C o n f i d e n t i a l

Course : BCA

Semester : IV

Subject Code : BC 0048

Subject Name : Computer Networks

Unit number : 4

Unit Title : Physical Layer

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C o n f i d e n t i a l

Unit-4 Physical Layer

Physical Layer

Objectives

After going through the presentation, you should be able to:

•Describe Network topologies

•Discuss switching technologies

•Describe different multiplexing techniques

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C o n f i d e n t i a l

Unit-4 Physical Layer

Lecture outline

•Introduction

•Network Topologies

•Switching

•Multiplexing

•Summary

Physical Layer

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C o n f i d e n t i a l

Unit-4 Physical Layer

Introduction

• Physical layer is the bottommost layer in the OSI/OSO reference model.

• It defines mechanical, electrical & timing interfaces to the network.

• Switching is another important task of physical layer.

• Two switching techniques are used. They are circuit switching & packet

switching.

• Multiplexing is the process in which two or more signals are combined

for transmission over a single communications path.

Multiplexing schemes, like TOM, FDM are developed by telephone

companies to reduce the cost.

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C o n f i d e n t i a l

Unit-4 Physical Layer

Network Topologies

• Topology is a term used to describe the way in which computers are

connected in network.

• The physical topology of a network refers to the configuration of cables,

computers, and other peripherals.

• Network Topologies are logical layouts of the network. The term

"logical" used here marks a great significant.

• That means network topologies depends not on the "physical" layout of

the network. No matter that computer on a network are placed in a

linear format, but if they connected via a hub they are forming a Star

topology, not the Bus Topology.

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C o n f i d e n t i a l

Unit-4 Physical Layer

Network Topologies

Linear Bus Topology :

The type of network topology in which all of the nodes of the network are

connected to a common transmission medium which has exactly two

endpoints is called the 'bus'.

It consists a single main cable connects each node.

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Unit-4 Physical Layer

Network Topologies

Ring Topology :

The type of network topology in which each of the nodes of the network is

connected to two other nodes in the network and with the first and last

nodes being connected to each other, forming a ring.

That is the nodes are connected in a circle using cable segments. Each

node is physically connected only to two others.

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Unit-4 Physical Layer

Network Topologies

Star Topology :

A star topology is designed with each node connected directly to a central

network hub or concentrator.

Data on a star network passes through the hub or concentrator before

continuing to its destination.

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Unit-4 Physical Layer

Network Topologies

Tree Topology :

A tree topology combines characteristics of linear bus and star topologies.

Tree topologies allow for the expansion of an existing network, and enable

schools to configure a network to meet their needs.

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Unit-4 Physical Layer

Network Topologies

Topology Comparisons :

•There are a number of factors to consider in making a choice of

a topology.

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Feature Bus Ring Star

Reliability High Low Low

Complexity Moderate Low Low

Flexibility High Moderate Low

Expandability High Moderate Low

Cost Low Moderate Moderate

C o n f i d e n t i a l

Unit-4 Physical Layer

Switching

What is the purpose of switching ?

Communication is typically achieved by transmitting data from source to

destination through a network of intermediate switching nodes.

The switching nodes are not concerned with the content of data. Rather

their purpose is to provide a switching facility that will move the data

from node to node until it reaches the destination.

There are two types of switching

1. Circuit switching

2. Message switching

3. Packet switching

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Unit-4 Physical Layer

Switching

1. Circuit switching :

A circuit switching network is one that establishes a dedicated circuit (or

channel) between nodes and terminals before the users may

communicate.

Circuit switching is used for ordinary telephone calls.

Communication using circuit switching involves three phases

1. Connection establishment: Before any signal can be transmitted, an

end to end circuit must be established.

2. Data transfer: Information can now be transmitted from source

through the network to the destination using the dedicated path

established.

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Unit-4 Physical Layer

Switching

Circuit switching : ( continued…)

2. Data transfer: Information can now be transmitted from source

through the network to the destination using the dedicated path

established.

3. Termination: After some period of data transfer, the connection is

terminated

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Unit-4 Physical Layer

Switching

2. Message switching :

Message switching was the precursor of packet switching, where

messages were routed in their entirety and one hop at a time.

Hop-by-hop Telex forwarding are examples of message switching systems.

E-mail is another example of a message switching system.

When this form of switching is used, no physical path is established in

advance in between sender and receiver. Instead, when the sender

has a block of data to be sent, it is stored in the first switching office

(i.e. router) then forwarded later at one hop at a time.

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Unit-4 Physical Layer

Switching

Message switching :

As the figure indicates, a complete message is sent from node A to node

B when the link interconnecting them becomes available.

The message is stored at B until the next link becomes available, with

another queuing delay before it can be forwarded. It repeats this

process until it reaches its destination.

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Unit-4 Physical Layer

Switching

3. Packet switching :

Packet switching splits traffic data (for instance, digital representation of

sound, or computer data) into chunks, called packets.

Packet switching is similar to message switching.

Any message exceeding a network-defined maximum length is broken up

into shorter units, known as packets, for transmission.

Packet switching is used to optimize the use of the channel capacity

available in a network, to minimize the transmission latency and to

increase robustness of communication.

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C o n f i d e n t i a l

Unit-4 Physical Layer

Switching

Packet switching : ( continued…)

The most well-known use of packet switching is the Internet.

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Unit-4 Physical Layer

Switching

Comparison of Communication Switching Techniques :

Sl No Circuit Switching Datagram Packet Switching Virtual

Circuit Packet

Switching

1. Dedicated transmission path No dedicated path No dedicated path

2. Continuous transmission of data Transmission of packets Transmission of packets

3. Message are not stored Packets may be stored until Delivered. Packets stored until

delivered

4. Fixed bandwidth Dynamic use of bandwidth Dynamic use of

bandwidth

5. No overhead bits after Overhead bits in each packet Overload bits in each

packet

call setup.

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Unit-4 Physical Layer

Multiplexing

What is Multiplexing ?

Multiplexing is the process in which two or more signals are combined for

transmission over a single communications path.

Multiplexing has made communications very economical by transmitting

thousands of independent sig nals over a single transmission line.

There are three predominant ways to multiplex:

1. Fre quency Division Multiplexing (FDM)

2. Wavelength Division Multiplexing (WDM).

3. Time Division Multiplexing (TDM)

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Unit-4 Physical Layer

Multiplexing

1. Fre quency Division Multiplexing (FDM):

Frequency Division Multiplexing. (FDM) is predominantly used in analog

communica tions.

In the FDM, modulated carrier frequencies are combined for transmission

over a single line by a multiplexer (MUX). There is always some

unused frequency range between channels, known as guard band.

At the receiving end of the communications link, a demultiplexer (DEMUX)

separates the channels by their fre quency and routes them to the

proper end users.

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Unit-4 Physical Layer

Multiplexing

Fig : Transmitting end of an FDM system

FDM was the first multiplexing scheme to enjoy wide-scale network

deployment. FDM is widely used in FM stereo broadcast.

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Unit-4 Physical Layer

Multiplexing

2. Wavelength Division Multiplexing (WDM) :

This is a form of frequency-division multiplexing (FDM) but it is commonly

called wavelength-division multiplexing (WDM). With WDM, the light

streaming through the fiber consists of many colors, or wavelengths,

each carrying a separate channel of data.

Wavelength Division Multiplexing (WDM) is a cost-effective way to

increase the capacity of fiber optic communications.

The key elements of a WDM optical system are tunable semiconductor

lasers, electro-optical modulators, multiplexing components, single-

mode optical Figures and optical amplifiers.

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Unit-4 Physical Layer

Multiplexing

2. Wavelength Division Multiplexing (WDM) : ( Continued..)

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Unit-4 Physical Layer

Multiplexing

3. Time division multiplexing (TDM) :

While FDM has been used to great advantage in increasing system

capacity, the use of TDM offers even greater system improvements.

TDM is protocol insensitive and is capable of combining various protocols

and different types of signals, such as voice and data, onto a single

high-speed transmission link.

It is more efficient than FDM, as there is no need for guard bands.

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Unit-4 Physical Layer

Multiplexing

Time division multiplexing (TDM) : (continued..)

The main disadvantages of TDM are the greater complexity of digital

systems and the greater transmission bandwidth required.

In order to use TDM, the transmission must be digital in nature so an

essential component of TDM is the process of sampling the analog

signal in time.

In order to transmit telephone conversations, speech, which is an analog

signal, is con verted to a digital signal, transmitted, and then

reconverted into analog at the receiving telephone.

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Unit-4 Physical Layer

Summary

Topology is a term used to describe the way in which computers are

connected in network.

The physical topology of a network refers to the configuration of cables,

computers, and other peripherals.

Different network topologies are Bus, Ring, Star and Tree

The purpose of switch is provide a switching facility that will move the

data from node to node until it reaches the destination.

Different switching technologies are circuit, message and packet

switching.

Multiplexing is the process in which two or more signals are combined for

transmission over a single communications path.

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