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C H A P T E R 2

Data Transmission

2.1 ANALOG AND DIGITAL DATA

Data is the form of facts which can be in the form of voice, picture or plain numerical numbers. In

the world of communications, we differentiate data by the fact that whether it is in the analogous

form or in the discrete form. When data is in the form analogous to some happening continuously,

then we call it analog data. However, when it is in the discrete form represented by the presence or

absence of some fact, then it is called digital data. From the point of communicating the data from

one end of the globe to the other end, we try to use the digital data because it becomes much easier

for us to connect different kinds of software and hardware when they employ the digital data

transmission methods. These two types of data i.e. analog and digital as well as their associated

characteristics are being studied in this Section.

2.1.1 Analog Data

Figure 2.1

A sample of

analog data

Let us take an example to understand the analog data. Suppose you are sitting in a concert hall

where many musical instruments are being played by different players. Say one musician is play-

ingSitarand the other is playingTabla. The harmony of sound coming out from these two instru-

ments gives you the pleasure of listening. If there is any mismatch between the timing of the tune

of Sitar and the Tabla, you get the unpleasantness and thus consider it a noise rather than music.

This is an analog data communication. Both Sitar and Tabla are sending sound waves in the same

time

Sound

level

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Data Transmission 11

sequence and there is a rhythm and harmony between the two. So long as it is there, you enjoy

listening to it. The moment, there is some disturbance say noise in the mike system, you feel dis-

turbed. This analog signal in its simplest form may be shown in the form as shown in Figure 1.1.

Old music systems played conveys the songs in the analog form. Casettes on ordinary tape

recorders record music using analog system and play music in analog sound waves.

2.1.2 Digital Data

You would have noticed on the hockey playground, the referee blows a whistle and all the players

in the field understand the message instantaneously. The whistle is blown in short bursts of high

pitched sound like PEE, PE, PE, PE or it may have a long burst PEEEEEEE. Both of these whistle

calls convey different meaning to the players. The first one is an indication to the players to start

the game. The second long whistle is to stop the match immediately. The message conveyed by theburst of these sound energy in short pulses is very clear

Figure 2.2

A sample of

digital pulses

of sound

blown using

a whistle

to all the players. There is no chance of any confusion even if the distance of a player is large from

the referee. This is an example ofDigitalData Transmission. Short burst of sound energy are sent

by the sender to the receiver and the receiver is able to understand it clearly. This may be shown

pictorially as in Figure 2.2.

2.1.3 Different Characteristics of Analog and Digital Data

You would have noticed one drawback of analog signal. It is very sensitive to disturbances. But

the digital data communication is not. For example, referees whistle bursts can always convey the

correct meaning to all the players on the hockey playground even when there is noise due to the

spectators in the stadium. But little noise on the mike in the concert hall spoils the entire music

program.

There is another example where both digital and analog sound data is conveyed. On the

Republic Day Parade, the band playing the march past music contains both the pipes music and the

drum beats. The drum beats is giving a digital signal so that the soldiers can keep their steps in

tune and march properly. At the same time, the marshal sound of the band conveys the message

that they have to fight the war and keep their spirits high.

As the PCs work on digital principle and the telephone lines carry analog signal most

efficiently, therefore, we convert the digital pulses to analog form using a modem.

tPE PE PE PEEEE

Sound

from

whistle

Burst of sound

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12 Data Communication

2.1.4 Advantages of Digital Data Transmission over Analog

Data

Transmission

(a) The voice data, music and images (e.g. television, fax and video) can be interspersed to

make more efficient use of the circuits and equipment.

(b) Much higher data rates are possible using existing telephone lines.

(c) Digital transmission is much cheaper than analog transmission, since it is not necessary to

accurately reproduce an analog waveform after it has passed through potentially hundreds

of amplifiers on a transcontinental call. Being able to correctly distinguish a 0 from a 1 is

enough.

(d) Maintenance of a digital system is easier than maintenance of analog one. A transmitted bit

is either received correctly or not, making it simpler to track down problems.

(e) A digital signal can pass through an arbitrary number of regenerators (amplifiers in analog

systems) with no loss in signal and thus travel long distances with no information loss. In

contrast, analog signals always suffer some information loss when amplified, and this loss

is accumulative. Hence digital transmission can be made to have low error rate.

2.2 ANALOG MODULATIONS

2.2.1 Concept of Modulation

To modulate means to mix data signal onto a carrier and modify the characteristics of the carrier

for transmission in a communication network. A carrier is an electromagnetic wave that vibrates at

a fixed frequency.

Thus, if the input signal ism(t)and a carrier at frequencyfcto propagate a signals(t)whose

band width is centered onfc. Thenm(t)will modify the characteristics offcand the resultant signal

s(t)will be passed on the transmission medium. This change is known as modulation. When the

input signal is analogous, then we call it as analog modulation.

Analog data modulate the carrier by any one of the following methods:

(a) Amplitude modulation (AM)

(b) Frequency modulation (FM)

(c) Phase modulation (PM)

2.2.1 Amplitude Modulation

Amplitude modulation is the simplest form of modulation and is shown in Figure 2.3. Mathemat-

ically, the process is expressed as follows:

Where is the carrier andx(t)is the input signal carrying data. The parameter na, is

known as the modulation index. Modulation index is the ratio of the amplitude of the input signal

to the amplitude of the carrier signal. Thus the input signal .

s(t) = [1 + nax(t)]cos2fct (2.1)

Cos2fct

m(t) =nax(t)

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In the signal , the component 1 is the DC (direct current) component

that prevents the loss of information.

Figure 2.3 elaborates the concept of amplitude modulation technique. The carrier signal as

seen in part (a) of this figure has a much higher frequency than the information signal shown in

part (b). By imposing the lower frequency information signal on the carrier, the amplitude of the

resulting compound signal is made to vary in the form of information signal. Part (c) of the figure

shows the resulting modulated signal. Radio Programs transmitted via Akashvani on medium

wave and shortwave frequencies in India are examples of amplitude modulation.

Advantages

(a) Amplitude modulation is easy to implement.

(b) It can be used both for analog and digital signal.

Figure 2.3

Generation of

amplitude

modulated

signal

Disadvantages

(a) It is affected by the noise signal that may add up with the information signal. Electrical

noise causes this problem.

s(t) = [1 + nax(t)]Cos 2fct

Time, t

Amplitude

Amplitude Time, t

Amplitude

Time, t

(a) Carrier signal

(b) Information signal

(c) Amplitude modulated resulting signal

InformationEnvelope

AM Signal

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(b) As the strength of the signal decreases in a channel with distance traveled, it reaches a

minimum level unacceptable for adequate communications. Before signal strength goes

down to this extent, it must be amplified. But amplifiers add noise and adversely effect the

characteristics of the information signal.

Example 2.1

Derive an expression fors(t)when the modulating signal is represented byx(t)and is given by

.

Solution

The resultings(t)i.e. output amplitude modulated signal when carrier(fc) is being modulated by

the input analog signalx(t)is given by:

Using the trigonometric identity, this is further simplified as:

Here the modulated signal contains the frequencies(fc+ fm)and(fcfm). It means, the band width

for the modulated signal will be from(fcfm)to(fc+ fm).

(fc - fm) is called the lower side band and (fc + fm) is called the upper band.

Suppose the voice frequency is in the range of 300 Hz and 3000 Hz and it modulates a carrier

of 60 kHz. The resulting signal contains the upper side band of 60.3 to 63 kHz and a lower side

band of 57 to 59.7 kHz.

Power Transmission

The relationship for the power transmission is given by the following relationship:

WherePtis the total transmitted power ins(t)andPcis the transmitted power in the carrier. Alsonais the modulation index and from equation [2.3], it is natural that nashould be large enough for

getting optimum value ofPt, which carries the information. However,nashould be less than 1.

In Single Side Band (SSB), only one of the band frequencies is used for transmitting the signal.

The other band as well as the carrier is filtered out. Therefore, less power is required because no

power is used to transmit the carrier or the other side band.2.2.3 Frequency Modulation

In frequency modulation, the modulated signals(t)is represented as the following:

Cos2fct

s(t) = [1 + na cos2fmt]Cos 2fct (2.2)

s(t) =cos2fct+n

a

2Cos 2(fcfm)t+

na

2Cos 2(fc+fm)t

Pt=Pc1 +

na2

2

(2.3)

s(t) =Ac Cos[2fct+ (t)]

where (t) =nfm(t)andnfis the frequency modulation index

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Figure 2.4 illustrates the principle of Frequency Modulation. An FM signal has a constant

amplitude but varies in frequency over time to convey information. Part (a) and (b) of this figure

show that the carrier has a frequency much higher than the information signal it has to transport.

After imposing the lower frequency information signal of the carrier, the frequency of the result-

ing compound signal varies to match the form of the information signal. Part (c) of this figure

shows the resulting modulated signal.

Figure 2.4

Generation of

frequency

modulated

signal

Advantage

Frequency modulated wave is least effected by the noise due to electrical disturbance.

Disadvantages

(a) Frequency signal has a wide spectrum of frequencies and therefore needs much higher

band width than amplitude modulation.

(b) The number of FM signals one can transmit over a channel with a fixed total band width is

smaller than the number of AM signals one can transmit through the same medium.

2.2.4 Phase Modulation

In phase modulation, the modulated signal is expressed in the following form:

Time, t

Amplitude

Amplitude Time, t

Amplitude

Time, t

(a) Carrier signal

(b) Information signal

(c) Frequency modulated resulting signal

s(t) =Ac Cos[2fct+ (t)] (2.4)

where (t) =npm(t).Herenpis the phase modulation index andAcis the carrier

index

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Phase modulation uses at least two analog signals. The first signal is a carrier, and the other

signals modify the carrier signal to convey information. In Phase modulation, the shape of the

carriers signal curve is made to change at given points in time. Figure 2.5 shows the process of

phase modulation. Both signals are sine waves that have the same fixed frequency and amplitude.

They are however offset from each other. The two cross the amplitude reference line at different

times are therefore, have different phase.

Figure 2.5

Carrier and

information

signals 180

degrees dif-ferent in

phase

The difference in phase between the two sine wave is a phase angle. As seen in the above fig-

ure, the two signals are offset by one-half cycle or 180 degrees out of phase. The resulting com-

pound phase modulated signal is shown in Figure 2.6.

Figure 2.6

Phase modu-

lated signal

Advantages

Phase modulation provides the signal modulation that allows computers to communicate at higher

data rates through telephone system.

Amplitude Time, t

Amplitude

PhaseDifference

Degrees180

Carrier signal

Time, t

Information signal

A

mplitude Time, t

PhaseChange

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Disadvantages

Phase modulation requires two signals with a phase difference between them. A reference pattern

and a signal pattern are both required.

Uses

This technique is used to convey colour information in colour television broadcasts.

Example 2.2

Derive an expression fors(t)if(t) is the phase modulating signal AssumeAcas 1.

Solution

The modulated signal is given as follows:

hereAc= 1 and

Substituting these values we get the expression s(t) as:

2.3 DIGITIZATION

Digitization is the process of converting any continuously varying source of input, such as the

lines in a drawing or a sound signal, into a series of discrete units represented (in a computer) by

the binary digits 0 and 1. A drawing or photograph, for example, can be digitized by a scanner that

converts lines and shading to combinations of 0s and 1s by sensing different intensities of lightand dark. Figure 2.7 shows an analog to digital converter IC chip.

Figure 2.7

Analog to

digital

converter

2.3.1 Digitization Process

The process of digitizing an analog signal starts by dividing the original signal into uniformly

spaced samples as shown in Figure 2.8. The amplitudes of the sample pulses rise and fall with the

amplitude of the original signal. The original signal is separated into individual pulses or samples

each sample having a different amplitude based on the amplitude of the original signal. At the

np Cos2fmt.

s(t) =Ac Cos[2fct+ (t)]

(t) =np Cos2fmt

s(t) =Cos[2fct+ np Cos2fmt]

A-D Converter

Analog In Digital Box

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receiving end, these samples are used to reconstruct the original signal. The more frequently the

samples are taken, the more accurate is the reconstructed waveform. Note the truth of this state-

ment in Figure 2.8(c).

To determine the minimum number of samples to use to replicate the original waveform is

given by Nyquist theorem . According to this theorem, for a given signal, fs, the minimum sam-

pling rate (Nyquist Sampling Rate Sr) to assure accurate recovery of the signal at the receiving end

is twice the frequency of the highest sine wave element sin(2fs) of the original signal, orSr = 2[sin(2fs)]

Figure 2.8

Sampled

signal

Original signals are sampled at ranges at or above the minimum sampling rate to assure that

the original signal is accurately replicated. If the sampling rate were less than twice the highest

fundamental sine wave frequency, then a distortion known asaliasingorfold-overoccurs.

(a) Sampled Signal

(b) Reconstructed Waveform (So)

(c) Reconstructed Waveform Using Twice as Many Samples

1/Sr Samples

fs

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Let us consider sampling the voice signals on the telephone lines, which contain signals from

300 to 3 kHz. Essentially, the sampling process causes mixing, which is similar to that used for

regular amplitude modulation (AM) to result. This process creates the sum and difference fre-

quencies as well as the original signals that were mixed. For a sampling circuit, these are Sr,

sin(2fs), Sr-sin(2fs), and Sr+sin(2fs).Filters are used to remove all but the difference and original fs signals. If the sampling rate is

higher than 2[sin(2fs)], there is a gap between one group and the other. If the sampling rate isless than this value, then fold-over error occurs. (Refer to Example 2.3.)

Example 2.3

Show the differences between sampling a voice channel (300 Hz to 3 kHz) i.e. fs using sampling

rates at and below 2[sin(2fs)].Solution

The minimum sampling rate is twice the highest frequency component of fs, or 23 kHz = 6 kHz.Mixing the voice band with 6 kHz and removing the higher-frequency elements produces the

original voice channel (300 Hz to 3kHz) and the difference band 6 kHz - (300 Hz to 3 kHz) = (3 to

5.7 kHz).

Figure 2.9 (c)

Aliasing

(fold-over)

distortion

These two bands are shown in Figure 2.9. Using 4.5 kHz for Sr as an arbitrary value that is less

than 2[sin(2fs)] results in the original voice band and a difference frequency band of 4.5 kHz -(300 Hz to 3 kHz) = 1.5 kHz to 4.2 kHz). (See part b of the figure) Note that there is a fold-over of

the original band and the difference frequency band from (1.5 to 3 kHz)

Baseband Differencef(Sr) - f(s)

0 300 3 5.7

freq. (kHz)

(a) Sampling Rate (Sr) = 2 x Signal Rate (s)

(b) Sampling Rate < 2 x Signal Rate

300 3

Baseband

Difference

freq. (kHz)

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2.3.2 Pulse Code Modulation (PCM)

Pulse Code Modulation abbreviated as PCM is a digitizing process in which an analog or continu-

ous signal is represented in digital or discrete form.

Quantization

Quantization is the process of approximating sample levels into their closest fixed value. The val-

ues are preselected and since they are fixed, they are easy to encode. The new waveform called the

quantized waveform has either quantum changes in amplitude or no change in amplitude.

Given a signal, fs, with peak voltage point of Vh and Vl, the size (S) of a quantum step is

determined by the following relationship:

S = (Vh - Vl)/nHere, n is the number of steps between Vh and Vl. Figure 2.10 shows the relationship between fs

and a quantized example. The quantized levels are those fixed levels that are the nearest to fs at the

point the sample is taken.

Figure 2.10

Quantized

signal

Pulse Amplitude Modulation (PAM)

The process of sampling and quantizing a signal is a form of pulse amplitude modulation (PAM)

where the samples produce pulses of varying amplitudes.

However, when these amplitudes are restricted to discrete quantized values and assigned spe-

cific binary codes which are to be transmitted, then a technique called pulse coded modulation is

being used.

S/2

S

Step Size

Time of a Step

Vl

Vh

fs

Quantum Jump

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2.3.3 Converting Voice to Ones and Zeros

The varying sounds of human speech must first be transformed into discrete pulses to be sent by

digital means. The device for making this transformation is called a codec(Coder Decoder), a

name derived from its function of coding an analog signal into digital form at the sending end and

then decoding it back to analog form at the receiving end. These are mainly used at exchanges for

routing calls over main trunk lines. A codec accomplishes its tasks in three stages.

Stage 1

In the first stage, codec does the sampling of the amplitude of the analog signal at very short

intervals. See Figure 2.11(a). The voltage of the signal is measured at discrete intervals.

Stage 2

This is the stage of quantizing or assigning decimal values to the amplitude samples. The result is

known as pulse amplitude modulation (PAM). The value of each voltage sample is quantized, or

assigned a specific measurement (bars of varying height in Figure 2.11(a), which is then converted

to a digital number expressed in the 1s and 0s of binary code. The digital numbers can then be

transmitted.

Figure 2.11(a)

Digitizing the

voice

Stage 3

In this stage, known as pulse code modulation (PCM) , the voltage values are converted, or coded,

into binary numbers for digital transmission. Encapsulated in eight-bit bytes, amplitude samples

zip through a communications link as a stream of digital bursts. Figure 2.11(b) shows the digital

encoding of speech signal.

Sampling Quantizing

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Figure 2.11(c)

PCM Trans-

mitter Block

Diagram

PCM Decoder

Figure 2.11(d)

PCM Decoder

A PCM decoder reverses the process of converting the digital to analog equivalent signal. Thedigital data are fed serially into the decoder. Each one of the data bits is reshaped to remove dis-

tortions caused by the transfer along the interconnecting medium used. After shaping, the data bits

are fed into a digital to analog converter to produce the quantized samples they represent. These

samples are held and filtered to recreate the original signal, fs. The differences between the origi-

nal signal and the recreated one result from quantization error and any possible bit errors that

might occur in the transmission. [See Figure 2.11(d)]

2.3.4 Sampling Rate

In order to make sure that speech remains intelligible, a great many samples must be taken. The

sampling rate as per Nyquist Theorem, must be twice that of the highest significant frequency to

be transmitted. Thus for a voice signal, with an upper frequency limit of 4,000 hertz over the

phone system, the codec must take 8,000 samples per second.

The speech lost between samples, known as the Nyquist interval, is unnoticeable

when the signal is decoded at the receiving end.

The sampling rate and the number of quantizing levels determine the bit rate of the digital

communications channel. To convey 128 discrete volume levels requires seven binary data bits.

The Pulse Code Modulation (PCM) component must generate all seven data bits each time the

Low

Pass

Filter

Sampler Quantizer EncoderADC

sin(2 fs)(Input Signal)

Binary Codes

Binary Decoderand Hold

Restorer DACand Filter

Sample

Code

Binary

(Replicated)fs

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Pulse Amplitude Modulation (PAM) component performs a sample of the analog signal. To oper-

ate the system, you need to sample at a rate of 8,000 samples per second and generate seven data

bits each time. This produces 78,000 = 56,000 bps.

Figure 2.11(e)

Recovering

the analog

signal

Besides, the 56 Kbps of digitized voice, these systems provide an additional 8 Kbps for the

system control. The total bit rate is 64 Kbps. For economies of scale for long-distance communi-

cations require vendors to combine several 64-Kbps channels into one channel of larger capacity.

Quantization Error

By sampling a signal of limited bandwidth at twice its highest frequency, which for speech istaken as 4000 Hz (8000 times per second), it is possible to reproduce the speech signal perfectly.

However, the process of assigning a discrete binary number to each sample introduces an error

known as the quantization error. This unavoidable error is the difference between the actual value

of the analog sample and the nearest value encoded by one of the binary numbers. The average

quantization error is a measure of the trade-off between using a scale with more bits per sample

(which yields smaller steps) versus using a coarser scale that requires fewer bits. The standard

scale used in USA is an 8-bit nonlinear scale known as -law 255. The required bit rate or digitalbandwidth for a PCM encoded speech signal using-law 255is then

8 bits 8000 samples/second = 64,000 bits/second.PCM encoding according to-255 is standard throughout the USA. European countries use a dif-

ferent encoding algorithm known as A-law.

2.3.5 Natural Sampling

As seen in Figure 2.12, samples are created by generating a short pulse at the specific time. The

amplitude of the pulse is determined equal to the amplitude of the signal at the time of the sample.

The width of the pulse is designated tpand the time period pulses (1/Sr) isTr. The shapes of the

pulses themselves come in two forms. One is called Natural Sampling, in which the peak of the

Coded Values Reconverted Signal

10 1011 0010 1010100 1 10101010

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pulse follows the signals actual shape. The second pulse form isFlattopshape in which the peak

amplitude is held flat by the sample and the hold circuit. (See Figure 2.12 part (d). For the flattop

sampling, the reconstructed signal (So) for a given signal f(s) is represented by the relationship:

where tp is the time period for the sampling pulse and Tr is the reciprocal of sampling rate (Sr).

Since tp/Tr is the duty cycle of the sampling signal, the relationship of So to sin(2fs) is a directfactor of that duty cycle.

Example 2.4

A 3.6 cosf signal is naturally sampled at the rate of 56 kHz using 1.25 microsecond sampling

pulses. What is the value of the reconstructed output signal?Substituting the values in the equation 2.5 we have

tp = 1.2510-6, Tr = 1/(56103)Therefore:

So = (1.25 10-63.6 cosf)/[(1/56 103)]Calculating we get So = 0.252 cosf

2.3.7 Sample and Hold

In order to reproduce the waveform accurately, we use the method of Sample and Hold. A sample

pulses amplitude is detected and that value retained until the occurrence of the next sample pulse.

(See Figure 2.13). For this method to be effective, the hold time between samples (TH) is rela-

tively small compared with the time period of the original signal.The most common method used for sample and hold circuits is to employ a capacitance at the

output of the buffer amplifier. The capacitor is charged to the sample pulse value. When the

amplitude falls to zero between pulses, the capacitor remains charged to the pulse value. The next

sample pulse causes the capacitor to charge or discharge to that value. Again the value is held until

the next pulse arrives. Figure 2.14 shows the circuit diagram for sample and hold waveform cre-

ation for accurate reproduction of the waveform.

2.3.8 Coding a Quantized Signal

The range of voltages for signalfsas illustrated in Figure 2.15 is divided into discrete quantized

steps (S). The signal is sampled at each step, with the resulting amplitude of the samples coded

into binary values. The binary equivalents are actually associated with analog values midway

between step amplitudes to minimize errors. These binary codes are shown at the bottom of thefigure. The original waveform is transmitted as a serial stream of binary bits representing the

quantized levels of each of the samples. At the receiving station the binary bits are decoded into

the quantized samples and the original signal is reproduced from the resulting samples.

So=tp

TrSin(2fs) (2.5)

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Figure 2.12

Sampling

types

t

V(t)

(a) Input Signal Sin(2 fs)

(b) Sampling Pulses at Rate Sr. (Pulse Exaggerated for Clarity)

V(t)

t

(c) Natural Sampling

V(t)

t

V(t)

(d) Flattop Sampling

t

Tr tp

Tr = 1/Sr

So

Leading

edge edge

TrailingMidway

Recovered Signal So = (tp/Tr) x sin(2 fs)

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Figure 2.13

Sampling and

Hold wave-

form

Figure 2.14

Sampling and

Hold Circuit

2.3.9 Differential Pulse Code Modulation (DPCM)

This method of digitization consists of out putting the difference between the current value and the

previous one and not the complete amplitude of the signal at the time of sampling. In this method

lessor number of bits would be needed because we are only going to see the difference of the level

and not the absolute value of the sample amplitude.

V(t)

time

Sample Hold

sin(2 fs)

V in

V out

Chold

Sample Time

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Figure 2.15

Codification

of a Signal

Figure 2.16

Delta modu-

lation method

of

Codification

of a Signal

2.3.10 Delta Modulation (DM)

Figure 2.16 shows the technique employed in Delta Modulation. In this method, each sampled

value differs from the predecessor value by either +1 or -1. A single bit is transmitted telling

001

3

010

2

011

1

100

0

101

1

110

2

V

3Code

010 011 100 101 110 110 110 100 100 100 100 011 011 100 100Binary Code

Time

0

5

10

15

1 1 1 1 1 1 1111110 0 0 0 0 0 0 0 0 0

Sampling

interval

Digitizationlevels

Consecutive samplesalways differ by Signal changed too

rapidly for encoding

+_ 1

to keep up

Bit streamsent

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whether the new sample is above or below the previous one. This technique assumes that small

level changes between consecutive samples occurs. This method will not work if the signal

changes too fast. If it is so, the information is likely to be lost.

2.4 DIGITAL MODULATION AND DEMODULATION

Modulation is the process of converting a digital signal from a computer into an analog signal the

telephone system will accept. When you pick up the phone while your computer modem is com-

municating, or while you are sending a fax from you fax machine, you hear the sound of digital

information that has been converted to analog signals. At the other end of the connections, whether

it be across town or across the world, another modem interprets those analog signals the telephone

system has conveyed and converts them back into digital form so the receiving computer can

understand them.

Due to the fact that both attenuation and propagation of speed are frequency

dependent, it is undesirable to have a wide range of frequencies in the signal. But square

waves in digital data have a wide spectrum and are subject of strong attenuation and delay

distortion. Each square wave consists of series of Fourier components. Each component is

attenuated by a different amount which results in a different Fourier spectrum at the

receiver and hence a different signal. These adverse effects make baseband (DC) signal-

ing unsuitable except at slow speeds and over short distance.

2.4.1 Amplitude Shift Keying

In ASK, the two binary values are represented by two different amplitudes of the carrier fre-quency. Commonly, one of the amplitudes is zero. Thus, one binary digit is represented by the

presence, at constant amplitude of the carrier and the other by the absence of the carrier. The

resulting signal is:

A cos(2fct) binary 1s(t)=

0 binary 0

In Figure 2.17 (b), there is a carrier frequency generated for the digital pulse with a value 1 and

no carrier is generated when the digital pulse amplitude is zero.

2.4.2 Frequency Shift Keying

In frequency shift keying, the two binary values are represented by two different frequencies near

the carrier frequency. The resulting signal is represented by the following:

A cos(2f1t) binary 1s(t)=

A cos(2f2t) binary 0FSK is less susceptible to error than ASK. On voice grade lines, it is typically used up to 1200

bps.

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Figure 2.17 (c) shows the conversion of the digital pulses into two different frequencies. For

magnitude 1, the frequency is high and for value 0, the frequency is low.

Figure 2.17

Different

modulation

techniques

used for digi-

tal signal

(a) Binary

data

(b) ASK(c) FSK

(d) PSK

Figure 2.18 shows the FSK system. The two pairs of frequencies are characteristic of modems

that transmit at 300 bps, using frequency-shift keying in full duplex mode (sending and receiving

at the same time.) Operating within the 4,000 hertz allocated for the telephone voice channel, the

modem that originates the session transmits data by generating a carrier wave at either 1070 hertz

(for 0s) or 1270 hertz (for 1s). Its counterpart transmits 0s at 2025 hertz and 1s at 2225 hertz. With

FSK, only one bit is encoded per frequency shift.

Bell-103 modems require matching modes for proper communications. Because ofthe difference in transmit and receive frequencies, one modem must operate in the answer

mode. The other modem must be in the originate mode. If the modes do not match, both

modems will try to transmit and receive using the same frequencies and no data can move

between them.

Phase changes

(a)

(b)

(c)

(d)

10 0 1 1 0 0 1 0 0 1 0 0

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Figure 2.18

FSK system

with full

duplex mode

The above method is used in Bell-103 modem system. The modem that initiates a communi-

cations link is in the originate mode, and the remote modem that responds to the initiation and

completes the communications link is in the answer mode. The receive frequencies for the answer

mode are the same as the transmit frequencies for the originate mode. Similarly, the receive fre-

quencies for the originate mode are the same as the transmit frequencies for the answer mode. (See

Figure 2.19)

Figure 2.19

Bell-103

modem using

FSK system

with different

frequencies

for transmit-

ting and

receiving

1 1 1 0 0 1 1 1 0 0

0 1 01 0 1 11 0 1

2,225 Hz

2,025 Hz

1,270 Hz

1,070 Hz

0 Hz

4,000 Hz3,400 Hz

250 Hz

Voice Channel

TransmitBandwidth Bandwidth

Receive

Frequency

(Hz)Space1070 1270

Mark Space2025 2225

Mark

(a) Bell-103 modem originate-mode signal frequencies

Telephone System Bandwidth

Mark2225

Telephone System Bandwidth

Bandwidth

Space1070

(b) Bell-103 modem answer-mode signal frequencies

(Hz)

Frequency

Mark1270

Receive

Space2025

BandwidthTransmit

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2.4.3 Phase Shift Keying (PSK)

In phase shift keying, the phase of the carrier signal is shifted to represent data. In Figure 2.17 (d),

the phase of the wave is shifting depending on the digital signal is changing from zero to one, or

from one to zero.

In the phase shifting keying, a binary 0 is represented by sending a signal burst of the same

phase as the previous signal burst. A binary 1 is represented by sending a signal burst of opposite

phase to the preceding one. In this method, the phase shift is with reference to the previous bit

transmitted rather than to some constant reference signal. The resulting signal is mathematically as

shown below:

A cos(2fct+) binary 1s(t)=

A cos(2fct) binary 0

Figure 2.20

(a)

Phase shift

Keying

Phase shift keying or PSK uses a transition or shift from one phase to another to encode data.

As in other state-transition encoding schemes, the presence or absence of a transition can be used

to encode data. Figure 2.20 shows an example of PSK in which a 1 is represented by the presenceof transition (in this case, a 180 phase shift), and 0 is represented by the absence of a transition (as

in no phase shift). This is the case of Binary Phase Shift Keying. (See Figure 2.20 (a))

The straightforward phase-shift keying, however, is useful only when each phase can be

measured against an unchanging reference value, so a more sophisticated technique called differ-

ential phase shift keying or DPSK is used. In DPSK, the phase of the carrier wave is shifted to

represent more than two possible states, and each state is interpreted as a relative change from the

state preceding it. No reference values or timing considerations are required, and because more

than two states are possible, more than one binary digit can represent each state.

Generation of Binary Phase Shift Keying (BPSK)

The BPSK is created using the balanced modulator. The circuit allows the phase of a carrier sine

wave (fc) to be altered by a modulating digital signal. Figure 2.20(b) is an example of a trans-former balanced modulator, which gives the concept behind the BPSK modulator.

The reference frequency (fc) is applied to T1 and is coupled through the secondary winding

to the diodes D1 and D2 on the high side and D3 and D4 on the return side. The digital data stream

is applied to the center taps of T1s secondary and T2s primary. The current level supplied by the

digital circuits is enough to cause the diodes to switch on when the correct polarity is applied. As a

Time

0 1 1 1

Voltage

Data 1 01

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Figure 2.20(b)

BPSK Modu-

lator and

Phasor

diagram

point of reference, a logic 1 is selected to be positive at input A and negative at input B. This for-

ward biases diodes 1 and 3 and switches off diodes 2 and 4. The signal coupled from T1s sec-

ondary is not large enough by itself to switch the diodes on, but once the diodes are on (from the

digital input), this signal easily passes through to the primary of T2. The logic 1 in this case,

causes fc to be passed to T2 and coupled so that the phase of the output signal is the same as the

input signal.

Reversing the polarity at inputs A and B to represent a 0 switches on D2 and D4 while back

biasing D1 and D3. This time fc is directed to the opposite end of T2. The output signal coupled to

T2s secondary is 180 out of phase with fc at the input. A vector diagram can be drawn to illus-

trate the phases representing a logic 1 and a logic 0. Zero degrees of phase lies on the positive side

of the x-axis and is used as a reference for any phase generated by the modulator. Logic 1 in this

case generated a signal that has the same phase as the input. This is represented as a vector at 0 inthe phasor diagram in Figure 2.20 (b). This means that the difference between the reference (fc at

the input) and the output is 0. Similarly, the phase of the signal for a logic 1 (180 out of phase

with reference, fc) is shown lying on the negative x-axis. The band width of the circuit driven by

the modulator is large enough to pass the carrier frequency whose phase is shifted at the modulat-

ing rate.

T1fc

Return

Binary inA

Return

BBinary return

T2fout

D1

D2

D4

D3

Logic 0 Logic 1

180 0

- sin ct sin ct

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Suppose there is a data rate of 2400 bits per second, using a non-return to zero signal format,

has a fundamental sine wave of 1,200 Hz. If the carrier frequency to be passed by the modulator is

1,650 Hz, then the band width of the system driven by the modulator must he sufficient to pass

both the 1,200 Hz switching rate and the 1,650 Hz carrier signal. Thus the minimum band width is

1,650 - 1,200 or 450 Hz. The normal voice band width of the telephone lines is 300 to 3000 Hz

which is quite adequate to handle the modulators signals.

BPSK Phase Detector

The phase detector is similar to the modulator. The capacitors and resistors replace the second

transformer found in the modulator and form a peak detector circuit.

The clock is fed into the one balanced modulator and 90 phase shifter. The shifted signal is

presented as the fc input to a second balanced modulator. [See Figure 2.20 (c)]. The amplitudes ofthese signals are large enough to switch on the diodes on. The incoming data stream is applied as

the other input to the balanced modulator. The output of the modulator is altered to perform the

function of an FM detector or phase detector.

The capacitors and resistors that replace the secondary transformer found in the modulator

of Figure 2.20 (b). The capacitors charge to the peak value of the applied sine wave signals and in

combination with the resistors, filter out the AC sine wave components. Signal fc is applied to

input A and B with sufficient amplitude to bias alternating pairs of diodes on for the positive and

negative alternations of fc. The positive alternation of fc switches on D1 and D4 while the nega-

tive half switches on D2 and D3. The effect is to pass the incoming signals (fin) to the RC circuit.

The amount the capacitors will charge to depends on the phase relationship between fin and fc.

Figure 2.20(c)

BPSK Phase

Detector

2.4.4 Quadrature Phase Shift Keying (QPSK)

More efficient use of bandwidth can be achieved if each signaling element represents more

than one bit. For example, instead of a phase shift of 180 degree as allowed in PSK, a common

encoding technique, known as quadrature phase shift keying (QPSK) uses phase shifts of multi-

ples of 90 as shown below:

T1fin

Ret

Ret

BClock

voutD1

D2

D4

D3

A

C1

C2

R1

R2

C

D

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Figure 2.21

QPSK tech-

nique used

for high

speed

modems

A cos(2fct+ 45) 11A cos(2fct+ 135) 10s(t)=

A cos(2fct+ 225) 00A cos(2fct+ 315) 01

Thus, each signal element represents two bits rather than one. This scheme can be extended. It

is possible to transmit bits four at a time using sixteen different phase angles. Further, each angle

can have more than one amplitude. This is shown in Figure 2.21.

2.4.5 Differential Phase-shift Keying

A modem employing differential phase-shift keying can encode eight or more bits of data at two

bits (one dibit) per shift on one frequency, compared with the single bit per frequency change ofthe FSK method. This is accomplished by manipulating the phase of the carrier wave. As shown in

Figure 2.22(a), a waves cycle may be measured form peak to peak, from zero point to zero point

or from through to trough. A cycle may be divided into phases, or points, typically expressed in

terms of degrees. For example, 0 degree represents the starting point of the cycle, 90 degree rep-

resents the one-quarter point, 180 represents the halfway point, etc.

0000

0001

0010

90

0

0110

0111

0100

180

1100

1101

1110

270

1010

1011

1000

00110101

1111 1001

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Figure 2.22(a)

Differential

Phase shifting

Figure 2.22(b)

Differential

Phase shift

keying

method

Table 2.1 V.26 Dibit Encoding

One Cycle

90 180 270 360 0 18090 3602700

One Cycle

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Dibit Encoding Phase Shift in degrees Phase Shift in degrees(CCITT) (AT & T)

00 0 45

01 90 135

11 180 225

10 270 315

In the DPSK method shown in Figure 2.22 (b), a carrier may in effect be split into four waves,

each with the frequency of the original but starting at a different phase of the original cycle. Four

phases can represent all possible combinations of two bits, the modem merely chooses the appro-

priate phase shift for the dibit to be encoded as shown in Table 2.1. The shift is always calculated

in relation to the starting point of the previous cycle. For example, if the previous cycle began at apeak, a 180 shift to encode the dibit 11 would jump two phases to start at a trough.

Binary (Dibit) Coding by Phase Shift

To encode a series of dibits, some modems first generate two complete cycles, measured from the

zero line. At the end of the second cycle, a 90 shift is made to encode the first dibit (01). Encoding

another 01 produces a wave cycle that represents a one-phase shift from the 01. The next shift, to

encode the dibit 11, jumps 180, or two phases, from the previous cycle to the end. Note that after

each shift, two full cycles are completed to confirm the new starting point. (See Figure 2.23) This

figure shows the phase change description for dibit encoding.

Figure 2.23

Binary (Dibit)

Coding by

Phase ShiftTo encode

01 01 11 10 00

requires a shift of

Dibit Table

00 0

9001

18011

27010

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2.5 TRANSMISSION IMPAIRMENTS

Communication signal consists of varying a voltage with time to represent an information stream.

If transmission media were free from limitations or were perfect, then the receiver would receive

exactly the same signal as the sent one. However, the media far from perfection, the digital signal

received at the receiving end may lead to error. The main impairments or limitations can be due to

following reasons:

(a) Noise in the surrounding of the media

(b) Attenuation caused by the media to the transmitted signal

(c) Phase distortion

2.5.1 Noise

Noise is the unwanted energy from sources other than the transmitter. Noise may be of the fol-lowing four types:

(a) Thermal noise

(b) Intermodulation noise

(c) Crosstalk

(d) Impulse noise

Thermal Noise

Thermal noise is due to thermal agitation of electrons in a conductor due to heat. It is present in all

electronic devices and the transmission media. It is a function of temperature.

The noise is considered to be independent of frequency. Thus, the thermal noise, in watts,

present in a bandwidth of W hertz can be expressed as follows:

N = kTWwhere

N = Noise power density

k = Boltzmanns constant = 1.380310-23 Joules per KT = Temperature in degree Kelvin

Expressed in decibel-watt, we can write

N = 10 log k + 10 log T + 10 log W

Substituting the values of k, we get

N = -228.6 dBW + 10 log T + 10 log W

The thermal noise is unavoidable. The only remedy is to keep the temperature low. For this reason,

we need to use the air conditioned environment.

Intermodulation noise

Intermodulation noise is caused when there is some nonlinearity in the transmitter, receiver, or

intervening transmission system. In a nonlinear system the output is a more complex function of

the input. Such a nonlinearity causes the component malfunction or the use of excessive signal

strength.

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Crosstalk

Crosstalk is caused by inductive coupling between two wires that are close to each other. When we

speak on a telephone, you can hear another conversation in the background. This is the crosstalk.

Crosstalk can occur by electrical coupling between nearby twisted pair or coaxial cable lines

carrying multiple signals. Crosstalk can also occur when unwanted signals are picked up by

microwaves antennas.

Impulse Noise

Impulse noise is caused by the spikes on the power line or lightning discharge occur due to thermal

storm. This may cause short clicks and crackles with no loss of intelligibility in the case of analog

signal. However, impulse noise is primary source of error in digital data communication. For

example, a sharp spike of energy of .015 second duration may not destroy any voice data but

would wash out about 70 bits of data being transmitted at 4800 bps.

2.5.2 Attenuation

Attenuation is the loss of energy as the signal propagates outward. If the attenuation is too much,

the received signal may not be detected and the signal may fall below the noise level. In some

cases, the attenuation properties of a medium are known so amplifiers can be put in to try to com-

pensate for the frequency-dependent attenuation. However, this approach can not restore the signal

exactly back to its original shape.

The attenuation distortion is much less of a problem with digital signals.

Example 2.5

A fiber optic system requires 5 micro watts of power for proper functioning at the receiver. The

cable is 10 km long and has an attenuation loss of 2 dB/km. There is a loss of 2 dB at both the

source and the receiver. Calculate the required level of optical power at the optical source.

Solution

We know that the loss in the = dB loss per km number of km length ofcable the cable

= 210 = 20 dB.Loss at the Source = 2 dB (given)

Loss at the Receiver = 2 dB (given)

Therefore total loss = 20 + 2 + 2 = 24 dB

If X is the transmitted power then, the received power = 24 dB downWe know that at every 3 dB loss, the power becomes half the value.

Therefore, if X watts is the transmitted power, then the received power = 3 + 3 + 3

+ 3 + 3 + 3 + 3 + 3 = 24 dB. (down)

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It is given to us that for proper working of the receiver, the received power at the

receiver should be = 510-6 watts.As indicated above, if X is the transmitted power, then the total received power

after losses = [X/(28)] watts = 510-6 wattsTherefore X = 510-6 28 = 1027 10-6 = 640 micro watts.

Example 2.6Why the digital communication systems are more resistant to channel noise than analog systems.

Solution

Digital communication systems are more resistant to channel noise because of the following rea-

son:

The detector in the digital system needs to find the presence and the absence of a pulse and

therefore even if there is noise, the detection is not very difficult. Moreover, noise is not a static

quantity. Sometimes, noise signal is large some times small but the pulses are of constant magni-

tude. Hence, their detection becomes easier. It is similar to the case of the sound of the whistle

blown by a referee in a play ground can be easily discriminated even when there is noise in the

field.

2.5.3 Phase distortion

Every digital pulse is made up of the fundamental frequency and the higher harmonics. This is

known as the different Fourier components of the digital wave which combined together give the

shape of the square wave. In the case of phase distortion, the impairment of the signal in a guided

transmission media is dependent on the frequency which travels through the media. This is due to

th fact that the velocity of propagation of a signal through the guided media varies with frequency.

Thus the various Fourier components of a square wave signal will arrive at the receiver at different

times.

For digital data, fast components from one bit may catch up and overtake slow components

from the bit ahead, mixing the two bits and increasing the probability of incorrect reception.

Delay distortion is particularly critical for digital data. Because of delay

distortion, some of the signal components of one bit position will spill over into other bit

positions, causing intersymbol interference, which is a major limitation to maximum bit

rate over a transmission control.

=1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

=

x

(28)

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Equalization Techniques

In order to overcome the problem of delay distortion, equalization techniques are employed where

the different frequencies are made to over come the effect of change in the speed by equalizing this

speed.

A transmitter equalizer is set to compensate for the nominal or average characteristics of the

transmission medium. The equalizer compensates for amplitude distortion in the medium and for

the problem called group dely. Group delay measures the amount by which a signal of one fre-

quency travels faster in the transmission medium than a signal of a different frequency.

Delay distortion has the greatest effect on the transmission of an analog signal. Analog signals

of different frequencies travel at different rates through a transmission medium. Because each sig-

naling element contains many frequencies, the signaling elements arrive at the receiver over aperiod of time rather than all at once. The frequencies that travel faster (leading frequencies) arrive

earlier, and those traveling slower (lagging frequencies) arrive later. The leading and lagging fre-

quencies not only fail to make their proper contribution to the proper signaling element, but also

cause interference with signaling elements behind and ahead of the proper element. The equalizer

must get the parts of each element back together and cancel their effects on other elements.

The adaptive equalizer compensates for delay distortion by temporarily storing

the analog signal in a tapped delay line. The signals from each tap, amplified by a differ-

ent amount as determined by the amount of error detected, are summed to form the cor-

rected signal.

2.6 TRANSMISSION MEDIA

Media is the general term used to describe the data path that forms the physical channel between

sender and the receiver. Media can be twisted-pair wire such as that used for telephone installa-

tions, coaxial cable of various sizes and electrical characteristics, fiber optics and wireless sup-

porting either light waves or radio waves.

Wire or fiber-optic media are referred to as bounded media. Wireless media

are sometimes referred to as unbounded media.

Media differ in the capability to support high data rates and long distances. The reasons for this

are noise absorption, radiation, attenuation and band width. Noise absorption is the susceptibility

of the media to external electrical noise that can cause distortion of the data signal and thus dataerrors. Radiation is the leakage of signal from the media caused by undesirable electrical charac-

teristics of the media. Radiation and the physical characteristics of the media contribute to attenu-

ation, or the reduction in signal strength as the signal travels down the wire or through free space.

Attenuation limits the usable distance that data can travel on the media.

Band width is similar to the concept of frequency response in a stereo amplifier the greater

the frequency response, the higher the band width. According to a fundamental principle of infor-

mation theory, higher band width communications channels support higher data rates.

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There are several types of physical channels (communication media) through which data can

be transmitted from one point to another. These are of the following types:

(a) Twisted pair

(b) Coaxial Cable

(c) Optical fiber

(d) Radio channels

(e) Satellite channel

2.6.1 Twisted-pair Wire

A twisted pair consists of two insulated copper wires, typically about 1 mm thick. The wires are

twisted together in a helical. The purpose of twisting the wires is to reduce electrical interference

from similar pairs close by.Twisted pair wires (Figure 2.24 (a)) are commonly used in local telephone communication,

and for digital data transmission over short distances up to 1 km. When many twisted pairs run in

parallel for a substantial distance, such as all the wires coming from a multistory apartment build-

ing to the telephone exchange, they are bundled together and encased in a protective sheath. The

pairs in these bundles would interfere with one another if it were not for the twisting.

Wire pairs are normally used to connect terminals to the main computer up to short distances

from the main computer. Data transmission speeds of up to 9600 bits per second can be achieved if

the distance is not more than 100 meters.

Advantages

(a) Being the oldest method of data transmission, trained manpower to repair and service this

media of communications are easily available.(b) In telephone system, signal can travel several kilometers without amplification when

twisted pair wires are used

(c) This media can be used for both analog and digital data transmission. The band width

depends on the thickness of the wire and the distance traveled, but several megabits per

second can be achieved for a few kilometers in many cases.

(d) It is the least expensive media of transmission for short distances.

(e) The effect on the network as a whole, if portion of a twisted-pair cable is damaged, the

entire network is not shut down, as well as it may be the case with coaxial cable.

Disadvantages

(a) Easily pickup noise signals which results in higher error rates when the line length exceeds

100 meters.

(b) Being thin in size, they are likely to break easily.

(c) It can support 19,200 bps up to 50 feet on RS-232 port. On a 10BASE-T, which supports

10Mbps, twisted pair wires can be used up to 100 meters.

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Figure 2.24

(a)

A twisted

pair of wires

Shielded wire typically is used in an electrically noisy environment to limit the effects of noise

absorption. Unshielded twisted pair, commonly referred to as UTP is by far the more common of

the two configurations. Twisted-pair wiring is more commonly used for LAN media. The twisted

pair version of Ethernet is designated as 10BASE-T, in which 10 refers to the Ethernet clock rate

of 10 Mbps.

Twisted pair cabling comes in several varieties. In computer networks, two of these are

important. Category 3 twisted pairs consist of two insulated wires gently twisted together. Four

such pairs are typically grouped together in a plastic sheath for protection and to keep the eight

wires together. Another more advanced category, 5 twisted pairs were introduced. They are simi-

lar to category 3 pairs, but with more twist per centimeter and Teflon insulation, which results in

less crosstalk and better quality signal over longer distances, making them more suitable for

high-speed computer communication.

2.6.2 Coaxial Cable

A coaxial cable consist of a stiff copper wire as the core, surrounded by an insulating material. The

insulator is encased by a cylindrical conductor, often as a closely woven braided mesh. The outer

conductor is covered in a protective plastic sheath. A cutaway view of a coaxial cable is shown in

Figure 2.25. The signal is transmitted by the inner copper wire and is electrically shielded by the

outer metal sleeve.

Figure 2.25

Coaxial cable

Copper

core

material

InsulatingconductorWire mesh plastic covering

Protective

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Two kinds of coaxial cable are widely used. One kind, 50-ohm cable, is commonly used for

digital transmission. The other kind, 75-ohm cables, is commonly used for analog transmission in

cable TV transmission.

Baseband networks are the networks where the entire bandwidth of the cable is

utilized for a single channel. Broadband is basically a frequency division multiplexed sit-

uation, where the coaxial cables bandwidth is separated into subchannels of either equal

or varying frequency ranges that can be treated as separate communication media.

Table 2.2 Different terms of Coaxial Cable Implementation

Terms Implementation10Base2 An implementation of the 802.3 Ethernet standard on thin Ethernet (RG-58) coaxial

cable. It has a data-transfer rate of 10 megabits per second and a maximum cablesegment length of 185 meters.

10Base5 An implementation of the 802.3 Ethernet standard on thick Ethernet coaxial cable.It has a data-transfer rate of 10 megabits per second and a maximum cable seg-ment length of 500 meters over a bus topology.

10BaseF Emerging 802.3 standards that define the use of on Ethernet over fiber-optic cable.

10BaseT An implementation of the 802.3 Ethernet standard over unshielded twisted-pair(UTP) wiring. It is similar to wiring used with modern telephone systems usingRJ-45 connectors. The standard is based on a star topology, with each node con-nected to a central wiring center and a maximum cable-segment length of 100meters.

Thick Ethernet Connecting coaxial cable used on an Ethernet network. The cable is 1 centimeter(0.4 inch) thick, and can be used to connect network nodes up to a distance ofapproximately 1006 meters.

Thin Ethernet Connecting coaxial cable used on an Ethernet network. The cable is 5 millimeters(0.2 inch) thick, and can be used to connect network nodes up to a distance ofapproximately 165 meters. Used for office installations.

IEEE uses the 10BASE5 designation for thick Ethernet coaxial cable and 10BASE2 for the

thin Ethernet coaxial cable. Coaxial cable can support data rates of up to several tens of Mbps at

distances up to several thousand feet. Certain types of signaling enable high data rates over dis-

tances of several miles.

Coaxial cable is difficult to connect to network devices and generally requires more planning

than twisted-pair system. Many coaxial systems require the connectors on the main cable to beattached directly to the adapter on the PCs. This reduces flexibility in locating workstations and

servers.

Installation

Coaxial cable is typically installed in tow configurations: daisy-chain (from device to device

Ethernet) and star (ARCnet). The daisy chain is shown in Figure 2.26.

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Figure 2.26

Coaxial cable

wiring con-

figuration

The Ethernet cabling shown in the Figure 2.26 is an example of Thinnet, which uses RG-58

type cable. Devices connect to the cable by means of T-connectors. Cables are used to provide

connections between T-connectors. One characteristic of this type of cabling is that the ends of the

cable run must be terminated by a special connector, called a terminator. The terminator contains

a resistor that is matched to the characteristics of the cable. The resistor prevents signals that reach

the end of the cable from bouncing back and causing interference.

Advantages of Coaxial Cable

(a) It has better shielding than twisted pairs, so it can span longer distances at higher data bps.

(b) It can be used for both analog data transmission as well as digital data transmission. For

analog, 75 ohm, broad band coaxial is used and for digital data transmission, 50 ohm cable,

baseband cable is used.

LAN Adapter card RG-58Alternative

Methods for

Thin Ethernet

Dual CoaxDrop Cable

Wet

Plate

LAN Adapter card

RG-62Ancent Hub

LAN Adapter card

UTPConcentrator

Wet

Plate

UTPw / RJ-45

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(c) Coaxial cable has higher bandwidth and excellent noise immunity. RG-58 cable (10BA-

SE2) is a thin coaxial cable (50-ohm) in widespread use for LAN connections. RG-11

(10BASE5) cable is a coaxial cable that is much thicker and sturdier and can withstand

more rugged surroundings and can be used with much longer segment lengths. RG-59, a

75-ohm coaxial cable and RG-62 (93-ohm) cable are used in ARCnet Local Area networks

or the IBM 3270 applications.

(d) It is relatively inexpensive as compared to fiber optic cables and easy to handle.

(e) Coaxial cable has a bandwidth in the range of 300-400 MHz, it is capable of carrying over

50 standard 6 MHz colour TV channels or thousands of channels of voice-grade and/or

low-speed data over a single cable. CD-quality audio (1.4 Mbps), or a digital bit stream at 3

Mbps can be mixed on coaxial cable for transmitting video signal. Broadband cable is

inferior to baseband cable for sending digital data but has the advantage that a huge amountof it is already available in the Cable TV and systems. Therefore, cable TV systems may

begin operating as Metropolitan Area Networks and offer telephone and Internet services

at low cost.

Trade-off between Coaxial Cable and Twisted-pair Wiring

Following factors give the comparison between the coaxial cable and twisted pair wiring as trans-

mission media. Table 2.3 gives a comparative study of the various cable media.

Table 2.3 Characteristics of Cable Media

Factor Unshielded Shielded Coaxial Cable Fiber opticTwisted- Twisted-pair

pair cable cable(UTP) (STP)

Cost Lowest Moderate Moderate Highest

Installation Easy Fairly easy Fairly easy Difficult

Bandwidth 1 to 155 1 to 155 Mbps typically 10 Mbps 2 GbpsCapacity Mbps (typically 16 (typically 100 Mbps)

(typically 10 Mbps)Mbps)

Attenuation High (Range High (Range Lower (range of a Lowest (range of tens of few few hundred few kilometers) kilometers)hundred meters)meters)

Electromagnetic Most vul- Less vulner- Less vulnerable than Not affected by EMI or

Interference (EMI) nerable to able than UTP UTP but still vulner- evesdroppingEMI and eav but still able to EMI and eves-esdropping vulnerable to dropping

EMI and eves-dropping

Cost In general, coaxial cable is more expensive by a factor of two or three than twisted pair,

and more expensive by a smaller factor than shielded twisted pair.

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Data Rate The difference between coaxial and twisted pair is more apparent in the data rate

that they support. For comparable distances to be spanned, twisted pair will typically be suitable

for data rates at least an order of magnitude less. If the data rate of choice is 1-Megabit per second,

then either coax or twisted pair will suffice at distances out to several hundred meters. At 10-Me-

gabits per second, only coaxial will serve.

Security Cables that employ copper conductors can easily be breached by listening equip-

ment. If the main consideration is security, then fiber cable is the only choice to avoid espionage.

However, it is to be remembered that no system can ever be perfectly secure. Even fiber-optic

lines can be tapped without detection.

Electromagnetic compatibility Coaxial cable emits less radiation which may

cause interference with the communication equipment as compared to twisted wires.

2.6.3 Optical Fiber

Fiber optic is the newest form of bounded media. This media is superior in data handling and

security characteristics. The fiber optic cable transmits lights signals rather than electrical signals.

It is enormously more efficient than the other network transmission media. Each fiber has an inner

core of glass or plastic that conduct light. There are two types of light sources for which fiber

cables are available. These sources of lights are:

(a) Light Emitting Diodes (LEDs)

(b) Light Amplification by Stimulated Emission Radiation (Lasers)

Figure 2.27

Transmission through

optical fibers

Figure 2.27 shows the principle of operation of the fiber optic system. The system basically

consists of fiber optic cables that are made of tiny threads of glass or plastics. In a single-mode

fibers the core is 8 to 10 microns (about the size of hair). In multimode fibers, the core is 50

microns in diameter.Towards its source side is a converter that converts electrical signals into light waves. These

light waves are transmitted over the fiber. Another converter placed near the sink converts the

light waves back to electrical signals by photoelectric diodes. These electrical signals are ampli-

fied and sent to the receiver.

A comparison of semiconductor laser diodes and LEDs is given in Table 2.4.

ElectricalSignal

to lightwave converter

Optical fibre

Light waves

Electrical

wave converter

Light toelectrical

ElectricalSignal

Light waves

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Table 2.4 Comparison of Semiconductor diodes laser and LEDs as light source

Item Light Emitting Diode (LED) Semiconductor Laser

Data rate Low High

Mode Multimode Multimode or single mode

Distance 3 Km. 30 Km.

Lifetime Long life Short life

Temperature Sensitivity Minor Substantial

Cost Low Substantial

Each fiber has an inner core of glass or plastic that conduct light. The inner core is surrounded

by cladding, a layer of glass that reflects the light back into the core. Each fiber is surrounded by a

plastic sheath. The sheath can be either tight or loose.Optical fibers may be multimode or single mode. Single mode fibers allow a single light path

and are typically used with laser signalling. Single mode fiber can allow greater bandwidth and

cable run than multimode but is more expensive. Multimode fibers use multiple light paths. The

physical characteristics of the multimode fiber make all parts of the signal (those from the various

paths) arrive at the same time, appearing to the receiver as though they were one pulse. Figure 2.28

shows the working of the single and multimode optical fiber.

Optical fibers are differentiated by core/cladding size and mode of operation. Micron is one

millionth of a meter = 1/25,000 inch (approximately)

The following are the common types of fiber-optic cable:

(a) 8.3-micron core/12.5-micron cladding, single-mode

(b) 62.5-micron core/125-micron cladding, multimode(c) 50-micron core/125-micron cladding, multimode

(d) 100-micron core/140-micron cladding, multimode

Characteristics of Fiber-optic Cable

Fiber-optic cable has the following characteristics.

Cost Fiber-optic cable is slightly more expensive than copper cable, but costs are falling.

Associated equipment costs can be much higher than for copper cable, making fiber-optic net-

works much more expensive. Single mode fiber devices are more expensive and more difficult to

install than multimode devices.

Installation Fiber optic cable is more difficult to install than copper cable. Every fiber

connection and splice must be carefully made to avoid obstructing the light path. Also the cableshave a maximum bend radius, which makes cabling more difficult.

Bandwidth capacity Because it uses light, which has higher frequency than electricity,

fiber optic cabling provides a data rates from 100 Mbps to 2 gigabits per second. The data rate

depends on the fiber composition, the mode, and the wavelength (frequency) of the transmitter

light. A common multimode installation can support 100 Mps over several kilometers.

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Figure 2.28

(a) Single

mode optical

fiber

(b) Step index

fiber

Node Capacity In the case of Ethernet network fiber optic cables have the useful upper

limit is around 75 nodes on a single collision domain.

Attenuation Fiber optic cable has much lower attenuation than copper wires, mainly

because the light is not radiated out in the way electricity is radiated from copper cables. It has adifferent problem namely, chromatic dispersion. Different wavelengths of light travel through

glass differently, and the colours of a single pulse of light will spread apart slightly as they travel

down a cable. At a distance of several miles, one bit may shift into the next bit, causing data to be

lost. Single-mode fiber-optic cable conveys only one frequency of light down the cable, so it does

not suffer from chromatic dispersion.

Cladding

Core 8 - 12 m

125 m

Light

Ray

(a) Single Mode Fiber Cable

125-400 m

Light

Rays

(b) Multimode Fiber Cable

Core

50-200

m

Cladding

Cladding

Cladding

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Electro Magnetic Interference Fiber-optic cable is not subject to electrical inter-

ference. In addition, it does not leak signals, so it is immune to evesdropping. Because it does not

require a ground, fiber-optic cable is not affected by potential shifts in the electrical ground, nor

does it produce sparks. This type of cable is ideal for high-voltage areas or in installations where

evesdropping could be a problem. Fiber is particularly appropriate for campus and multi-building

backbones and for high security applications such as financial transactions, military operations,

and public safety.

Mode of Transmission Fiber optic channels arehalf-duplex, meaning that light signals

can only move in one direction at a time. A full-duplex circuit would cause light wave interference

without special electronics and is generally not economically viable. Moreover, a bend radius that

is too tight causes distortion and attenuation of the light signal due to changes in the electrical and

physical characteristics of the inner core.

Uses of Optical Fiber

Figure 2.29

A fiber optic

ring with

active

repeaters

Fiber-optic media can support high bandwidth applications including video conference to the

desktop, digital voice/image/graphics networking in the local area network environment. Fiber-optic media are the basis for several high bandwidth networking standards such as Fiber Distrib-

uted Data Interface (FDDI) and Synchronous Optical Network (SONET)

Fiber optic can be used for LANs as well as for long transmission although tapping onto it is

more complex than connecting to an Ethernet. One way around the problem is to realize that a ring

network is really just a collection of a point-to-point links as shown in Figure 2.29.

Computer

Fiber

Optical

receiver

(photodiode)

Signalregenerator

(electrical)

Optical

transmitter

(LED)

propagation

of lightDirection

Optical

fiber

Interface

Detailofinterface

To / from computer

Copper wire

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The interface at each computer passes the light pulse stream through to the next link and also

serves as a T junction to allow the computer to send and accept messages. The figure shows the

active repeater. The incoming light is converted to an electrical signal regenerated to full strength

if it has been weakened, and re-transmitted as light. The interface with the computer is an ordinary

copper wire that comes into the signal regenerator. Purely optical repeaters are now being used

too.

Comparison of Fiber Optics and Copper Wire

Advantages

Fiber has many advantages over copper wire as a transmission media. These are:

(a) It can handle much higher band widths than copper. Due to the low attenuation, repeatersare needed only about every 30 km on long lines, versus about every 5 km for copper.

(b) Fiber is not being affected by power surges, electromagnetic interference, or power fail-

ures. Nor it is affected by corrosive chemicals in the air, making it ideal for harsh factory

environments.

(c) Fiber is lighter than copper. One thousand twisted pairs copper cables of 1 km long weight

8000 kg. But two fibers have more capacity and weigh only 100 kg, which greatly reduces

the need for expensive mechanical support systems that must be maintained.

(d) Fibers do not leak light and are quite difficult to tap. This gives them excellent security

against potential wiretappers.

Disadvantages

Fibers have the following disadvantages over copper wires:(a) Fiber is an unfamiliar technology requiring skills most engineer do not have.

(b) Since optical transmission is inherently unidirectional, two-way communication requires

either two fibers or two frequency bands on one fiber.

(c) Fiber interfaces cost more than electrical interfaces.

2.6.4 Radio, VHF, Microwave and Satellite Link

Radio waves have frequencies between 10 kilohertz (KHz) and 1 giga hertz (GHz). Radio waves

include the following types:

(a) Short-wave

(b) Very-high-frequency (VHF) television and FM radio.

(c) Ultra-high-frequency (UHF) radio and television

The range of frequency and type of medium used for their transfer is shown in Figure 2.30.

Radio waves can be broadcast omni directional or directional. Various kinds of antennas can be

used to broadcast radio signals. The power of the radio frequency (RF) signal is determined by the

antenna and trans-receiver (a device that TRANSmits and reCEIVEs a signal over a medium such

a copper, radio waves, or fiber-optic cables).

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In vacuum, all electromagnetic waves travel at the same speed, no matter what their frequency

is. This speed, usually called the speed of light, c, and it is approximately 3108 meters per sec-ond or about 1 foot per nanosecond. In copper or fiber the speed slows to about 2/3 of this value

and becomes slightly frequency dependent. The fundamental relation between frequency, (f), wave

length, andc(in vacuum) is

For example, 1 MHz waves are about 300 meters long and 1 cm waves have a frequency of 30

GHz.

Figure 2.30

Radio

frequency

range and

type of trans-

mission

media

Radio Transmission

Some of the characteristics of radio waves are as follows:

(a) Radio waves are easy to generate.

(b) They can travel long distances

(c) They can penetrate buildings easily so they are widely used for communications both

indoors and outdoors.

(d) Radio waves are omni directional, meaning that they travel in all directions from the

source, so that the transmitter and receiver do not have to be carefully aligned physically.

The properties of radio waves are frequency dependent. At low frequencies,

radio waves pass through obstacles well, but the power falls off sharply with distance

from the source, roughly, as 1/r3 in air. At high frequencies, radio waves tend to travel in

straight lines and bounce off obstacles. They are absorbed by rain. At all frequencies,

radio waves are subject to interference from motors and other electrical equipment.

f=c (2.7)

Gamma

f (Hz)10 100 2

104

106

108

1010

1012

1014

1016

1018

1020

1022

1024

Twisted pair

f (Hz)10 104 5

1076

10 108

109

1010

1011

1012

1013 14

1015

10

X - rayUVInfraredMicrowave

RadioRay

1016

Visible light

Coax

MaritineAMradio radio

FM

Satellite

microwaveTerrestrial

TV

opticsFiber

LFBand MF HF VHF UHF SHF EHT THF

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In Very Low Frequency (VLF), Low Frequency (LF) and Medium Frequency (MF) bands,

radio waves follow the ground. Amplitude modulated radio broadcasting uses the MF band. This

band of frequencies can not be used for data transfer because they offer relativelylow bandwidth.

The amount of information that an electromagnetic wave can carry is related to its bandwidth.

With current technology, it is possible to encode a few bits per Hertz at low frequencies, but often

as many as 40 under certain conditions at high frequencies. So a cable with a 500 MHz bandwidth

can carry several gigabits/sec.

In the HF and VHF bands, the ground waves tend to be absorbed by the earth.

However, the waves that reach the ionosphere, a layer of charged particles circling the

earth at a height of 100 to 500 km, are refracted by it and sent back to earth.

Microwave Transmission

Above 100 MHz, the waves travel in straight lines and can therefore be narrowly focused. Con-

centrating all the energy into a small beam using a parabolic antenna (like the satellite TV dish)

gives a much higher signal to noise ratio, but the transmitting and receiving antennas must be

accurately aligned with each other. Before the advent of fiber optics, these microwaves formed the

heart of the long distance telephone transmission system.

In order to overcome the problems of line-of-sight and power amplification of weak signals,

microwave systems use repeaters at intervals of about 25 to 30 km in between the transmitting and

receiving stations (Figure 2.5). The first repeater is placed in line-of-sight of the transmitting sta-

tion and the last repeater is placed in line-of-sight of the receiving station. Two consecutive

repeaters are also placed in line-of-sight of each other. The data signals are received, amplified,and re-transmitted by each of these stations.

Unlike radio waves, at lower frequencies, microwaves do not pass through buildings

well. In addition, even though the beam may be well focused at the transmitter, there is

still some divergence in space. Some waves may be refracted off low-lying atmospheric

layers and may take slightly longer to arrive than direct waves. The delayed waves may

arrive out of phase with the direct wave and thus cancel the signal. This effect is called

Multipath Fading. It is often a serious problem in Microwave communication systems.

Since microwaves travel in a straight line, if the towers are too far apart, the earth will get in

the way. Consequently, repeaters are needed periodically. The higher the towers are, the furtherapart they can be. The distance between repeaters goes up very roughly with the square root of the

tower height. For 100 meter high towers, repeaters can be spaced 80 km apart.

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Figure 2.31

Illustrating

microwave co

mmunication

from one

point to

another

Characteristics of Microwave Communications

Microwave transmission is weather and frequency dependent. The frequency band of 10 GHz is in

the routine use. Microwave communication is widely used for long-distance telephone communi-

cation, cellular telephones, television distribution and other uses that a severe shortage of spectrum

has developed. The following are the characteristics of Microwave communications:

(a) Microwave is relatively inexpensive as compared to fiber optics system. For example,

putting up two simple towers and antennas on each one may be cheaper than burying 50

km of fiber through a congested area or up tower a mountain, and it may also be cheaper

than leasing the telephone line.

(b) Microwave systems permit data transmission rates of about 16 Giga (1 giga = 109) bits per

second. At such high frequencies, microwave systems can carry 250,000 voice channels at

the same time. They are mostly used to link big metropolitan cities which have heavy tele-

phone traffic between them.

Types of Microwave Communication Systems

There are two types of microwave data communication systems. These are:

(a) Terrestrial

(b) Satellite

Line of sight Line of sight Line of sight

In betweenRepeaters

AntennasTransmitting

antennasReceiving

Transmittingsta

tion

Receivingstation

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Terrestrial Microwave

Terrestrial Microwave systems typically use directional parabolic antennas to send and receive

signals in the lower giga hertz range. The signals are highly focussed and the physical path must

be line-of-sight. Relay towers are used to extend signals. Terrestrial microwave systems are typi-

cally used when using cabling is cost-prohibitive.

Because, terrestrial microwave system does not use cables, microwave links often

connect separate buildings where cabling would be too expensive, difficult to install or

prohibited. For example, if two buildings are separated by a public road, you may not be

able to get permission to install cable over or under the road. Microwave links would be a

good choice in this type of situation.

Terrestrial Microwave systems have the following characteristics:

Frequency Range:

Most terrestrial microwave systems produce signals in the low giga hertz range usually at 4 to 6

GHz and 21 to 23 GHz.

Cost:

Short-distance systems can be relatively inexpensive and they are effective in the range of

hundreds of meters. Long distance systems can be very expensive.

Installation:

Line-of-sight requirements for microwave systems can make installation difficult. Antennas must

be carefully aligned. Also because the transmission must be line of sight, suitable trans-receiver

sites could be a problem. If your organization does not have a clear line of sight between two

antennas, you must either purchase or lease a site.

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