unit 1 physical layer

8/11/2019 Unit 1 Physical Layer

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Physical Layer

Unit 1


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One of the major functions of the physical layer is to movedata in the form of electromagnetic signals across atransmission medium.

Whether you are collecting numerical statistics from anothercomputer, sending animated pictures from a designworkstation, or causing a bell to ring at a distant controlcenter, you are working with the transmission of data acrossnetwork connections.

Generally, the data usable to a person or application are not

in a form that can be transmitted over a network. For example, a photograph must first be changed to a form

that transmission media can accept.

Transmission media work by conducting energy along aphysical path.

To be transmitted, data must be transformed to


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Analog and Digital Data

Data can be analog or digital.

The term analogdata refers to information that is

continuous; digi ta ldata refers to information that has

discrete states.

For example, an analog clock that has hour, minute,

and second hands gives information in a continuous

form; the movements of the hands are continuous.

On the other hand, a digital clock that reports thehours and the minutes will change suddenly from 8:05

to 8:06.


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Analog and Digital Signals Like the data they represent, signals can be either analog

or digital.

An analog signal has infinitely many levels of intensity over

a period of time.

As the wave moves from value A to value B, it passesthrough and includes an infinite number of values along its

path.

A digital signal, on the other hand, can have only a limited

number of defined values. Although each value can be anynumber, it is often as simple as 1 and 0.


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Periodic and Nonperiodic A per iodic signal completes a pattern within a measurable

time frame, called a period, and repeats that pattern over

subsequent identical periods.

The completion of one full pattern is called a cyc le.

A nonper iodic signal changes without exhibiting a patternor cycle that repeats over time. Both analog and digital

signals.

In data communications, we commonly use periodic

analog signals and nonperiodic digital signals.


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PERIODIC ANALOG SIGNALS

The sine wave is the most fundamental form of aperiodic analog signal.

A sine wave can be represented by three parameters:

thepeak amplitude, the frequency, and thephase.

These three parameters fully describe a sine wave.


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Peak Ampl i tude: The peak amplitude of a signal isthe absolute value of its highest intensity, proportional

to the energy it carries.

Figure: Two signals with the same phase and frequency,

but different amplitudes


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Period and Frequency

Periodrefers to the amount of time, in seconds, asignal needs to complete 1 cycle.

Frequencyrefers to the number of periods in I s.

Note that period and frequency are just onecharacteristic defined in two ways. Period is the

inverse of frequency, and frequency is the inverse of

period.

Period is formally expressed in seconds. Frequency is

formally expressed in Hertz (Hz), which is cycle per


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Figure: Two signals with the same amplitude and phase,

but different frequencies


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Table: Units of period and frequency


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Example The power we use at home has a frequency of 60 Hz.

The period of this sine wave can be determined as

follows:

Express a per iod of 100 ms in microseconds.


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The period of a signal is 100 ms. What is its frequency in

kilohertz?

Solution

F irst we change 100 ms to seconds, and then we

calculate the frequency from the period (1 Hz = 103

kHz).


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Frequency is the rate of change with respect totime.

Change in a short span of time means high

frequency. Change over a long span of time means low

frequency.

If a signal does not change at all, its frequency is

zero.

If a signal changes instantaneously, its frequency

is infinite.


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Phase

Phase describes the position of the waveformrelative to time 0.


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Figure: Three sine waves with the same amplitude and frequency,

but different phases


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wavelength

The wavelength is the distance a simple signal cantravel in one period.


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Composite Signals

A single-frequency sine wave is not useful in datacommunications; we need to send a composite

signal, a signal made of many simple sine waves.

According to Fourier analysis, any compositesignal is a combination of simple sine waves with

different frequencies, amplitudes, and phases.

If the composite signal is periodic, the

decomposition gives a series of signals withdiscrete frequencies; if the composite signal is

nonperiodic, the decomposition gives a

combination of sine waves with continuous

frequencies.


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Bandwidth

The range of frequencies contained in a compositesignal is its bandwidth.

The bandwidth is normally a difference between

two numbers. For example, if a composite signalcontains frequencies between 1000 and 5000, its

bandwidth is 5000 - 1000, or 4000


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If a periodic signal is decomposed into five sine waveswith frequencies of 100, 300, 500, 700, and 900 Hz, what

is its bandwidth? Draw the spectrum, assuming all

components have a maximum amplitude of 10 V.

SolutionLetfhbe the highest frequency,flthe lowest frequency, and

Bthe bandwidth. Then

Example

The spectrum has only five spikes, at 100, 300, 500, 700,

and 900 Hz


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DIGITAL SIGNALS

Information can also be represented by a digital signal.

For example, a 1 can be encoded as a positive voltage

and a 0 as zero voltage. A digital signal can have more

than two levels.

In this case, we can send more than 1 bit for each level.


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Figure: Two digital signals: one with two signal levels and the

other with four signal levels


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Bit Rate

Most digital signals are nonperiodic, and thus periodand frequency are not appropriate characteristics.

Another termbit rate(instead of frequency)is used

to describe digital signals.

The bit rate is the number of bi ts sent in 1s,

expressed in bits per seconds (bps).

Example:Assume we need to download text

documents at the rate of 100 pages per minute. What isthe required bit rate of the channel?

Ans.A page is an average of 24 lines with 80

characters in each line. If we assume that one character

requires 8 bits, the bit rate is


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Bit Length

Wavelength is concerned about for an analog signal:

the distance one cycle occupies on the transmission

medium.

Bit length: for digital signal.

The bit length is the dis tance one bi t occup ies on the

transm iss ion medium .


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TRANSMISSION IMPAIRMENT

Signals travel through transmission media, which arenot perfect.

The imperfection causes signal impairment. (harm,

destruction)

This means that the signal at the beginning of the

medium is not the same as the signal at the end of the

medium.

What is sent is n ot w hat is received. Three causesof impairment are attenuation, distortion, and noise


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Attenuation:Attenuation means a loss of energy.

When a signal, simple or composite, travels through a

medium, it loses some of its energy in overcoming the

resistance of the medium.

That is why a wire carrying electric signals gets warm.

Some of the electrical energy in the signal is converted

to heat.

To compensate for this loss, ampl i f iers are used toamplify the signal.


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Figure 3.27: Attenuation and amplification


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Distortion: Distortion means that the signal changes itsform or shape. Distortion can occur in a compos i te

signal made of different frequencies.


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Noise:Noise is another cause of impairment. Several types of noise, such as thermal noise, induced

noise, crosstalk, and impulse noise, may corrupt thesignal.

Thermal noise is the random motion of electrons in a wire,which creates an extra signal not originally sent by thetransmitter.

Induced noise comes from sources such as motors.

Crosstalk is the effect of one wire on the other.


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DATA RATE LIMITS

A very important consideration in data communications

is how fast we can send data, in bits per second, over

a channel. Data rate depends on three factors:

1. The bandwidth available

2. The level of the signals we use

3. The quality of the channel (the level of noise)


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Noiseless Channel: Nyquist bit Rate

BitRate = 2 X bandwidth X Log2L

In this formula, bandwidth is the bandwidth of the channel, L

is the number of signal levels used to represent data, and

BitRate is the bit rate in bits per second.

According to the formula, we might think that, given a specific

bandwidth, we can have any bit rate we want by increasing the

number of signal levels.

Although the idea is theoretically correct, practically there is a

limit. When we increase the number of signal levels, we

impose a burden on the receiver.


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Example: Consider a noiseless channel with a bandwidth of3000 Hz transmitting a signal with two signal levels. The

maximum bit rate can be calculated as

Consider the same noiseless channel transmitting a signal with

four signal levels (for each level, we send 2 bits). The maximum

bit rate can be calculated as

N i Ch l Sh


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Noisy Channel: Shannon

Capacity

Capacity = bandwidth X Log2(1 + SNR)

In this formula, bandwidth is the bandwidth of the

channel, SNR is the signal-to-noise ratio, and capacity

is the capacity of the channel in bits per second.

Note that in the Shannon formula there is no indication

of the signal level, which means that no matter how

many levels we have, we cannot achieve a data rate

higher than the capacity of the channel.

In other words, the formula defines a characteristic of


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Using Both Limits: In practice, we need to use bothmethods to find the limits and signal levels.

From Shannon Capacity, we can find capacity of

channel. And using that capacity in Nyquist bit Rate we

can find number of level to be send for goodperformance.


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Transmission medium and physical layer


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Computers and other telecommunication devices usesignals to represent data.

These signals are transmitted from one device toanother in the form of electromagnetic energy, which is

propagated through transmission media. In telecommunications, transmission media can be

divided into two broad categories: guided andunguided.

Guided media include twisted-pair cable,

coaxial cable, and

fiber-optic cable.

Unguided medium is free space


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Classes of transmission media


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GUIDED MEDIA

Guided media, which are those that provide a conduitfrom one device to another.

There are three categories of guided media:

1. Twisted-pair cable

2. Coaxial cable

3. Fiber-optic cable


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Twisted-pair cable

Twisted pair consists of twoconductors (normally copper),each with its own plasticinsulation, twisted together.

One of the wires is used tocarry signals to the receiver,and the other is used only as aground reference.

Twisted-pair cable comes in twoforms: unshielded andshielded

The twisting helps to reduce the

interference (noise) and


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UTP Vs. STP

U hi ld d T i d i (UTP)


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Unshielded Twisted-pair (UTP)

cable

UTP cable is the most common type oftelecommunication medium in use today.

The range is suitable for transmitting both data and

video.

Advantages of UTP are its cost and ease of use.

UTP is cheap, flexible, and easy to install.


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UTP Standards

Category Bandwidth Data Rate Digital/Analog Use

1 very low < 100 kbps Analog Telephone

2 < 2 MHz 2 Mbps Analog/digital T-1 lines

3 16 MHz 10 Mbps Digital LANs



6 (draft) 200 MHz 200 Mbps Digital LANs

7 (draft) 600 MHz 600 Mbps Digital LANs


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UTP connecto rs

The most common UTP connector is RJ45 (RJ stands forRegistered Jack).


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Shielded Twisted (STP) Cable

STP cable has a metal foil or braided-mesh coveringthat enhances each pair of insulated conductors.

The metal casing prevents the penetration of

electromagnetic noise.

Materials and manufacturing requirements make STP

more expensive than UTP but less susceptible to noise.

Applications of Twisted Pair


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Applications of Twisted Pair

Cables

Twisted-pair cables are used in telephones lines toprovide voice and data channels.

The DSL lines that are used by the telephone

companies to provide high data rate connections also

use the high-bandwidth capability of unshieldedtwisted-pair cables.

Local area networks, such as 10Base-T and 100Base-

T, also used UTP cables.


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Coaxial Cable (or coax)

Coaxial cable (or coax) carries signals of higherfrequency ranges than those in twisted pair cable.

Instead of having two wires, coax has a central core

conductor of solid or stranded wire (usually copper)

enclosed in an insulating sheath, which is, in turn,encased in an outer conductor of metal foil, braid, or a

combination of the two.

The outer metallic wrapping serves both as a shield

against noise and as the second conductor, whichcompletes the circuit.


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Categories of coaxial cables


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Fiber-Optic Cable

A fiber-optic cable is made of glass or plastic andtransmits signals in the form of light.

To understand optical fiber, we first need to explore

several aspects of the nature of light.

Light travels in a straight line as long as it is moving

through a single uniform substance.

If a ray of light traveling through one substance

suddenly enters another (less or more dense)

substance, its speed changes abruptly, causing the ray

to change direction.

This change is called refract ion.


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Critical angle

If the angle of incidence increases, so does the angle of

refraction.

The critical angleis defined to be an angle of incidence

for which the angle of refraction is 90 degrees.


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Reflection

When the angle of incidence

becomes greater than the

critical angle, a new

phenomenon occurs called

reflection.

Light no longer passes into

the less dense medium at all.


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Bending of light ray


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Optical fibers use ref lect ion to guide light through achannel.

A glass or core is surrounded by a cladding of lessdense glass or plastic. The difference in density of the

two materials must be such that a beam of lightmoving through the core is reflected off the claddinginstead of being into it.

Information is encoded onto a beam of light as a seriesof on-off flashes that represent 1 and 0 bits.


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Fiber Construction


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Types of Optical Fiber

There are two basic types of fiber: multimode fiber and

single-mode fiber.

Multimode fiber is best designed for short transmission

distances, and is suited for use in LAN systems andvideo surveillance.

Single-mode fiber is best designed for longer

transmission distances, making it suitable for long-

distance telephony and multichannel television

broadcast systems.

Propagation Modes (Types of


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Propagation Modes (Types of

Optical Fiber )


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Multimode: In this case multiple beams from a light sourcemove through the core in different paths.

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 lineuntil it reaches the interface of the core and cladding. At the

interface there is an abrupt change to a lower density that

alters the angle of the beams motion.

In a multimode graded-index fiberthe density is highest atthe center of the core and decreases gradually to its lowest

at the edge.

Single modeuses 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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Fib Si


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Fiber SizesOptical fibers are defined by the ratio of the diameter of

their core to the diameter of their cladding, both

expressed in microns (micrometers)

Type Core Cladding Mode

50/125 50 125 Multimode, graded-index

62.5/125 62.5 125 Multimode, graded-index

100/125 100 125 Multimode, graded-index

7/125 7 125 Single-mode


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Advantages of Optical Fiber

The major advantages offered by fiber-optic cable overtwisted-pair and coaxial cable are noise resistance, lesssignal attenuation, and higher bandwidth.

Noise Resistance: Because fiber-optic transmission useslight rather than electricity, noise is not a factor. External

light, the only possible interference, is blocked from thechannel by the outer jacket.

Less signal attenuation

Fiber-optic transmission distance is significantly greater

than that of other guided media. A signal can run for mileswithout requiring regeneration.

Higher bandwidth

Currently, data rates and bandwidth utilization over fiber-

optic cable are limited not by the medium but by the signal


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Disadvantages of Optical Fiber

The main disadvantages of fiber optics are cost,installation/maintenance, and fragility (weakness).

Cost. Fiber-optic cable is expensive. Also, a laser light

source can cost thousands of dollars, compared to

hundreds of dollars for electrical signal generators. Fragility. Glass fiber is more easily broken than wire,

making it less useful for applications where hardware

portability is required.


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Unguided Media

Unguided media, or wireless communication, transportelectromagnetic waves without using a physical

conductor.

Signals are broadcast though air or water, and thus are

available to anyone who has a device capable ofreceiving them.

The section of the electromagnetic spectrum defined

as rad io communicat ionis divided into eight ranges,

called bands,each regulated by governmentauthorities.


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Propagation of Radio Waves

Radio technology considers the earth as surrounded bytwo layers of atmosphere: thetroposphereand theionosphere.

The troposphere is the portion of the atmosphere

extending outward approximately 30 miles from theearth's surface.

The troposphere contains what we generally think ofas air. Clouds, wind, temperature variations, andweather in general occur in the troposphere.

The ionosphere is the layer of the atmosphere abovethe troposphere but below space.


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Ground propagation. In ground propagation, radiowaves travel through the lowest portion of theatmosphere, hugging the earth. These low-frequencysignals emanate in all directions from the transmittingantenna and follow the curvature of the planet. Thedistance depends on the power in the signal.

In Sky propagation, higher-frequency radio wavesradiate upward into the ionosphere where they are

reflected back to earth. This type of transmissionallows for greater distances with lower power output.

In Line-of-Sight Propagation, very high frequency

signals are transmitted in straight lines directly from


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Bands

Band Range Propagation Application

VLF 330 KHz Ground Long-range radio navigation

LF 30300 KHz GroundRadio beacons and

navigational locators

MF 300 KHz

3 MHz Sky AM radio

HF 330 MHz SkyCitizens band (CB),

ship/aircraft communication

VHF 30300 MHzSky and

line-of-sight

VHF TV,

FM radio

UHF 300 MHz3 GHz Line-of-sightUHF TV, cellular phones,

paging, satellite

SHF 330 GHz Line-of-sight Satellite communication

EHF 30300 GHz Line-of-sight Long-range radio navigation


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We can divide wireless transmission into three broadgroups:

radio waves,

microwaves, and

infrared waves


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Radio Waves

Although there is no clear-cut demarcation betweenradio waves and microwaves, electromagnetic wavesranging in frequencies between 3 kHz and 1 GHz areno rmally cal led radio waves; waves ranging infrequencies between 1 and 300 GHz are calledmicrowaves.

Radio waves, are omnid i rect ional. When an antennatransmits radio waves, they are propagated in alldirections.

This means that the sending and receiving antennas donot have to be aligned. A sending antenna sendswaves that can be received by any receiving antenna.

Radio waves, can travel long distances. This makes

radio waves a good candidate for long-distance


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Omnidirectional antenna


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Microwaves

Electromagnetic waves having frequencies between 1 and300 GHz are called microwaves.

Microwaves are unidi rect ional.

When an antenna transmits microwaves, they can be

narrowly focused. This means that the sending and receiving antennas need

to be aligned. The unidirectional property has an obvious

advantage.

A pair of antennas can be aligned without interfering withanother pair of aligned antennas.

Microwaves are used for unicast communication such

as cellular telephones, satellite networks, and wireless

LANs.

Unidirectional antenna


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Infrared

Infrared waves, with frequencies from 300 GHz to 400 THz(wavelengths from 1 mm to 770 nm), can be used for short-

range communication.

Infrared waves, having high frequencies, cannot penetrate

walls. This advantageous characteristic prevents interference

between one system and another; a short-range

communication system in one room cannot be affected by

another system in the next room.

When we use our infrared remote control, we do not

interfere with the use of the remote by our neighbors.


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Switching: Introduction

A network is a set of connected devices. Whenever wehave multiple devices, we have the problem of how to

connect them to make one-to-one communication

possible.

The solution is swi tch ing. A swi tched netwo rkcons ists o f a ser ies o f inter linked nodes, cal led

switches.

Switches are devices capable of creating temporary

connections between two or more devices linked to theswitch.

In a switch, some of these nodes are connected to the

end system Others are used only for routing.


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Switched network


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Three Methods of Switching

Traditionally, three methods of switching have beendiscussed: circuit switching,

packet switching, and

message switching. The first two are commonly used today. The third

has been phased out in general communicationsbut still has applications.

Packet switching can further be divided into twosubcategories, virtual-circuit approach and

datagram approach

Switching can happen at several layers of the


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CIRCUIT-SWITCHED NETWORKS

A circuit-switched network consists of a set of switchesconnected by physical links.

A connection between two stations is a dedicated path

made of one or more links.

However, each connection uses only one dedicatedchannel on each link.

Each link is normally divided into n channels by using

FDM(frequency division multiplexing) or TDM(time

division multiplexing)


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A trivial circuit-switched network


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Three Phases

The actual communication in a circuit-switched networkrequires three phases: connection setup, data transfer,

and connection teardown.

Setup: Before the two parties can communicates, a

dedicated circuits needs to be established. The end systems are normally connected through

dedicated lines to the switches , so connection setup

means creating dedicated channels between the

switches.

Data-Transfer: After the establishment of the dedicated

circuit the two parties can transfer data.

Teardown Phase: When one of the parties needs to


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unit 1 physical layer

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