03-datatransmission.ppt

8/20/2019 03-DataTransmission.ppt

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Data and ComputerData and Computer

CommunicationsCommunications

Eighth EditionEighth Editionby William Stallingsby William Stallings

Lecture slides by Lawrie BrownLecture slides by Lawrie Brown

Chapter 3 – Data TransmissionChapter 3 – Data Transmission


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Data TransmissionData Transmission

Toto, I've got a feeling we're not in KansasToto, I've got a feeling we're not in Kansas

anymoreanymore. Judy Garland in. Judy Garland in The Wizard ofThe Wizard of

Oz Oz


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TransmissionTransmission TerminologyTerminology

data transmission occurs between adata transmission occurs between a

transmitter & receiver via some mediumtransmitter & receiver via some medium

guided mediumguided medium eg. twisted pair, coaial cable, optical !iber eg. twisted pair, coaial cable, optical !iber

unguided " wireless mediumunguided " wireless medium

eg. air, water, vacuumeg. air, water, vacuum


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direct lin#direct lin# no intermediate devicesno intermediate devices

point$to$pointpoint$to$point direct lin#direct lin#

only % devices share lin#only % devices share lin#

multi$pointmulti$point more than two devices share the lin#more than two devices share the lin#


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simplesimple one directionone direction

eg. televisioneg. television

hal! duplehal! duple either direction, but only one way at a timeeither direction, but only one way at a time

eg. police radioeg. police radio

!ull duple!ull duple both directions at the same timeboth directions at the same time

eg. telephoneeg. telephone


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Frequency, Spectrum andFrequency, Spectrum and

BandwidthBandwidth time domain conceptstime domain concepts analog signalanalog signal

various in a smooth way over timevarious in a smooth way over time

digital signaldigital signal maintains a constant level then changes to anothermaintains a constant level then changes to another

constant levelconstant level

periodic signalperiodic signal pattern repeated over timepattern repeated over time

aperiodic signalaperiodic signal pattern not repeated over timepattern not repeated over time


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eriodiceriodic

SignalsSignals


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Sine !a"eSine !a"e

pea# amplitude '()pea# amplitude '() maimum strength o! signalmaimum strength o! signal voltsvolts

!re*uency '!)!re*uency '!) rate o! change o! signalrate o! change o! signal +ert '+) or cycles per second+ert '+) or cycles per second period - time !or one repetition ')period - time !or one repetition ') - /"! - /"!

phase 'phase 'φφ)) relative position in timerelative position in time


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#arying Sine !a"es#arying Sine !a"es

s$t% A sin$'s$t% A sin$'

π

(t )(t )

%%


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!a"elength $!a"elength $

%%

is distance occupied by one cycleis distance occupied by one cycle

between two points o! correspondingbetween two points o! corresponding

phase in two consecutive cyclesphase in two consecutive cycles assuming signal velocityassuming signal velocity vv havehave λ λ = vT = vT

or e*uivalentlyor e*uivalently λ λ f = v f = v

especially whenespecially when v=c v=c c - 01/2c - 01/233 msms$/$/ 'speed o! light in !ree space)'speed o! light in !ree space)


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Frequency Domain ConceptsFrequency Domain Concepts

signal are made up o! many !re*uenciessignal are made up o! many !re*uencies

components are sine wavescomponents are sine waves

4ourier analysis can shown that any signal4ourier analysis can shown that any signalis made up o! component sine wavesis made up o! component sine waves

can plot !re*uency domain !unctionscan plot !re*uency domain !unctions


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Addition o(Addition o(

FrequencyFrequencyComponentsComponents

$T*+(%$T*+(%

cc is sum o!is sum o! f & 3f f & 3f

π

π

ππ π


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FrequencyFrequency

DomainDomain

epresentationsepresentations

!re* domain !unc o!!re* domain !unc o!4ig 0.5c4ig 0.5c

!re* domain !unc o!!re* domain !unc o!

single s*uare pulsesingle s*uare pulse

π π π

< <


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Spectrum & BandwidthSpectrum & Bandwidth

spectrumspectrum range o! !re*uencies contained in signalrange o! !re*uencies contained in signal

absolute bandwidthabsolute bandwidth width o! spectrumwidth o! spectrum

e!!ective bandwidthe!!ective bandwidth

o!ten 6usto!ten 6ust bandwidthbandwidth

narrow band o! !re*uencies containing most energynarrow band o! !re*uencies containing most energy 78 8omponent78 8omponent

component o! ero !re*uencycomponent o! ero !re*uency


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Data ate and BandwidthData ate and Bandwidth

any transmission system has aany transmission system has a limited band o!limited band o!!re*uencies!re*uencies

thisthis limits the data ratelimits the data rate that can be carriedthat can be carried

s*uare have in!inite components and hences*uare have in!inite components and hencebandwidthbandwidth

but most energy in !irst !ew componentsbut most energy in !irst !ew components

limited bandwidth increases distortionlimited bandwidth increases distortion have ahave a direct relationship between data rate &direct relationship between data rate &

bandwidthbandwidth


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Figure 3-. $a% & $/%Figure 3-. $a% & $/%


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Bit time T + '


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Data ate CalculationData ate Calculation 8ase /8ase /

Bandwidth 59+Bandwidth 59+,, use the sine wave o! use the sine wave o! 4ig. 0$: 'a)4ig. 0$: 'a)

59+ - ;! < !59+ - ;! < ! ! - /9+! - /9+

7ata rate - % 9bps7ata rate - % 9bps

8ase %8ase % Bandwidth 39+Bandwidth 39+,, use the sine wave o! use the sine wave o! 4ig. 0$: 'a)4ig. 0$: 'a)

39+ - ;! < !39+ - ;! < ! ! - %9+! - %9+

7ata rate - 5 9bps7ata rate - 5 9bps

8ase 08ase 0 Bandwidth 59+Bandwidth 59+,, use the sine wave o! use the sine wave o! 4ig. 0$5 'c)4ig. 0$5 'c)

59+ - 0! < !59+ - 0! < ! ! - %9+! - %9+

7ata rate - 5 9bps7ata rate - 5 9bps


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Data ate "s- BandwidthData ate "s- Bandwidth

BandwidthBandwidth 00 7ata rate7ata rate 00 'compare case / & %)'compare case / & %)

Same signal *ualitySame signal *uality

Same bandwidthSame bandwidth +igher signal *uality+igher signal *uality lower data ratelower data rate

8ompare case / & 08ompare case / & 0

Same data rateSame data rate BandwidthBandwidth 00 better signal *ualitybetter signal *uality

8ompare case % & 08ompare case % & 0


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Analog and Digital DataAnalog and Digital Data

TransmissionTransmission datadata

entities that convey meaningentities that convey meaning

signals & signalingsignals & signaling electric or electromagnetic representations o!electric or electromagnetic representations o!

data, physically propagates along mediumdata, physically propagates along medium

transmissiontransmission communication o! data by propagation andcommunication o! data by propagation and

processing o! signalsprocessing o! signals


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Acoustic Spectrum $Analog%Acoustic Spectrum $Analog%


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Audio SignalsAudio Signals

!re* range %2+$%2#+ 'speech /22+$:#+)!re* range %2+$%2#+ 'speech /22+$:#+) easily converted into electromagnetic signalseasily converted into electromagnetic signals varying volume converted to varying voltagevarying volume converted to varying voltage

can limit !re*uency range !or voice channel tocan limit !re*uency range !or voice channel to022$0522+022$0522+


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#ideo Signals#ideo Signals

=S( $ 530 lines per !rame, at !rames per sec=S( $ 530 lines per !rame, at !rames per sec have ;%; lines but 5% lost during vertical retracehave ;%; lines but 5% lost during vertical retrace

;%; lines 02 scans - /;:;2 lines per sec;%; lines 02 scans - /;:;2 lines per sec >0.;>0.;µµs per lines per line

////µµs !or retrace, so ;%.;s !or retrace, so ;%.; µµs per video lines per video line

ma !re*uency i! line alternates blac# and whitema !re*uency i! line alternates blac# and white

horiontal resolution is about 5;2 lines givinghoriontal resolution is about 5;2 lines giving%%; cycles o! wave in ;%.;%%; cycles o! wave in ;%.; µµss

ma !re*uency o! 5.%9+ma !re*uency o! 5.%9+


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Digital DataDigital Data

as generated by computers etc.as generated by computers etc.

has two dc componentshas two dc components

bandwidth depends on data ratebandwidth depends on data rate


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Analog SignalsAnalog Signals


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Digital SignalsDigital Signals


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Ad"antages & Disad"antagesAd"antages & Disad"antages

o( Digital Signalso( Digital Signals cheaper cheaper less susceptible to noiseless susceptible to noise

but greater attenuationbut greater attenuation digital now pre!erred choicedigital now pre!erred choice


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Transmission 1mpairmentsTransmission 1mpairments

signal received may di!!er !rom signalsignal received may di!!er !rom signaltransmitted causing?transmitted causing? analog $ degradation o! signal *ualityanalog $ degradation o! signal *uality digital $ bit errorsdigital $ bit errors

most signi!icant impairments aremost signi!icant impairments are attenuation and attenuation distortionattenuation and attenuation distortion

delay distortiondelay distortion noisenoise


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AttenuationAttenuation

where signal strength !alls o!! with distancewhere signal strength !alls o!! with distance depends on mediumdepends on medium received signal strength must be?received signal strength must be?

strong enough to be detectedstrong enough to be detected su!!iciently higher than noise to receive without error su!!iciently higher than noise to receive without error

so increase strength using ampli!iers"repeatersso increase strength using ampli!iers"repeaters is also an increasing !unction o! !re*uencyis also an increasing !unction o! !re*uency

so e*ualie attenuation across band o!so e*ualie attenuation across band o!!re*uencies used!re*uencies used eg. using loading coils or ampli!ierseg. using loading coils or ampli!iers


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Delay DistortionDelay Distortion

only occurs in guided mediaonly occurs in guided media

propagation velocity varies with !re*uencypropagation velocity varies with !re*uency

hence various !re*uency componentshence various !re*uency componentsarrive at di!!erent timesarrive at di!!erent times

particularly critical !or digital dataparticularly critical !or digital data

since parts o! one bit spill over into otherssince parts o! one bit spill over into others causing intersymbol inter!erencecausing intersymbol inter!erence


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2oise2oise

additional signals inserted betweenadditional signals inserted betweentransmitter and receiver transmitter and receiver

thermalthermal due to thermal agitation o! electronsdue to thermal agitation o! electrons uni!ormly distributeduni!ormly distributed white noisewhite noise

intermodulationintermodulation signals that are the sum and di!!erence o!signals that are the sum and di!!erence o!

original !re*uencies sharing a mediumoriginal !re*uencies sharing a medium


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2oise2oise

crosstal#crosstal# a signal !rom one line is pic#ed up by another a signal !rom one line is pic#ed up by another

impulseimpulse irregular pulses or spi#esirregular pulses or spi#es

eg. eternal electromagnetic inter!erenceeg. eternal electromagnetic inter!erence

short durationshort duration

high amplitudehigh amplitude a minor annoyance !or analog signalsa minor annoyance !or analog signals but a ma6or source o! error in digital databut a ma6or source o! error in digital data

a noise spi#e could corrupt many bitsa noise spi#e could corrupt many bits


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Channel CapacityChannel Capacity

ma possible data rate on comms channelma possible data rate on comms channel

is a !unction o! is a !unction o!

data rate $ in bits per seconddata rate $ in bits per second bandwidth $ in cycles per second or +ertbandwidth $ in cycles per second or +ert

noise $ on comms lin#noise $ on comms lin#

error rate $ o! corrupted bitserror rate $ o! corrupted bits

limitations due to physical propertieslimitations due to physical properties

want most e!!icient use o! capacitywant most e!!icient use o! capacity


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2yquist Bandwidth2yquist Bandwidth

consider noise !ree channelsconsider noise !ree channels i! rate o! signal transmission is %B then can carryi! rate o! signal transmission is %B then can carry

signal with !re*uencies no greater than Bsignal with !re*uencies no greater than B

ie. given bandwidth B, highest signal rate is %Bie. given bandwidth B, highest signal rate is %B !or binary signals, %B bps needs bandwidth B +!or binary signals, %B bps needs bandwidth B +

can increase rate by using 9 signal levelscan increase rate by using 9 signal levels

@y*uist 4ormula is? 8 - %B log@y*uist 4ormula is? 8 - %B log%%99

so increase rate by increasing signalsso increase rate by increasing signals at cost o! receiver compleityat cost o! receiver compleity

limited by noise & other impairmentslimited by noise & other impairments


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Shannon Capacity FormulaShannon Capacity Formula

consider relation o! data rate, noise & error rateconsider relation o! data rate, noise & error rate !aster data rate shortens each bit so bursts o! noise!aster data rate shortens each bit so bursts o! noise

a!!ects more bitsa!!ects more bits

given noise level, higher rates means higher errorsgiven noise level, higher rates means higher errors Shannon developed !ormula relating these toShannon developed !ormula relating these to

signal to noise ratio 'in decibels)signal to noise ratio 'in decibels)

S@AS@Adbdb--

/2 log/2 log/2/2

'signal"noise)'signal"noise)

8apacity 8-B log8apacity 8-B log%%'/S@A)'/S@A) theoretical maimumtheoretical maimum

capacitycapacity

get lower in practiseget lower in practise


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SummarySummary

loo#ed at data transmission issuesloo#ed at data transmission issues

!re*uency, spectrum & bandwidth!re*uency, spectrum & bandwidth

analog vs digital signalsanalog vs digital signals transmission impairmentstransmission impairments

03-datatransmission.ppt

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