Unit 2 the Physical Layer

7/30/2019 Unit 2 the Physical Layer

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The Physical Layer

Unit-2


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Transmission Media

Guided Media

Magnetic Media

Twisted Pair

Co-axial CableFiber Optic Cable

Unguided Media

Satellite Transmission

Radio Transmission

Microwave Transmission

Light wave Transmission


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Magnetic Media

Most common way to transport data form one

system to another system using magnetic

tape, floppy disk. These also called as

Secondary storage.

Problems

Unreliability.

Unusually much delay tales place

For large amount of data, cost is also more


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Guided Media

Twisted Pair

Available in 2 forms..

Advantages.

Because of the helical form, the electrical

interference is reduced

Used in Telephony System, but for longer distancerepeaters are needed

Cheap, Flexible easy to install


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Guided Media

Co-Axial Cable

Carries Signal of higher frequency range than

Twisted pair cable

Commonly used for analog transmission

Two Categories

Base Band

Broad Band


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Guided Media

Optical Fiber

A beam of light moving from a rarer medium to a

denser medium it will bent toward the vertical axis.

This is called Refraction.

If the angle of incidence becomes greater than thecritical angle it is known as Reflection.

Optical fibers use Total Internal Reflection to guide

light through a channel.


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Guided Media

Optical Fiber

Optical fiber is a thin flexible medium capable

of conducting optical rays.


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Guided Media

Optical Fiber

Three ways of Propagation

Multimode Step-Index: The received signal has a wider pulse width than the

input signal with decrease in maximum bit rate.

Multimode Graded-Index: The received signal has a narrow pulse width than the

input signal with increase in maximum bit rate.

Single mode step-index: The received signal is ofcomparable width to the

input signal. It can operate at hundreds of megabitsper second.


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Unguided

Satellite Transmission A communication satellite is a microwave relay station.

A communication satellite is used to link 2 or more grounded-

based microwave transmitter or receiver known as earth station

or ground station

A single orbitary satellite will operate on a no.of frequency bands

called transponders or transponder channels. A single satellite many transponders, each covering a particular

band of frequencies

Two Configuration can be for satellite communication

Point-to-Point Communication Multi Point Communication

Satellite used for Communication purpose are GeoStationary.

The Signal power is higher allowing smaller-diameter receiver

called antennas or dishes known as very small aperture or VSAT to

be used.


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Application

1. Television

2. Long-Distance

Telephone

3. Private bussiness

network

Unguided

Satellite Transmission


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It is used to cordless link between fixed-wire

termination point and the distributed

computer

The principle difference between radio and

microwave is that radio is Omni directional

and microwave is directional

Application is Data-Networking Applications

Unguided

Radio Transmission


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These links are widley used to providecommunication links when it is impractical or tooexcess to install physical transmission media.

Light-of-sight microwave communication throughthe earths atmosphere cab be used distance inexceed of 50km

Application Long-haul trunks

Short-haul trunks

Unguided

Microwave Transmission


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Coherent optical signalingusing lasers isunidirectional, so eachbuilding needs its own laserand its own photo detector.This scheme offers veryhigh bandwidth and verylow cost. It is also easy isinstall

Drawback is that laserbeams cannot penetratethrough rain of thick fog

Unguided

Lightwave Transmission


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Circuit Switching

(a) Circuit switching.

(b) Packet switching.


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The Knockout Switch

There is one ATM switch design, that uses outputqueuing, called knockout switch. Each input line isconnected to a bus on which incoming cells arebroadcasted in the cycle they arrive

h h h


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The Batcher-Banyan Switch The problem with the knockout switch is that the number of

crosspoints is quadratic in the number of lines. As with thecrossbar switches for circuit switching, the solution is the space

division switching requiring a multistage switch. This solution iscalled the Batcher-banyan switch.


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Batcher Switch

(a) Cells colliding in a banyan switch.

(b) Collision-free routing through a banyan switch.


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The Batcher-Banyan Switch


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Integrated Service Digital Network

(ISDN)

A good example for circuit-switchnetwork is ISDN It has provided universal end-to-end connected over a digital lines. The

primary goal of ISDN is to integrate voice andnon-voice services.

The Idea behind ISDN is the use of aconceptual pipe called the digital pipe throughwhich bits flow between the customer and theISDN Exchange.


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ISDN Objectives To make telephone system fully digital

To integrate Voice services such as Call transfer and call forward

Display callers information such as ph no, name address.

Instant call to another telephone

Automatic wake-up call services

To Integrate non-voice services

Connecting computer in the world as telephone are

connected Provide telemetry services such as meter reading, smoke

alarm

Provide teletext services (sending and receiving e-mails)

Access to remote db (banking, reservations, etc.,)


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ISDN Services


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ISDN Architecture

There is a n/w terminal device NT1 placed by the carriers at

the customers office connected to the ISDN exchange usingthe twisted pair.

The NT1 has a connector to which a bus cable can be

connected as cable to provide services to ISDN terminals.


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ISDN Architecture

In this system new Device called Private Branch

eXchange(PBX) which can have many conversations

simultaneously.


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ISDN ComponentsReference Points

The U reference point definesthe connection between ISDN

exchange and NT1

The T reference point defines

the connection between

network termination, NT1 andNT2

The S reference point defines

the connection between NT2

and ISDN terminals

The R reference point defines

the connection between non-

ISDN terminal and Terminal

Adapter

ISDN Channels

Bearer channel (B-channel) isdefined a rate of 64kbps. It is

used to carry digital data,

digitized voice in full duplex

mode

Data Channel (D-Channel) canbe 16kbps or 64 kbps rate.

Primarily it is designed to carry

out control signal for the B-

Channel

Hybrid Channel (H-Channel)

are available with data rate of

384kbps and moreThese

channels are mostly suited foe

video tele-conferencing...


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ISDN InterfacesBasic Rate Interface (BRi)

It specifies a digital pipeconsisting of2B+1D andadditional overload of

48kbps. Therefore each BRI

requires a digital pipe of

192kbps

Primary Rate interface (PRI)

It specifies a digital pipewith 23B+1D and In

additional overload of 8

kbps. Therefore PRI requires

a digital pipe of 1.544 Mbps


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Perspective on N-ISDN

N-ISDN was an attempt to replace the analog

telephone system with a digital one N-ISDN basic rate is too low so for home as for

business today.

N-ISDN may be partly saved, but by an

unexpected application: Internet access. Various companies now sell ISDN adapters that

combine the 2B + D channels into a single 144kbps digital channel.

Many Internet providers also support theseadapters. So the people can access Internet overa 144 kbps digital link


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ISDN Layers

Unit 2 the Physical Layer

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