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.

Chapter 7

Physical Layer and

Transmission Media

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Chapter 7: Outline

7.17.1

DATA AND SIGNAL DATA AND SIGNAL

7.27.2 DIGITAL TRANSMISSION DIGITAL TRANSMISSION

7.37.3 ANALOG TRANSMISSION ANALOG TRANSMISSION

7.47.4 BANDWIDTH UTILIZATION BANDWIDTH UTILIZATION

7.57.5TRANSMISSION MEDIATRANSMISSION MEDIA

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Chapter 7: Objective

We first discuss the relatioshi! "et#ee data ad si$als% We

the sho# ho# data ad si$als ca "e "oth aalo$ ad di$ital%

We the cocetrate o di$ital tras&issio% We sho# ho# to

co'ert di$ital ad aalo$ data to di$ital si$als%

Ne(t) #e cocetrate o aalo$ tras&issio% We sho# ho# to

co'ert di$ital ad aalo$ data to aalo$ si$als%

We the tal* a"out &ulti!le(i$ techi+ues ad ho# the, caco&"ie se'eral chaels%

-iall,) #e $o "elo# the !h,sical la,er ad discuss the

tras&issio &edia%

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7-1 DATA AND SIGNALS7-1 DATA AND SIGNALS

At the physical layer, thecommunication is node-to-node,but the nodes exchangeelectromagnetic signals. Figure7.1 uses the same scenario weshowed in four earlier chapters,

but the communication is now atthe physical layer.

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Figure 7.1: Communication at the physical layer

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.%/%/ Aalo$ ad Di$ital

Data ca "e aalo$ or di$ital% The ter& aalo$ data Data ca "e aalo$ or di$ital% The ter& aalo$ datarefers to ifor&atio that is cotiuous% Di$ital datarefers to ifor&atio that is cotiuous% Di$ital data

ta*e o discrete 'alues%ta*e o discrete 'alues%

Li*e the data the, re!reset) si$als ca "e either Li*e the data the, re!reset) si$als ca "e either

aalo$ or di$ital% A aalo$ si$al has ifiitel,aalo$ or di$ital% A aalo$ si$al has ifiitel,&a, le'els of itesit, o'er a !eriod of ti&e% A&a, le'els of itesit, o'er a !eriod of ti&e% A

di$ital si$al) o the other had) ca ha'e ol, adi$ital si$al) o the other had) ca ha'e ol, a

li&ited u&"er of defied 'alues% Althou$h eachli&ited u&"er of defied 'alues% Althou$h each

'alue ca "e a, u&"er) it is ofte as si&!le as /'alue ca "e a, u&"er) it is ofte as si&!le as /

ad 0%ad 0%

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7.7

.%/%/ 1cotiued2

Aalo$ Si$als Aalo$ Si$als Ti&e ad -re+uec, Do&aisTi&e ad -re+uec, Do&ais 3o&!osite Si$als3o&!osite Si$als Bad#idth Bad#idth

Di$ital Si$als Di$ital Si$als

Bit Rate Bit Rate Bit Le$th Bit Le$th

Di$ital Si$al as a 3o&!osite Aalo$ Si$al Di$ital Si$al as a 3o&!osite Aalo$ Si$al Tras&issio of Di$ital Si$alsTras&issio of Di$ital Si$als Base"ad Tras&issio Base"ad Tras&issio Broad"ad Tras&issio Broad"ad Tras&issio

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7.8

Figure 7.: Comparison of analog and digital signals

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7.9

Figure 7.!: A sine wa"e

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Figure 7.#: $a"elength and period

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12

The frequency domain is more compact and useful when we

are dealing with more than one sine wave. For example,

Figure 7.6 shows three sine waves, each with different

amplitude and frequency. All can be represented by three

spies in the frequency domain.

(xample 7.1

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Figure 7.): &he time domain and fre'uency domain ofthree sine wa"es

Figure 7 7: &he bandwidth of periodic and nonperiodic

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Figure 7.7: &he bandwidth of periodic and nonperiodiccomposite

signals

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.le"els and the

other with four signal le"els

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Assume we need to download text documents at the rate of

!"" pages per minute. #hat is the required bit rate of the

channel$ A page is an average of %& lines with '" characters

in each line. (f we assume that one character requires ' bits,

the bit rate is

(xample 7.

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. periodic and

nonperiodic digital signals

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Figure 7.1: aseband transmission

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An example of a dedicated channel where the entire

bandwidth of the medium is used as one single channel is a

)A*. Almost every wired )A* today uses a dedicated

channel for two stations communicating with each other.

(xample 7.!

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Figure 7.11: andwidth of a band-pass channel

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.transmission on a

band-pass channel

( l 7 #

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An example of broadband transmission using modulation is

the sending of computer data through a telephone subscriber

line, the line connecting a resident to the central telephone

office. Although this channel can be used as a low+pass

channel, it is normally considered a band+pass channel. ne

reason is that the bandwidth is so narrow -& /0 that if wetreat the channel as low+pass and use it for baseband

transmission, the maximum bit rate can be only ' bps

-explained later0. The solution is to consider the channel a

band+pass channel, convert the digital signal from thecomputer to an analog signal, and send the analog signal.

(xample 7.#

( l 7 %

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A second example is the digital cellular telephone. For better

reception, digital cellular phones digiti/e analog voice.

Although the bandwidth allocated to a company providing

digital cellular phone service is very wide, we still cannot

send the digiti/ed signal without conversion. The reason is

that we have only a band+pass channel available betweencaller and callee. For example, if the available bandwidth is

# and we allow !""" couples to tal simultaneously, this

means the available channel is #1!""", 2ust part of the

entire bandwidth. #e need to convert the digiti/ed voice to acomposite analog signal before transmission.

(xample 7.%

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.%/%4 Tras&issio I&!air&et

Si$als tra'el throu$h tras&issio &edia) #hich Si$als tra'el throu$h tras&issio &edia) #hichare ot !erfect% The i&!erfectio causes si$alare ot !erfect% The i&!erfectio causes si$al

i&!air&et% This &eas that the si$al at thei&!air&et% This &eas that the si$al at the

"e$ii$ of the &ediu& is ot the sa&e as the"e$ii$ of the &ediu& is ot the sa&e as the

si$al at the ed of the &ediu&% What is set is otsi$al at the ed of the &ediu&% What is set is ot#hat is recei'ed% Three causes of i&!air&et are#hat is recei'ed% Three causes of i&!air&et are

atteuatio) distortio) ad oise%atteuatio) distortio) ad oise%

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.%/%4 1cotiued2

Atteuatio Atteuatio

Distortio Distortio

Si$al5to5Noise Ratio 1SNR2 Si$al5to5Noise Ratio 1SNR2

Noise Noise

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Figure 7.1%: oise

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. / 6 D t R t Li it

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.%/%6 Data Rate Li&its

A 'er, i&!ortat cosideratio i data A 'er, i&!ortat cosideratio i data

co&&uicatios is ho# fast #e ca sed data) ico&&uicatios is ho# fast #e ca sed data) i

"its !er secod) o'er a chael% Data rate de!eds"its !er secod) o'er a chael% Data rate de!eds

o three factors7o three factors7

/%/% The "ad#idth a'aila"leThe "ad#idth a'aila"le

4%4% The le'el of the si$als #e useThe le'el of the si$als #e use

6%6% The +ualit, of the chael 1the le'el of oise2The +ualit, of the chael 1the le'el of oise2

T#o theoretical for&ulas #ere de'elo!ed toT#o theoretical for&ulas #ere de'elo!ed to

calculate the data rate7 oe ", N,+uist for acalculate the data rate7 oe ", N,+uist for a

oiseless chael) aother ", Shao for a ois,oiseless chael) aother ", Shao for a ois,

chael%chael%

. / 6 1 ti d2

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.%/%6 1cotiued2

Noiseless 3hael7 N,+uist Bit Rate Noiseless 3hael7 N,+uist Bit Rate

Nois, 3hael7 Shao 3a!acit, Nois, 3hael7 Shao 3a!acit,

Usi$ Both Li&itsUsi$ Both Li&its

(xample 7 7

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#e need to send %6 bps over a noiseless -ideal0 channel

with a bandwidth of %" /. ow many signal levels do we

need$ #e can use the *yquist formula as shown:

(xample 7.7

3ince this result is not a power of %, we need to either

increase the number of levels or reduce the bit rate. (f we

have !%' levels, the bit rate is %'" bps. (f we have 6&levels, the bit rate is %&" bps.

(xample 7 *

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;onsider an extremely noisy channel in which the value of

the signal+to+noise ratio is almost /ero. (n other words, the

noise is so strong that the signal is faint. For this channel the

capacity ; is calculated as shown below.

(xample 7.*

This means that the capacity of this channel is /ero

regardless of the bandwidth. (n other words, the data is socorrupted in this channel that it is useless when received.

(xample 7 +

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#e can calculate the theoretical highest bit rate of a regular

telephone line. A telephone line normally has a bandwidth of

""" / -"" to "" /0 assigned for data

communications. The signal+to+noise ratio is usually !6%.

For this channel the capacity is calculated as shown below.

(xample 7.+

This means that the highest bit rate for a telephone line is

&.''! bps. (f we want to send data faster than this, we caneither increase the bandwidth of the line or improve the

signal+to noise ratio.

(xample 7 1

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#e have a channel with a !+</ bandwidth. The 3*= for

this channel is 6. #hat are the appropriate bit rate and

signal level$

(xample 7.1

. / 8 9 f

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.%/%8 9erfor&ace

U! to o#) #e ha'e discussed the tools ofU! to o#) #e ha'e discussed the tools oftras&itti$ data 1si$als2 o'er a et#or* ad ho#tras&itti$ data 1si$als2 o'er a et#or* ad ho#

the data "eha'e% Oe i&!ortat issue i et#or*i$the data "eha'e% Oe i&!ortat issue i et#or*i$

is the !erfor&ace of the et#or*:ho# $ood is it;is the !erfor&ace of the et#or*:ho# $ood is it;

We discuss +ualit, of ser'ice) a o'erallWe discuss +ualit, of ser'ice) a o'erall&easure&et of et#or* !erfor&ace) i detail i&easure&et of et#or* !erfor&ace) i detail i

3ha!ter <%3ha!ter <%

. / 8 1cotiued2

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.%/%8 1cotiued2

Bad#idth Bad#idth Bad#idth i Hert= Bad#idth i Hert= Bad#idth i Bits !er Secods Bad#idth i Bits !er Secods Relatioshi! Relatioshi!

Throu$h!ut Throu$h!ut

Latec, 1Dela,2 Latec, 1Dela,2

Bad#idth5Dela, 9roduct Bad#idth5Dela, 9roduct

>itter >itter

(xample 7 11

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The bandwidth of a subscriber line is & / for voice or

data. The bandwidth of this line for data transmission can be

up to 6 bps, using a sophisticated modem to change the

digital signal to analog. (f the telephone company improves

the quality of the line and increases the bandwidth to ' /,

we can send !!% bps.

(xample 7.11

Figure 7 17: Filling the lin4 with bits for cases 1 and

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Figure 7.17: Filling the lin4 with bits for cases 1 and

(xample 7 1

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#e can thin about the lin between two points as a pipe.

The cross section of the pipe represents the bandwidth, and

the length of the pipe represents the delay. #e can say the

volume of the pipe defines the bandwidth+delay product, as

shown in Figure 7.!'.

(xample 7.1

Figure 7 1*: Concept of bandwidth delay product

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Figure 7.1*: Concept of bandwidth-delay product

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7-2 DIGITAL TRANSMISSION7-2 DIGITAL TRANSMISSION

A computer networ4 is designed tosend information from one point toanother. &his information needs tobe con"erted to either a digitalsignal or an analog signal fortransmission. 5n this section, we

discuss the /rst choice, con"ersionto digital signals6 in the next section,we discuss the second choice,

con"ersion to analog signals.

. 4 / Di$ital to Di$ital 3o'ersio

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.%4%/ Di$ital5to5Di$ital 3o'ersio

I this sectio) #e see ho# #e ca re!reset I this sectio) #e see ho# #e ca re!resetdi$ital data ", usi$ di$ital si$als% Thedi$ital data ", usi$ di$ital si$als% The

co'ersio i'ol'es three techi+ues7 lieco'ersio i'ol'es three techi+ues7 lie

codi$) "loc* codi$) ad scra&"li$% Liecodi$) "loc* codi$) ad scra&"li$% Lie

codi$ is al#a,s eeded? "loc* codi$ adcodi$ is al#a,s eeded? "loc* codi$ ad

scra&"li$ &a, or &a, ot "e eeded%scra&"li$ &a, or &a, ot "e eeded%

. 4 / 1cotiued2

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.%4%/ 1cotiued2

Lie 3odi$ Lie 3odi$ 9olar Sche&es 9olar Sche&es Bi!olar Sche&es Bi!olar Sche&es Multile'el Sche&es Multile'el Sche&es

Bloc* 3odi$ Bloc* 3odi$ 8B@B 3odi$ 8B@B 3odi$ <B@/0B 3odi$ <B@/0B 3odi$

Scra&"li$ Scra&"li$

B<ZS 3odi$ B<ZS 3odi$ HDB6 3odi$ HDB6 3odi$

Figure 7 1+: ine coding and decoding

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Figure 7.1+: ine coding and decoding

Figure 7 : 8olar schemes 98art 5: 3;

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Figure 7.: 8olar schemes 98art 5: 3;


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Figure 7.: 8olar schemes 98art 555: anchesters;

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Figure 7.: 8olar schemes 98art 555: anchesters;

Figure 7.1: ipolar schemes: A5 and pseudoternary

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Figure 7.1: ipolar schemes: A5 and pseudoternary

Figure 7.: ultile"el: 1< and *)&

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Figure 7.: ultile"el: 1< and *)&

Figure 7.!: loc4 coding concept

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Figure 7.!: loc4 coding concept

Figure 7.#: =sing bloc4 coding #>% with 3-5 linedi h

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coding scheme

Figure 7.%: *>1 bloc4 encoding

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g > g

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Figure 7.7: &wo cases of *2 scrambling techni'ue

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g g '

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Figure 7.+: Components of 8C encoder

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Figure 7.!: y'uist sampling rate for low-pass andbandpass signals

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bandpass signals

Figure 7.!: <uantiation and encoding of a sampledsignal

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signal

(xample 7.1!

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64

#e want to digiti/e the human voice. #hat is the bit rate,

assuming ' bits per sample$

Solution

The human voice normally contains frequencies from " to

&""" /. 3o the sampling rate and bit rate are calculated as

follows.

p

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Figure 7.!#: &he process of delta modulation

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.%6%/ Di$ital5to5Aalo$ 3o'ersio

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$ $

Di$ital5to5aalo$ co'ersio is the !rocess of Di$ital5to5aalo$ co'ersio is the !rocess of

cha$i$ oe of the characteristics of a aalo$cha$i$ oe of the characteristics of a aalo$

si$al "ased o the ifor&atio i di$ital data%si$al "ased o the ifor&atio i di$ital data%

-i$ure .%6 sho#s the relatioshi! "et#ee the-i$ure .%6 sho#s the relatioshi! "et#ee the

di$ital ifor&atio) the di$ital5to5aalo$di$ital ifor&atio) the di$ital5to5aalo$&odulati$ !rocess) ad the resultat aalo$&odulati$ !rocess) ad the resultat aalo$

si$al%si$al%

.%6%/ 1cotiued2

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1 2

A&!litude Shift Ce,i$ A&!litude Shift Ce,i$

Biar, ASC 1BASC2 Biar, ASC 1BASC2 Multile'el ASC Multile'el ASC Biar, -SC 1B-SC2 Biar, -SC 1B-SC2

Multile'el -SC Multile'el -SC 9hase Shift Ce,i$ 9hase Shift Ce,i$

Biar, 9SC 1B9SC2 Biar, 9SC 1B9SC2 uadrature 9SC 19SC2uadrature 9SC 19SC2

3ostellatio Dia$ra&3ostellatio Dia$ra&

uadrature A&!litude Modulatiouadrature A&!litude Modulatio

Bad#idth for AM Bad#idth for AM

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Figure 7.!): inary amplitude shift 4eying

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Figure 7.!+: Concept of a constellation diagram

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Figure 7.#: Constellation diagrams for some <As

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.%6%4 Aalo$5to5Aalo$ 3o'ersio

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Aalo$5to5aalo$ co'ersio) or aalo$ Aalo$5to5aalo$ co'ersio) or aalo$

&odulatio) is the re!resetatio of aalo$&odulatio) is the re!resetatio of aalo$ifor&atio ", a aalo$ si$al% Oe &a, as*ifor&atio ", a aalo$ si$al% Oe &a, as*

#h, #e eed to &odulate a aalo$ si$al? it is#h, #e eed to &odulate a aalo$ si$al? it is

alread, aalo$% Modulatio is eeded if thealread, aalo$% Modulatio is eeded if the&ediu& is "ad!ass i ature or if ol, a&ediu& is "ad!ass i ature or if ol, a

"ad!ass chael is a'aila"le to us%"ad!ass chael is a'aila"le to us%

A&!litude Modulatio A&!litude Modulatio -re+uec, Modulatio-re+uec, Modulatio

9hase Modulatio 9hase Modulatio

Figure 7.#1: Amplitude modulation

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Figure 7.#: Fre'uency modulation

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7 4 BANDWIDTH UTILIZATION7 4 BANDWIDTH UTILIZATION

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7-4 BANDWIDTH UTILIZATION7-4 BANDWIDTH UTILIZATION

5n real life, we ha"e lin4s withlimited bandwidths. 2ometimeswe need to combine se"eral low-bandwidth channels to ma4e useof one channel with a largerbandwidth. 2ometimes we need to

expand the bandwidth of achannel to achie"e goals such as

pri"acy and anti-Bamming.

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.%8%/ 1cotiued2

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-re+uec,5Di'isio Multi!le(i$ -re+uec,5Di'isio Multi!le(i$

Wa'ele$th5Di'isio Multi!le(i$ Wa'ele$th5Di'isio Multi!le(i$

S,chroous TDM S,chroous TDM Statistical Ti&e5Di'isio Multi!le(i$ Statistical Ti&e5Di'isio Multi!le(i$

Ti&e5Di'isio Multi!le(i$ Ti&e5Di'isio Multi!le(i$

Figure 7.##: 0i"iding a lin4 into channels

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Figure 7.#%: Fre'uency-di"ision multiplexing

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(xample 7.1#

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Assume that a voice channel occupies a bandwidth of & /.

#e need to combine three voice channels into a lin with a

bandwidth of !% /, from %" to % /. 3how theconfiguration, using the frequency domain. Assume there

are no guard bands.

Figure 7.#): (xample 7.1#

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Figure 7.#7: $a"elength-di"ision multiplexing

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Figure 7.#+: 2ynchronous time-di"ision multiplexing

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(xample 7.1)

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Telephone companies implement T@< through a hierarchy

of digital signals, called digital signal -@30 service or digital

hierarchy. Figure 7.! shows the data rates supported byeach level. The commercial implementations of these

services are referred to as T lines.

❑ DS-0 service is a single digital channel of 64 kbps.

❑ DS-1 is a 1.544-Mbps service.

❑ DS-2 is a 6.12-Mbps service.

❑ DS- is a 44.!6-Mbps service.

❑ DS-4 is a 2!4.1!6-Mbps service.

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Figure 7.%#: Fre'uency hopping spread spectrum9F@22;

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Figure 7.%%: F@22 cycles

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Figure 7.%7: 0222

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7-5 TRANSMISSION MEDIA7-5 TRANSMISSION MEDIA

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7 5 TRANSMISSION MEDIA7 5 TRANSMISSION MEDIA

$e discussed many issues relatedto the physical layer in thischapter. 5n this section, we discuss

transmission media. &ransmissionmedia are actually located belowthe physical layer and are directly

controlled by the physical layer.$e could say that transmissionmedia belong to layer ero.

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Figure 7.%+: &wisted-pair cable

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Figure 7.)!: odes

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Figure 7.)#: (lectromagnetic spectrum for wirelesscommunication

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Ch07_2.ppt