ch1data communication & computer networks

7/25/2019 Ch1Data Communication & Computer Networks

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Data Communication &Computer Networks

Chapter 1Introduction


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DATA COMMUNICATION

Communication:

Communication is a process of establishing

connection or link between two points for information

exchanges.

Telecommunication

Telecommunication means communication at a

distance.

Data communication

Data communication is the exchange of data betweentwo devices via some form of transmission medium such

as a wire cable.


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General Communication Model

General Communication Model

Source TransmitterTransmission

SystemReceiver Destination

Microphone

TelephoneComputerScanner

Transformer

EncoderCompressModulator

Line/Cable

iber/!irSatellite"et#or$

Transformer

Decoder%ncompressDemodulator

Spea$erEarphoneComputer&rinter

'asic Communication Criteria( &erformance) Reliability) Security


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General Communications Model


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Communications Model

Source

*enerates data to be transmitted

Transmitter

converts data into transmittable si*nals

Transmission System carries data

Receiver

converts received si*nal into data Destination

ta$es incomin* data from the receiver


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What is Data Communications?

E+chan*e of di*ital informationbet#een t#o di*ital devices is datacommunication


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Data Communications Model


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Components of a datacommunications system

l, Messa*eThe messa*e is the information -data. to becommunicated,

&opular forms of information include te+t) numbers)pictures) audio)

and video,

,SenderThe sender is the device that sends the datamessa*e,

It can be a computer) #or$station) telephone handset)video camera,0, ReceiverThe receiver is the device that receives the

messa*e, It can be a computer) #or$station) telephone handset)television,,Transmission mediumThe transmission medium is thephysical path

by #hich a messa*e travels from sender to receiver,Some e+amples of transmission media include t#isted2pair#ire)

coa+ial cable) 3ber2optic cable) and radio #aves,

4, &rotocol ! protocol is a set of rules that *overn datacommunications,

It represents an a*reement bet#een the communicatin*devices,

5ithout a protocol) t#o devices may be connected but not

communicatin*) 6ust as a person spea$in* rench cannot beunderstood


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i*ure ( Components of a datacommunication system


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Four fundamental characteristics

our fundamental characteristics of data communication(

delivery) accuracy) timeliness) and 6itter,I, Delivery, The system must deliver data to the correct

destination, Data must be received by the intended deviceor user and only by that device or user,

, Accuracy, The system must deliver the data accurately,

Data that have been

altered in transmission and left uncorrected are unusable,

0, imeliness, The system must deliver data in a timelymanner, Data delivered late are useless, In the case ofvideo and audio) timely delivery means deliverin* data as

they are produced) in the same order that they areproduced) and #ithout si*ni3cant delay, This $ind ofdelivery is called real-time transmission.

! "itter, 7itter refers to the variation in the pac$et arrivaltime, It is the uneven delay in the delivery of audio or video

pac$ets,


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Fi#ure $ Data fow (simplex, hal-duplex,and ull-duplex)


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%imple ransmission and Dupleransmission

%imple ransmission and Dupleransmission

Device ! Device 'Simple+Transmission

Direction of data

Device ! Device '8alf Duple+Transmission

Direction of data at time 1

Device ! Device 'ull Duple+Transmission

Direction of data all the time

Direction of data at time

9ne can send and the other can receive

'oth can send and receive but in di:erent time

'oth can send and receive simultaneously


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Simplex

In simple+ mode) the communication isunidirectional,9nly one of the t#o devices on a lin$ can transmit;the other can only receive


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Hal-duplex

In half2duple+ mode) each station can bothtransmit and receive) but not at the same time,5hen one device is sendin*) the other can onlyreceive) and vice versaln a half2duple+ transmission) the entire capacityof a channel is ta$en over by #hichever of the

t#o devices is transmittin* at the time,In half2duple+) the entire capacity of the channelis ta$en over by the transmittin* -sendin*.,5al$ie2tal$ies and C' -citi=ens band. radios are

both half2duple+ systems


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Full-duplexIn full2duple+ mode both stations can transmit andreceive simultaneously

In full2duple+ mode) si*nals *oin* in one directionshare the capacity of the lin$ #ith si*nals *oin* inthe other direction,This sharin* can occur in t#o #ays( either the

lin$ must contain t#o physically separatetransmission paths) one for sendin* and the otherfor receivin*; or the capacity of the channel isdivided bet#een si*nals travelin* in bothdirections,

9ne common e+ample of full2duple+communication is the telephone net#or$, 5hent#o people are communicatin* by a telephone line)both can tal$ and listen at the same time


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Analo# and Di#ital Data ransmission

Data

Entities that convey Some meanin* Si*nals

Electric or electroma*netic or optical

representations of data, So #hen ever data needs to be sent )

it has to be converted into si*nal ofsome form for transmission over

suitable medium Transmission

Communication of data bypropa*ation and processin* of si*nals


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Data

!nalo*

Continuous values #ithin some interval

e,*, sound) video

Di*ital Discrete values

e,*, te+t) inte*ers


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%i#nals

Means by #hich data are propa*ated

!nalo* Continuously variable

>arious media 5ire) 3ber optic) space

Speech band#idth 1??h= to @$h=

Telephone band#idth 0??h= to 0??h=

>ideo band#idth


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Data and %i#nals

%sually use di*ital si*nals for di*italdata and analo* si*nals for analo*data

Can use analo* si*nal to carry di*italdata Modem

Can use di*ital si*nal to carry analo*data Compact Disc audio


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Analo# %i#nals Carryin# Analo#and Di#ital Data


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Di#ital %i#nals Carryin# Analo#and Di#ital Data


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Analo# ransmission

!nalo* si*nal transmitted #ithoutre*ard to content

May be analo* or di*ital data

!ttenuated over distance

%se ampli3ers to boost si*nal

!lso ampli3es noise


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Di#ital ransmission

Concerned #ith content,

Inte*rity endan*ered by noise) attenuationetc,

Repeaters used, Repeater receives si*nal,

E+tracts bit pattern,

Retransmits,

!ttenuation is overcome, "oise is not ampli3ed,


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Advanta#es of Di#ital ransmission

Di*ital technolo*y Lo# cost LSI/>LSI technolo*y

Data inte*rity

Lon*er distances over lo#er Auality lines Capacity utili=ation

8i*h band#idth lin$s economical

8i*h de*ree of multiple+in* easier #ith di*ital

techniAues Security B &rivacy

Encryption

Inte*ration

Can treat analo* and di*ital data similarly


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ransmission 'mpairments

Impairment is imperfections inmedium cause impairment

Si*nal received may di:er from si*naltransmitted

!nalo* 2 de*radation of si*nal Auality

Di*ital 2 bit errors

Caused by-three causes of impairment.

!ttenuation and attenuation distortion Delay distortion

"oise


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Causes of impairment


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Attenuation

Means loss of ener*y 2 #ea$ersi*nal

5hen a si*nal travels throu*h amedium it loses ener*yovercomin* the resistance of themedium

!mpli3ers are used to compensatefor this loss of ener*y byamplifyin* the si*nal,


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Measurement of Attenuation

To sho# the loss or *ain of ener*ythe unit decibel is used,

d' F 1?lo*1?&/&1

&12 input si*nal

&2 output si*nal


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Fi#ure $ Attenuation


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Suppose a si*nal travels throu*h a transmissionmedium and its po#er is reduced to one2half,

This means that & is -1/.&1, In this case) theattenuation -loss of po#er. can be calculated as

E+ample (1

! loss of 0 d' -G0 d'. is eAuivalent to losin* one2half the po#er,


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9ne reason that en*ineers use the decibel tomeasure the chan*es in the stren*th of a si*nal isthat decibel numbers can be added -orsubtracted. #hen #e are measurin* several points-cascadin*. instead of 6ust t#o, In i*ure 0,@ asi*nal travels from point 1 to point , In this case)the decibel value can be calculated as

E+ample(0


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0,00

Decibels for E+ample 0


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Sometimes the decibel is used to measure si*nalpo#er in milli#atts, In this case) it is referred to asd'mand is calculated as d'mF 1? lo*1? &m ) #here

&m is the po#er in milli#atts, Calculate the po#er

of a si*nal #ith d'mF H0?,

Solution5e can calculate the po#er in the si*nal as

(ample $)


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The loss in a cable is usually de3ned in decibels per $ilometer -d'/$m.,

If the si*nal at the be*innin* of a cable #ith H?,0 d'/$m has a po#er of m5) #hat is the po#er of the si*nal at 4 $mSolutionThe loss in the cable in decibels is 4 J -H?,0. F H1,4 d', 5e cancalculate the po#er as

E+ample ( 4


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Distortion

Means that the si*nal chan*es its formor shape

Distortion occurs in compositesi*nals

Each freAuency component has its o#npropa*ation speedtravelin* throu*h amedium,

The di:erent components therefore

arrive #ith di:erent delaysat thereceiver,

That means that the si*nals havedi:erent phasesat the receiver than

they did at the source,


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i*ure ( Distortion


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Noise

There are di:erent types of noiseThermal2 random noise of electrons

in the #ire creates an e+tra si*nal

Induced2 from motors andappliances) devices act aretransmitter antenna and medium asreceivin* antenna,

Crosstal$2 same as above butbet#een t#o #ires,

Impulse2 Spi$es that result frompo#er lines) li*hnin*) etc,


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i*ure ( "oise


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Si*nal to "oise Ratio -S"R. To measure the Auality of a system the S"R is

often used, It indicates the stren*th of thesi*nal #rt the noise po#er in the system,

It is the ratio bet#een t#o po#ers,

It is usually *iven in d' and referred to asS"Rd',

S"R is actually the ratio of #hat is #anted-si*nal. to #hat is not #anted -noise.,

! hi*h S"R means the si*nal is less corruptedby noise;

a lo# S"R means the si*nal is more corruptedby noise,


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The po#er of a si*nal is 1? m5 and the po#erof the noise is 1 K5; #hat are the values ofS"R and S"Rd'

SolutionThe values of S"R and S"Rd' can becalculated as follo#s(

E+ample (


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The values of S"R and S"Rd' for anoiseless channel are

(ample $*

5e can never achieve this ratio in real life; it

is an ideal,


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i*ure (T#o cases of S"R( a hi*h S"R and a lo# S"R


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Data rate limits

Important consideration in data communicationis ho# fast #e can send data) in bits per secondover a channel,

Data rate depends on three factors(

1, 'and#idth available

, Level of si*nals #e use

0, uality of the channel-level of noise.

. T#o theoretical formulas for data rate

calculation1,'y"Auist for noiseless channel -"Auist bit rate.

,'y shannon for a noisy channel-Shannoncapacity theorem.


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Ny+uist ,andwidth

C F ' lo* M5here

C F Capacity of the channel

' F 'and#idth

M F "umber of discrete si*nal orvolta*e level


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%hannon Capacity Formula

C F ' lo*-1N S"R.,

5here,

C F Capacity of the channel in bits per sec,

' F 'and#idth in 8ert=, S"R F Si*nal2to2"oise ratio in d',

-S"R.d' F 1? lo*1?-si*nal po#er/noise po#er.,

The formula assumes 5hite noise -thermal noise.,

Impulse noise) attenuation distortion or delaydistortion is not accounted for,

The #ider the band#idth) the more noise) thus as 'increases) S"R decreases,


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(ample

Spectrum of the channel F 0 to M8= S"R F d'

Then ' F M8= G 0M8= F 1M8=

S"Rd'F d' F 1? lo*1?-S"R.

S"R F 41 %sin* ShannonOs formula)

C F 1?P lo*-1N41. Q 1?P F Mbps

'ased on "yAuistOs formula

C F ' lo*M

P 1?F P -1?. P lo*M

F lo*M

M F 1


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he (pression (-.No

The ratio of si*nal ener*y per bit to noisepo#er density per hert=

1 #att F 1 7/s

EbF STb EbF ener*y per bit in a si*nal

S F si*nal po#er

TbF time reAuired to send one bit

R F data rate F 1 / Tb

Eb/ "oF -S/R. / "oF S / $TR) or in decibel notation

-Eb/"o.d'F Sd'5G 1?lo* R G 1?lo* $ G 1?lo* T

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