that connects
computers within a metropolitan area, which could be a single large
city, multiple cities and towns, or any given large area with multiple
buildings.
▪ A MAN is larger than a local area network (LAN) but smaller than a wide
area network (WAN). It is commonly used on large companies or school
campuses with multiple buildings.
▪ It serves as a high-speed network to permit the sharing of regional
resources.
▪ The most common examples of MAN are cable TV networks and
telephone company networks.
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Internetwork
▪ A network of networks is called an internet. ( inter-network)
▪ As an example, assume that an organization has two offices, one on the
east coast and the other on the west coast.
▪ Each office has a LAN that allows all employees in the office to
communicate with each other.
▪ To make the communication between employees at different offices
possible, the management leases a point-to-point dedicated WAN from a
service provider, such as a telephone company, and connects the two
LANs.
▪ Now the company has an internetwork, or a private internet.
Communication between offices is now possible.
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▪ When a host in the west coast office sends a message to another host in the
same office, the router blocks the message, but the switch directs the message
to the destination.
▪ On the other hand, when a host on the west coast sends a message to a host on
the east coast, router R1 routes the packet to router R2, and the packet reaches
the destination.
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Fig: A heterogeneous network made of four WANs and three LANs

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Switching
▪ An internet is a switched network in which a switch connects at least two
links
. together.
▪ A switch needs to forward data from a network to another network
when required.
▪ The two most common types of switched networks are
1. circuit-switched
2. packet-switched networks.
• A switch is a hardware component in network infrastructure that
performs the switching process.
• The switch connects network devices, such as computers and servers, to
one another.
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• A switch enables multiple devices to share a network while preventing each

device's traffic from interfering with other devices' traffic.
• The switch acts as a traffic cop at a busy intersection.
• When a data packet arrives at one of its ports, the switch determines which
direction the packet is headed.
• It then forwards the packet through the correct port for its destination.
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1. Circuit-Switched Network
▪ In a circuit-switched network, a dedicated connection, called a circuit, is always
available
. between the two end systems; the switch can only make it active or
inactive.

▪In Figure, the four telephones at each side are connected to a switch. The switch
connects a telephone set at one side to a telephone set at the other side.
▪The thick line connecting two switches is a high-capacity communication line
that can handle four voice communications at the same time; the capacity can be
shared between all pairs of telephone sets.
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2. Packet-Switched Network
▪ In a computer network, the
communication between the
two ends is done in blocks
of data called packets.
▪A router in a packet-switched network has a queue that can store and forward the
packet.
▪Now assume that the capacity of the thick line is only twice the capacity of the
data line connecting the computers to the routers.
▪If only two computers (one at each site) need to communicate with each other, there
is no waiting for the packets.
▪However, 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.
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The Internet
▪ Internet is composed of thousands of interconnected networks.

Fig: The Internet today

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▪ At the top level, the backbones are large networks owned by some communication
companies such as Sprint, Verizon (MCI), AT&T, and NTT.

▪ The backbone networks are connected through some complex switching systems, called
peering points.

▪ At the second level, there are smaller networks, called 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 are networks at the edge of the Internet that 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.
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Accessing the Internet

1. Using Telephone Networks
▪ Dial up Service : To the telephone line add a modem that converts data to voice. But it is
very slow when line used for internet connection.
▪ DSL Service : Telephone companies have upgraded their telephone lines to provide higher
speed internet services .
2. Using Cable Networks
▪ The cable companies have been upgrading their cable networks to provide internet
connection. But speed varies depending on the number of neighbors that use the same
cable.
3. Using Wireless Networks
▪ A household or small business can be connected to the internet through a wireless LAN.
4. Direct Connection to the internet
▪ A large organization can become a local ISP and be connected to internet.
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Protocol Layering
Network Models
▪ A protocol defines the rules that both the sender and receiver and all intermediate devices need to follow
to be able to communicate effectively.
▪ When communication is
‣ Simple -only one simple protocol.
‣ Complex- need to divide the task b/w different layers.
‣ need a protocol at each layer, or protocol layering.
▪ Elements of a Protocol
‣ Syntax
• Structure or format of the data
• Indicates how to read the bits - field delineation
‣ Semantics
• Interprets the meaning of the bits
• Knows which fields define what action
‣ Timing
• When data should be sent and what
• Speed at which data should be sent or speed at which it is being received.
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Scenarios
First Scenario
‣ Communication is so simple that it 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

Fig: single layer protocol

➢ Even in this simple scenario, we can see that a set of rules needs to be followed.
1. Maria and Ann know that they should greet each other when they meet.

2. They know that they should confine their vocabulary to the level of their friendship.
3. Each party knows that she should refrain from speaking when the other party is
speaking.
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Second Scenario
▪ Assume that Ann is offered a higher-level position in her company, but needs to move to
another branch located in a city very far from Maria.
▪ The two friends still want to continue their communication and exchange ideas because
they have come up with an innovative project to start a new business when they both
retire.

▪ They decide to continue their conversation using regular mail through the post office.

▪ They do not want their ideas to be revealed by other people if the letters are

intercepted. They agree on an encryption/decryption technique

▪ Now we can say that the communication between Maria and Ann takes place in
three layers
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Fig: A three-layer protocol

▪ Let us assume that Maria sends the first letter to Ann.

At Maria side:

‣ Maria talks to the machine at the third layer as though the machine is Ann and is
listening to her.

‣ The third layer machine listens to what Maria says and creates the plaintext which is
passedto the second layer machine.
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‣ The second layer machine takes the plaintext, encrypts it, and creates the
ciphertext, which is passed to the first layer machine.

‣ The first layer machine, presumably a robot, takes the ciphertext , puts it in an
envelope, adds the sender and receiver addresses, and mails it.

At Ann’s side

‣ The first layer machine picks up the letter from Ann’s mail box,
recognizing the letter from Maria by the sender address.
‣ The machine takes out the ciphertext from the envelope and delivers it to
the second layer machine.

‣ The second layer machine decrypts the message, creates the plaintext and passes
the plaintext to the third-layer machine.

‣ The third layer machine takes the plaintext and reads it as though Maria is
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‣ Protocol layering enables us to divide a complex task into several smaller and
simpler tasks.
‣ For example, in Figure 2.2, we could have used only one machine to do the job
of all three machines.
‣ However, if Maria and Ann decide that the encryption/ decryption done by the
machine is not enough to protect their secrecy, they would have to change the
whole machine.
‣ In the present situation, they need to change only the second layer machine; the
other two can remain the same. This is referred to as modularity.

‣ Modularity in this case means independent layers.

‣ A layer (module) can be defined as a black box with inputs and outputs,
without concern about how inputs are changed to outputs
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Advantages of protocol layering

▪ Allows to separate the services from the implementation.

‣ A layer needs to be able to receive a set of services from the lower layer and to
give the services to the upper layer; we don’t care about how the layer is
implemented.

‣ For example, Maria may decide not to buy the machine (robot) for the first layer; she
can do the job herself. As long as Maria can do the tasks provided by the first layer, in
both directions, the communication system works.

▪ Reduces the complexity at the intermediate system

‣ Communication does not always use only two end systems; there are
intermediate systems that need only some layers, but not all layers.

‣ If we did not use protocol layering, we would have to make each intermediate
system as complex as the end systems, which makes the expensive.
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Principles of Protocol Layering

First Principle
▪ The first principle dictates that if we want bidirectional communication, we need to
make each layer so that it 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 that we need to follow in protocol layering is that the two
objects under each layer 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.
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Logical Connections

▪ Two protocols at the same layer can have a logical Connection

▪ This means that we have layer-to-layer communication.

▪ Maria and Ann can think that there is a logical (imaginary) connection at each layer

through which they can send the object created from that layer.
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TCP/IP PROTOCOL SUITE

▪ TCP/IP (Transmission Control Protocol/Internet Protocol) is a

protocol-suite used in the Internet today.
▪ Protocol-suite refers a set of protocols organized in different
layers.
▪ It is a hierarchical protocol made up of interactive modules,
each of which provides a specific functionality.
▪ The term hierarchical means that each upper level protocol is
supported by the services provided by one or more lower level
protocols.
▪ TCP/IP is thought of as a five-layer model.
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Fig: Layers in TCP/IP protocol suite

Layered Architecture
▪ To show how the layers in the TCP/IP protocol suite are involved in
communication between two hosts, we assume that we want to use the suite in a
small internet made up of three LANs (links), each with a link- layer switch.

▪ We also assume that the links are connected by one router

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▪ As the figure shows, we have five

Fig: Communication through an internet communicating devices in this
communication:
1. source host(computer A)
2. The link-layer switch in link 1
3. The router
▪Let us assume that computer A 4. The link-layer switch in link 2
5. destination host (computer B).
communicates with computer B.
 Department of Computer Science & Engineering

▪ Each device is involved with a set of layers depending on the role of the

device in the internet.

▪ The two hosts are involved in all five layers.

▪ The source host needs to create a message in the application layer and send

it down the layers so that it is physically sent to the destination host.

▪ The destination host needs to receive the communication at the physical

layer and then deliver it through the other layers to the application layer.

▪ The router is involved in only three layers; link-layer switch in a link is

involved only in two layers, data-link and physical.

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Layers in the TCP/IP Protocol Suite

▪ To better understand the duties of each layer, we need to think about the
logical connections between layers.

Fig: Logical connections between layers of the TCP/IP protocol suite

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▪ The duty of the application, transport, and network layers is end-to- end.

▪ The duty of the data-link and physical layers is hop-to-hop, in which a hop is a
host or router.

▪ The domain of duty of the top three layers is the internet, and the domain of duty
of the two lower layers is the link.

▪ Another way of thinking of the logical connections is to think about the data unit
created from each layer.

▪ In the top three layers, the data unit (packets) should not be changed by any
router or link-layer switch.

▪ In the bottom two layers, the packet created by the host is changed only by the
routers, not by the link-layer switches.
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Fig: Identical objects in the TCP/IP protocol suite

▪ Figure 2.7 shows the second principle the identical objects at each layer related to
each device.
▪ Although the logical connection at the network layer is between the two hosts, we can
only say that identical objects exist between two hops in this case because a router may
fragment the packet at the network layer and send more packets than Received.
▪ The link between two hops does not change the object.
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Description of each Layer

Physical Layer
▪ The lowest level in the TCP/IP protocol suite, responsible for carrying
individual bits in a frame across the link

▪ Two devices are connected by a transmission medium (cable or air).

▪ The transmission medium does not carry bits; it carries electrical or optical
signals.

▪ So the bits received in a frame from the data-link layer are transformed and sent
through the transmission media.
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Data-link Layer
▪ Responsible for taking the datagram and moving it across the link.

▪ The routers are responsible for choosing the best links. When the next link to
travel is determined by the router, the data-link layer is responsible for taking the
datagram and moving it across the link.
▪ The link can be a wired LAN with a link-layer switch, a wireless LAN, a wired
WAN, or a wireless WAN.
▪ TCP/IP does not define any specific protocol for the data-link layer. It
supports all the standard and proprietary protocols.
▪ The data-link layer takes a datagram and encapsulates it in a packet called a
frame.

▪ Each link-layer protocol may provide a different service.

▪ Some link-layer protocols provide complete error detection and correction, some
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Network Layer
▪ Responsible for creating a connection between the source computer and the

destination computer.
▪ Responsible for host-to-host communication and routing the packet through
possible routes.
▪ The network layer in the Internet includes the main protocol, Internet Protocol (IP),
‣ defines the format of the packet, called a datagram at the network layer.
‣ defines the format and the structure of addresses used in this layer.

‣ responsible for routing a packet from its source to its destination, which is
achieved by each router forwarding the datagram to the next router in its path.

▪ IP is a connectionless protocol that provides no flow control, no error control, and

no congestion control services.

▪ This means that if any of theses services is required for an application, the application
should rely only on the transport-layer protocol.
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Network Layer cont.…

▪ The network layer also includes unicast (one-to-one) and multicast (one-to-many)
routing protocols.

▪ A routing protocol does not take part in but it creates forwarding tables for routers to help
them in the routing process.

▪ The network layer also has some auxiliary protocols that help IP in its delivery and routing
tasks.

‣ The Internet Control Message Protocol (ICMP)- helps IP to report some problems
when routing a packet.

‣ The Internet Group Management Protocol (IGMP)-helps IP in multitasking.

‣ The Dynamic Host Configuration Protocol (DHCP)-helps IP to get the network-layer

address for a host.

‣ The Address Resolution Protocol (ARP)-IP to find the link-layer address of a host or a
router when its network-layer address is given.
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Transport Layer
▪ Responsible for giving services to the application layer: to get a message from an

application program running on the source host encapsulates it in a transport layer

packet (called a segment or a user datagram and deliver it to the corresponding
application program on the destination host through the logical connection

▪ The logical connection at the transport layer is also end-to-end.

▪ There are a few transport-layer protocols in the Internet, each designed for some

specific task.

▪ Transmission Control Protocol (TCP)

‣ connection-oriented protocol that first establishes a logical connection between

transport layers at two hosts before transferring data.

‣ creates a logical pipe between two TCPs for transferring a stream of bytes.

‣ provides flow control, error control and congestion

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▪ User Datagram Protocol (UDP)

‣ Connectionless protocol that transmits user datagrams without first creating a

logical connection
‣ each user datagram is an independent entity without being related to the previous
or the next one

‣ does not provide flow, error, or congestion control

‣ simplicity, which means small overhead, is attractive to an application program that
needs to send short messages and cannot afford the retransmission of the packets
involved in TCP, when a packet is corrupted or lost.

▪ Stream Control Transmission Protocol (SCTP)

‣ designed to respond to new applications that are emerging in the multimedia.

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Application Layer
▪ The logical connection between the two application layers is end to- end.

▪ Communication at the application layer is between two processes (two

programs running at this layer).

▪ To communicate, a process sends a request to the other process and receives a

response.

▪ Process-to-process communication is the duty of the application layer.

▪ The application layer in the Internet includes many predefined protocols, but a

user can also create a pair of processes to be run at the two hosts.

▪ The Hypertext Transfer Protocol (HTTP) : accessing the World Wide Web

(WWW).
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▪ The Simple Mail Transfer Protocol (SMTP): e-mail service.

▪ The File Transfer Protocol (FTP): File Transfer.

▪ The Terminal Network (TELNET) and Secure Shell (SSH): Remote login

▪ The Simple Network Management Internet at global and local levels

▪ The Domain Name System (DNS): to find the network-layer address of a

computer

▪ The Internet Group Management Protocol (IGMP): to collect membership in a

group.
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Encapsulation and Decapsulation

Encapsulation at the Source Host

1. At the application layer, the data to be exchanged is referred to as a message.

• It does not contain header or trailer and message is passed to transport layer.

Fig: Encapsulation/Decapsulation
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2. The transport layer takes the message as the payload, the load that the

transport layer should take care of.

• It adds the transport layer header to the payload, which contains the

identifiers of the source and destination application programs plus some

more information that is needed for the end-to end delivery of the

message, such as information needed for flow, error control, or congestion

control.

• The result is the transport-layer packet, which is called the segment(in

TCP) and the user datagram (in UDP).

Department
• The transport layer ofthe
then passes CSE- DatatoScience
packet the network layer.
 Department of Computer Science & Engineering

3. The network layer takes the transport-layer packet as data or

payload and adds its own header to the payload.

• The header contains the addresses of the source and destination

hosts and some more information used for error checking of the

header, fragmentation information, and so on.

• The result is the network- layer packet, called a datagram.

• The network layer then passes the packet to the data-link layer.

Department of CSE- Data Science

 Department of Computer Science & Engineering

4. The data-link layer takes the network-layer packet as data or

payload and adds its own header, which contains the link-layer

addresses of the host or the next hop (the router).

• The result is the link-layer packet, which is called a frame.

• The frame is passed to the physical layer for transmission.

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Decapsulation and Encapsulation at the Router

▪ At the router, we have both decapsulation and encapsulation because the router is
connected to two or more links.

1. After the set of bits are delivered to the data-link layer, this layer decapsulates the datagram
from the frame and passes it to the network layer.

2. The network layer only inspects the source and destination addresses in the datagram header
and consults its forwarding table to find the next hop to which the datagram is to be
delivered.

• The contents of the datagram should not be changed by the network layer in the router
unless there is a need to fragment the datagram if it is too big to be passed through the next
link.

• The datagram is then passed to the data-link layer of the next link.

2. The data-link layer of the next link encapsulates the datagram in a frame and passes it to
the physical layer for transmission.
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Decapsulation at the Destination Host

▪ At the destination host, each layer only decapsulates the packet received, removes
the payload, and delivers the payload to the next- higher layer protocol until the
message reaches the application layer.

▪ decapsulation in the host involves error checking.

Addressing

▪ we have logical communication between pairs of layers in this model.

▪ Any communication that involves two parties needs two addresses: source
address and destination address.

▪ Although it looks as if we need five pairs of addresses, one pair per layer, we
normally have only four because the physical layer does not need addresses; the
unit of data exchange at the physical layer is a bit, which definitely cannot have an
address.
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Fig: Addressing in
the TCP/IP protocol
suite

▪ There is a relationship between the layer, the address used in that layer, and the
packet name at that layer.
▪ At the application layer, we normally use names to define the site that provides
services, such as someorg.com,or the e-mail address, such as
somebody@coldmail.com.
▪ At the transport layer, addresses are called port numbers, and these define the
application-layer programs at the source and destination.
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▪ Port numbers are local addresses that distinguish between several programs
running at the same time.

▪ At the network-layer, the addresses are global, with the whole Internet as the
scope.

▪ A network-layer address uniquely defines the connection of a device to the

Internet.

▪ The link-layer addresses, sometimes called MAC addresses, are locally defined
addresses, each of which defines a specific host or router in a network (LAN or
WAN).
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Fig: Multiplexing
and demultiplexing

Multiplexing and Demultiplexing

▪ Since the TCP/IP protocol suite uses several protocols at some layers, we can say that
we have multiplexing at the source and demultiplexing at the destination.

▪ Multiplexing in this case means that a protocol at a layer can encapsulate a packet
from several next-higher layer protocols (one at a time)

▪ Demultiplexing means that a protocol can decapsulate and deliver a packet to

several next-higher layer protocols (one at a time).

▪ To be able to multiplex and demultiplex, a protocol needs to have a field in its header
to identify to which protocol the encapsulated packets belong.
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▪ At the transport layer, either UDP or TCP can accept a

message from several application-layer protocols.

▪ At the network layer, IP can accept a segment from TCP or a

user datagram from UDP.

▪ IP can also accept a packet from other protocols such as ICMP,

IGMP, and so on.

▪ At the data-link layer, a frame may carry the payload coming

from IP or other protocols such as ARP
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The OSI Model

▪ The International Organization for Standardization (ISO) is a multinational
body dedicated to worldwide agreement on international standards.

▪ An ISO standard that covers all aspects of network communications is the Open
Systems Interconnection (OSI) model. It was first introduced in the late 1970s.

▪ ISO is the organization; OSI is the model.

▪ An open system is a set of protocols that allows any two different systems to
communicate regardless of their underlying architecture.

▪ The purpose of the OSI model is to show how to facilitate communication

between different systems without requiring changes to the logic of the
underlying hardware and software
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▪ The OSI model is not a protocol; it is a model for understanding and designing a
network architecture that is flexible, robust, and interoperable.

▪ The OSI model was intended to be the basis for the creation of the protocols in the
OSI stack.

▪ The OSI model is a layered framework for the design of network systems that
allows communication between all types of computer systems.

▪ It consists of seven separate but related layers, each of which defines a part of the
process of moving information across a network

Fig: The OSI Model

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OSI versus TCP/IP

Fig: TCP/IP and OSI model

▪ Two layers, session and presentation are missing from the TCP/IP protocol suite.
▪ The application layer in the suite is usually considered to be the combination of three
layers in the OSI model
▪ TCP/IP has more than one transport-layer protocol. Some of the functionalities of the
session layer are available in some of the transport-layer protocols.
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▪ The application layer is not only one piece of software. Many applications can be
developed at this layer.
▪ If some of the functionalities mentioned in the session and presentation layers are needed
for a particular application, they can be included in the development of that piece of
software.
Lack of OSI Model’s Success
▪ OSI was completed when TCP/IP was fully in place and a lot of time and money had
been spent on the suite; changing it would cost a lot.
▪ Some layers in the OSI model were never fully defined.
▪ For example, although the services provided by the presentation and the session
layers were listed in the document, actual protocols for these two layers were not fully
defined, nor were they fully described, and the corresponding software was not fully
developed.
▪ when OSI was implemented by an organization in a different application, it did not
show a high enough level of performance to entice the Internet authority to switch
from the TCP/IP protocol suite to the OSI model.
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Introduction to Physical Layer

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Introduction to Physical Layer

▪ One of the major functions of the physical layer is to move data in the form of
electromagnetic signals across a transmission medium
▪ Transmission medium can be broadly defined as anything that can carry
information from sender to receiver.
▪ We use different types of cables or waves to transmit data. Data is transmitted
normally through electrical or electromagnetic signals.
▪ Transmission media are located below the physical layer.
▪ Computers use signals to represent data. Signals are transmitted in form of
electromagnetic energy.

Fig: Transmission medium and

physical layer
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Classification of Transmission media

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Guided Media
▪ Guided media, which are those that provide a conduit from one device to another
include twisted pair cable, coaxial cable, and fiber optic cable.
Twisted-pair cable
▪ A twisted pair consists of two conductors (normally copper), each with its own
plastic insulation, twisted together

Fig: Twisted-pair cable

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Unshielded versus Shielded Twisted–pair cable

UTP- Unshielded Twisted–pair cable
▪ Pair of unshielded wires wound around each other
▪ Easiest to install
▪ Applications
‣ Telephone subscribers connect to the central telephone office
‣ DSL lines
‣ LAN – 10Mbps or 100Mbps
STP - Shielded Twisted–pair cable
▪ Pair of wires wound around each other placed inside a protective foil wrap
▪ Metal braid or sheath foil that reduces interference
Harder to handle (thick, heavy)
▪ Applications
‣ STP is used in IBM token ring networks.
‣ Higher transmission rates over longer distances.
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Categories
▪ The Electronic Industries Association (EIA) has developed
standards to classify unshielded twisted-pair cable into seven
categories.
▪ Categories are determined by cable quality, with 1 as the lowest
and 7 as the highest. Each EIA category is suitable for specific
uses.
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 Department of Computer Science & Engineering

UTP connector( Registered Jack:RJ45)

▪ RJ45 is a keyed connector , meaning the connector can be inserted in only one way

Applications of Twisted pair

Advantages of Twisted pair:
Used in
‣ Cheap
1. Telephone lines to provide voice and data channels
‣ Easy to work with
(local loop)
Disadvantages of Twisted pair:
2. The DSL lines that are used by the telephone
‣ Low data rate
companies to provide high-data-rate
‣ Short range
connections
3. Local area networks, such as 10-base-Tand 100base-T
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Co-axial Cable
▪ Co-axial cable carries signal of higher frequency ranges than twisted pair cable
▪ Inner conductor is a solid wire
▪ Outer conductor serves as a shield against noise and a second conductor
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Categories of coaxial cables

▪ Coaxial cables are categorized by Radio Government (RG) ratings.

▪ Each RG number denotes a unique set of physical specifications, including the wire gauge
of the inner conductor, the thickness and type of the inner insulator, the construction of the
shield, and the size and type of the outer casing.

▪ Each cable defined by an RG rating is adapted for a specialized function, as shown in Table
7.2.
 Department of Computer Science & Engineering

Coaxial Cable Connectors

BNC Connectors – Bayonet Neil Concelman
▪ To connect coaxial cable to devices we need coaxial connectors
▪ BNC Connector is used at the end of the cable to a device
‣ Example: TV set conenction
▪ BNC T connector used to Ethernet networks to branch out
connection to computer or other devices
▪ BNC terminator is used at the end of the cable to prevent the
reflection of the signal
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Applications of coaxial cable

1. Analog telephone network where a single cable could carry 10,000 voice
signals. Later it was used in Digital telephone networks where cable can carry
600Mbps
2. Cable TV network: hybrid network use coaxial cable only at the network
boundaries , near the consumer. Cable TV use RG-59
Advantages
‣ Easy to wire
‣ Easy to expand
‣ Moderate level of Electro Magnetic Interference
Disadvantage
‣ Single cable failure can take down an entire network
‣ Cost of installation of a coaxial cable is high due to its thickness and stiffness
‣ Cost of maintenance is also high
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Fiber-Optic Cable
▪ A fiber optic cable is made of glass or plastic and transmit signals in the form of light.
▪ Light travels in a straight line
▪ If light goes from one substance to another then the ray of light changes direction
▪ Ray of light changes direction when goes from more dense to a less dense substance

Bending of light ray

▪ Angle of Incidence (I): the angle the ray makes with the line perpendicular to the
interface between the two substances

▪ Critical Angle: the angle of incidence which provides an angle of refraction of 90-
degrees.
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Optical fiber

Jacket
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▪ An optical fiber cable has a cylindrical shape and consists of three concentric sections: the
core, the cladding, and the jacket(outer part of the cable).
▪ Uses reflection to guide light through a channel
▪ Core is of glass or plastic surrounded by Cladding
▪ Cladding is of less dense glass or plastic
Fiber Construction
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Propagation Modes
• When signal goes from one point to another they use propagation modes.
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▪ In multimode step-index fiber, the density of the core remains constant from the center
to the edges. A beam of light moves through this constant density in a straight line until it
reaches the interface of the core and the cladding.The term step-index refers to the
suddenness of this change, which contributes to the distortion of the signal as it passes
through the fiber.
▪ A second type of fiber, called multimode graded-index fiber, decreases this distortion of
the signal through the cable. The word index here refers to the index of refraction. A
graded index fiber, therefore, is one with varying densities. Density is highest at the center
of the core and decreases gradually to its lowest at the edge.

▪ 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.
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Applications of Fiber-optic cable

Used in
1. Cable TV network: hybrid network use a combination of optical fiber and coax cable.
Optical provides the backbone while coaxial cable provide the connation to the user.
2. Local area networks such as 100base-FX(fast Ethernet) and 1000base-XLANs.
3. Backbone networks because its wide bandwidth
Advantages
‣ Higher Bandwidth
‣ Less signal attenuation
‣ Immunity to electromagnetic interference (noise)
‣ Resistance to corrosive materials. Glass is more resistance to corrosive material
‣ Light weight. Fiber cables are much lighter than copper cables
‣ Greater immunity to tapping: copper cables create antenna effects that can easily
tapped
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Disadvantages
‣ Installation and maintenance. It’s a new technology. Its installation and maintenance
require expertise that is not yet available every where
‣ Unidirectional light propagation. If we need bidirectional , two fibers are needed.
‣ Cost. The cable and the interfaces are more expensive than those of other guided
media. If the demand of BW is not high , often use of optical fiber can not be justified
 Department of Computer Science & Engineering

Unguided Media: Wireless

▪ Unguided media transport electromagnetic waves without using a physical conductor it is
known as wireless communication.
▪ Signals broadcast through free space and available to capable receiver
▪ Electro magnetic spectrum for wireless communication:

Propagation methods
▪ Unguided signals travels from the source to destination in several ways it is known as
propagation.
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They are three types:

1. Ground propagation
2. Sky propagation
3. Line-of-Sight Propagation
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Ground propagation:
▪ Radio waves travel through the
lowest portion of the atmosphere
▪ Touching the earth.
Sky propagation:
▪ Radio waves radiate to the
ionosphere then they are reflected
back to earth.
▪ Line-of-Sight Propagation:
In straight lines directly from
antenna to antenna.
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 Department of Computer Science & Engineering

Radio Waves
▪ Radio waves, for the most part, are omnidirectional. When an antenna transmits

▪ radio waves, they are propagated in all directions. Frequencies between 3 KHz and 1
GHz.
▪ Used for multicasts(multiple way) communications, such as radio and television, and
paging system.
▪ Radio waves can penetrate buildings easily, so that widely use for indoors & outdoors
communication.

▪ Radio waves, particularly those waves that propagate in the sky mode, can travel long
distances. This makes radio waves a good candidate for long-distance broadcasting such
as AM radio.
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Omnidirectional Antenna
▪ Radio waves use omnidirectional antennas that
send out signals in all directions.
▪ Based on the wavelength, strength, and the purpose
of transmission, we can have several
▪ types of antennas.
Applications
▪ The omnidirectional characteristics of radio waves
make them useful for multicasting, in which there is
one sender but many receivers.
Fig: Omnidirectional antenna
▪ AM and FM radio, television, maritime radio,
cordless phones, and paging are examples of
multicasting.
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Microwaves
▪ Electromagnetic waves having frequencies between 1 and 300 GHz are called
microwaves.
▪ Microwaves are unidirectional.
▪ There are two types of micro waves data communication system : terrestrial and
satellite
▪ Micro waves are widely used for one to one communication between sender and
receiver, example: cellular phone, satellite networks and in wireless LANs(wifi),
WiMAX, GPS
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▪ The following describes some characteristics of microwave propagation:

1. Microwave propagation is line-of-sight. Since the towers with the mounted antennas
need to be in direct sight of each other, towers that are far apart need to be very tall. The
curvature of the earth as well as other blocking obstacles do not allow two short towers
to communicate by using microwaves. Repeaters are often needed for long distance
communication.
2. Very high-frequency microwaves cannot penetrate walls. This characteristic can be a
disadvantage if receivers are inside buildings.
3. The microwave band is relatively wide, almost 299 GHz. Therefore wider subbands can
be assigned, and a high data rate is possible.
4. Use of certain portions of the band requires permission from authorities.
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Unidirectional Antenna
▪ Microwaves need unidirectional antennas that send out signals in one direction.
▪ Two types of antennas are used for microwave communications: the parabolic dish and the
Horn

▪ A parabolic dish antenna is based on the geometry of a parabola: Every line parallel to
the line of symmetry (line of sight) reflects off the curve at angles such that all the lines
intersect in a common point called the focus.
▪ A horn antenna looks like a gigantic scoop. Outgoing transmissions are broadcast up a
stem (resembling a handle) and deflected outward in a series of narrow parallel beams by
the curved head.
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Applications
▪ Microwaves are used for unicast communication such as cellular telephones,
satellite networks, and wireless LANs.
Infrared
▪ Infrared waves, with frequencies from 300 GHz to 400 THz (wavelengths from 1
mmto 770 nm), can be used for short-range communication.

▪ Infrared waves, having high frequencies, cannot penetrate walls.

▪ Example: Remote control, File sharing between two phones, Communication between a
PC and peripheral device,
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Applications
▪ Infrared signals can be used for short-range communication in a closed area using line-
of-sight propagation
▪ The Infrared Data Association (IrDA), an association for sponsoring the use of infrared
waves, has established standards for using these signals for communication between
▪ devices such as keyboards, mice, PCs, and printers.
▪ For example, some manufacturers provide a special port called the IrDA port that allows
a wireless keyboard to communicate with a PC.
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Switching
▪ how to connect Whenever we have multiple devices?
– make a point-to-point connection between each pair of devices (a mesh
topology) ?
– make a connection between a central device and every other device (a star
topology)?
▪ impractical and wasteful when applied to very large networks.
– The number and length of the links require too much infrastructure to be cost-
efficient
– the majority of those links would be idle most of the time.
▪ A better solution is switching.
▪ A switched network consists of a series of interlinked nodes, called switches
creating temporary connections between two or more devices linked to the
switch.
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Figure : Switched network

▪ The end systems (communicating devices) are labeled A, B, C, D, and so on, and the
switches are labeled I, II, III, IV, and V. Each switch is connected to multiple links.
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Packet switching
▪ In a packet-switched network, there is no resource reservation;
resources are allocated on demand.
▪ The size of the packet is determined by the network and the governing
protocol.
▪ When a switch receives a packet, no matter what the source or
destination is, the packet must wait if there are other packets being
processed.
▪ As with other systems in our daily life, this lack of reservation may
create delay. For example, if we do not have a reservation at a restaurant,
we might have to wait.
▪ Two types of packet-switched networks: datagram networks and virtual
circuit networks.
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Datagram Networks
▪ Each packet is treated independently of all others.
▪ Even if a packet is part of a multipacket transmission, the network treats it as
though it existed alone.
▪ Packets in this approach are referred to as datagrams.
▪ Datagram switching is normally done at the network layer.

3 1
4 3 2 1
4
1

2 3
1
4
2 2 3 4 1

Figure : A Datagram network with four switches (routers)

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▪ The datagram networks are sometimes referred to as connectionless networks.

▪ The term connectionless here means that the switch (packet switch) does not keep
information about the connection state.

▪ There are no setup or teardown phases.

▪ Each packet is treated the same by a switch regardless of its source or

destination.

▪ In this type of network, each switch (or packet switch) has a routing table which
is based on the destination address.

▪ The routing tables are dynamic and are updated periodically.

▪ The destination addresses and the corresponding forwarding output ports are
recorded in the tables.
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▪ Every packet in a datagram network carries a header that contains, among other
information, the destination address of the packet.
▪ When the switch receives the packet, this destination address is examined; the
routing table is consulted to find the corresponding port through which the
packet should be forwarded.
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▪ The efficiency of a datagram network is better than that of a circuit-switched

network; resources are allocated only when there are packets to be transferred.

▪ If a source sends a packet and there is a delay of a few minutes before another
packet can be sent, the resources can be reallocated during these minutes for
other packets from other sources.
Delay
▪ There may be greater delay in a datagram network than in a virtual-circuit
network.

▪ Although there are no setup and teardown phases, each packet may
experience a wait at a switch before it is forwarded.

▪ In addition, since not all packets in a message necessarily travel through the
same switches, the delay is not uniform for the packets of a message.
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Figure Delays in a datagram network

➢ Delay will be greater .

▪ Here there are no setup and teardown phases, each packet may
experience a wait at a switch before it is forwarded.
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Virtual-Circuit Networks
▪ Virtual – circuit network is a category of packet switching network,
where a virtual path is established between the source and the
destination systems for data communication to occur.
▪ This path appears to the user as if it is a dedicated physical path, but
actually is a logical circuit allocated from a managed pool of circuit
resources as per traffic requirements.
▪ The network resources forming parts of this path can be shared by
other communications, however, is not visible to this user.
▪ As in a circuit-switched network, there are setup and teardown phases
in addition to the data transfer phase.
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Virtual-Circuit Networks
– Resources can be allocated during the setup phase, as in a circuit-
switched network, or on demand, as in a datagram network.
– As in a datagram network, data are packetized and each packet
carries an address in the header.
– all packets follow the same path established during the connection.
– A virtual-circuit network is normally implemented in the data-link
layer, while a circuit-switched network is implemented in the
physical layer and a datagram network in the network layer.
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Figure : Virtual-circuit network

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➢ Addressing :
▪ Global Address : A source or a destination needs to have a global
address—an address that can be unique in the scope of the
network .
▪ Virtual-Circuit Identifier(Local Address):
• The identifier that is actually used for data transfer is called the
virtual-circuit identifier (VCI) .
• A VCI is a small number that has only switch scope; it is used by a
frame between two switches.

Figure : Virtual-circuit identifier

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Data-Transfer Phase
▪ Three phases.
▪ To transfer a frame from a source to its destination, all switches need to have a
table entry for this virtual circuit.
▪ The table, in its simplest form, has four columns. This means that the switch
holds four pieces of information for each virtual circuit that is already set up.
▪ The data-transfer phase is active until the source sends all its frames to the
destination.
▪ The procedure at the switch is the same for each frame of a message.
▪ The process creates a virtual circuit, not a real circuit, between the source and
destination.
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Figure : Switch and table for a virtual-circuit network

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Setup Phase
In the setup phase, a switch creates an entry for a virtual circuit.

For example, suppose source A needs to create a virtual circuit to B. Two steps are
required: the setup request and the acknowledgment.
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Acknowledgment
A special frame, called the acknowledgment frame, completes the entries in the switching
tables.
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Teardown Phase

• In this phase, source A, after sending all frames to B, sends a special frame
called a teardown request.

• Destination B responds with a teardown confirmation frame.

• All switches delete the corresponding entry from their tables.

Efficiency

• In virtual-circuit switching, all packets belonging to the same source and

destination travel the same path, but the packets may arrive at the destination
with different delays if resource allocation is on demand.

• Resource reservation in a virtual-circuit network can be made during the setup

or can be on demand during the data-transfer phase.

• In the first case, the delay for each packet is the same; in the second case, each
packet may encounter different delays.
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Delay

➢ In a virtual-circuit network, there is a one-time delay for setup and

a one-time delay for teardown.

Total Delay=
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