3 data transmission

8/12/2019 3 Data Transmission

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UGANDA CHRISTIAN UNIVERSITY

FACULTY OF SCIENCE AND TECHNOLOGY

Department of Information Technology

Computer Networks

Program: BSCS 2

January Semester2013

3. Data Transmissions

Lecturer: Rebecca Asiimwe

Phone Number:+256 712-997- 544 /0704 522 081

Email: [email protected]


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Successful transmission of data depends principally on

two factors:

1. 1. The quality of the signal being transmitted and

2. 2. The characteristics of the transmission medium.

The purpose of this lesson (and the next) is to provide

students with knowledge on these two factorsbeginning with the concepts and terms that will be refer

red to throughout this lesson and subsequent lessons.3


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Concepts and Terminology (1)

Data transmission occurs between transmitter and

receiver over some transmission mediumwhich canbe classifiedasguidedor unguidedmedia. In both

cases, communication is in the form of electromagnetic

waves.

Guidedmedia(waves are guided along a physical

path)

e.g. twisted pair, optical fiber, coaxial cable

Unguidedmedia(wireless transmission- waves not

guided)

e.g. air, water, vacuum 4


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Terminology (2)

Direct link Refers to transmission path between two devices in

which signals propagate directly from transmitter to

receiver with no intermediate devices other than

repeaters or amplifiers used to increase signal strength

Point-to-pointmedium in guided media provides a;

Direct link between two devices and those are theonly devices sharing the medium.

Multi-point

More than two devices share the link5


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Terminology (3)

Transmission may be simplex, half duplex of full duplex.

Simplex transmission

Signals are transmitted in only one direction

e.g. Television Half duplex transmission

Either direction, but only one way at a time

e.g. police radio

Full duplex transmission Both directions at the same time

e.g. telephone6


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Data Flow

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Data Flow

Communication between two devices can be simplex, half-

duplex, or Full-duplex.

Simplex mode: The communication is unidirectional, as

on a one-way street. Only one of the two devices on a

link can transmit; the other can only receive e.g.

Keyboards and traditional monitors. The keyboard can only

introduce input; the monitor can only accept output.

The simplex mode can use the entire capacity of the

channel to send data in one direction 8


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Data Flow contd

In half-duplex mode, each station can both transmit andreceive, but not at the same time.:

When one device is sending, the other can only receive and

vice versa.

The half-duplex mode is like a one-lane road with trafficallowed in both directions. I.e. When cars are travelling in

one direction, cars going the other way must wait.

The entire capacity of a channel is taken over by

whichever of the two devices is transmitting at the time

. Walkie-talkies and CB (citizens band) radios are both half-

duplex systems.9


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Data Flow contd

In full-duplex mode, (duplex), both stations can transmit

and receive simultaneously/ at the same time.

Signals are transmitted in both directions at the same time

and share the capacity of the link. This sharing can occurin two ways: Either the link must contain two physically

separate transmission paths, one for sending and the other

for receiving; or the capacity of the channel is divided

between signals travelling in both directions. E.g telephone

network.

Used when communication in both directions is required all

the time.10


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Frequency and Bandwidth

Time domain concepts Analog signal

Varies in a smooth way over time

Digital signal

Maintains a constant level then changes to

another constant level

Periodic signal

Pattern repeated over time Aperiodic signal

Pattern not repeated over time11


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Analogue & Digital Signals

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Periodic

Signals

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Sine Wave

The sine wave is a fundamental periodic signal and is

represented by the following parameters.

Peak Amplitude (A)

maximum strength of signal over time and is measured in volts

Frequency (f)

Rate of change of signal / the rate at which the signal repeats

in cycles per second or Hertz (Hz)

Frequency can also be defined as the number of oscillations/

cycles per second.

Period= amount of time taken for one repetition (T) T = 1/f so F=1/T

Phase () Relative position in time of the signal. 14


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Varying Sine Waves

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In figure a)

Frequency= 1Hz

Period=1second

Figure b) has the same frequency and phase but a

peak amplitude of 0.5 volts

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In figure c)

Frequency= 2 Hz

Period T=0.5 seconds

In d) we notice the effect of a phase shift

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Wavelength ()

Distance occupied by one cycle

Assuming signal velocity v = vT f = v v = 3*108 ms-1 (speed of light in free space)

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Key SI multiples for Hertz (Hz)

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SI = International System of Units


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Exercise:

1. Given a wave moving at a frequency of 5MHz, find:

a) The period and

b) Wave length of the signal.

2. Given a signal with wave length of 3,000,000,0

00cm

a) Find the frequency at which this signal is moving.

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Frequency Domain Concepts, Spectrum

& Bandwidth

In practice, signalsare usually made up of many frequ

encies

Components of a signal are sine waves

Spectrum This is the range of frequencies contained in signal

Absolute bandwidth

width of spectrum

Effective bandwidth Often just bandwidth

Narrow band of frequencies containing most of theenergy/ band within which most of the signal energy isconcentrated

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Data Rate and Bandwidth

Any transmission system has a limited band of frequencies

This limits the data rate that can be carried

Although a given waveform may contain frequencie

s over a very broad range, as a practical matter, an

y transmission system (transmitter plus medium p

lus receiver) will be able to accommodate only a

limited band of frequencies. This can be evidentwhen listening to your radio stations, there are parti

cular frequencies at which the strength of the signal

is high and you can clearly get the transmission. 23


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READ ABOUT ANALOG AND DIGITAL:

DATA

SIGNALS

AND TRANSMISSION

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Analog and Digital Data Transmission

Data Entities that convey meaning or information

Signals

Electric or electromagnetic representations of dat

a

Signaling-Physical propagation of the signal alo

ng a suitable medium.

Transmission Communication of data by propagation and proce

ssing of signals which can either be analog or digi

tal. 25


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Analog and Digital Data

AnalogAnalog data takes on continuous values withi

n some interval

e.g. sound, video are continuously varying pat

terns of intensity. Most data collected by sens

ors such as temperature and pressure are co

ntinuous.

Digital Digital data takes on discrete values

e.g. text, integers 26


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Analog and Digital Signals

In a communications system, data are propagated from

one point to another by means of electromagnetic signals.

An analog signal is a continuously varying electromagne

tic wave that may be propagated over a variety of mediadepending on the spectrum for example wire media such

as twisted pair and coaxial cable, fiber optic, space for u

nguided media.

Digital signals is a sequence of voltage pulses that may

be transmitted over a wire medium for example a consta

nt positive voltage level may represent binary 0 and a co

nstant negative voltage level may represent binary 127


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Advantages & Disadvantages of Digital

Principal Advantages:

Cheaper

Less vulnerable to noise interference

Principal Disadvantages:

Suffer more from attenuation than do analogy si

gnals. Pulses become rounded and smaller

Leads to loss of information 28


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Attenuation of Digital Signals

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Data and Signals

Digital signals are usually used for digital data and analog

signals for analog data

But we can use an analog signal to carry digital data and t

his can be possible through the use of a modem that conv

erts a series of binary voltage pulses into an analog signal

by encoding the digital data onto a carrier frequency.

The resulting signal occupies a certain spectrum of freque

ncy centered about the carrier and may be propagated acr

oss a medium suitable for that carrier. The most common

modems represent digital data in the voice spectrum and hence allow those data to be propagated over ordinary voic

e-grade telephone lines. At the other end of the line, anoth

er modem demodulates the signal to recover the original d

ata.

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In an operation very similar to that performed bya modem, analog data can be represented by di

gital signals.

The device that performs this function for voice d

ata is a codec (coder-decoder). In essence the c

odec takes the analog signal that directly repres

ents the voice data and approximates that signalby a bit stream. At the receiving end the bit strea

m is used to reconstruct the analog data.31


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Analog Signals Carrying Analog and Digital

Data

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Digital Signals Carrying Analog and Digital D

ata

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Analog Transmission

Analog signal transmitted without regard to cont

ent

May be analog or digital data

Attenuated over distance

Use amplifiers to boost signal-Also amplifies noi

se

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Digital Transmission

Concerned with content Integrity endangered by noise, attenuation etc.

Repeaters used

Repeater receives signal

Extracts bit pattern and recovers pattern

Retransmits new signal

Attenuation is overcome

Noise is not amplified

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Advantages of Digital Transmission

Digital technology Low cost LSI/VLSI technology (Large scale integra

tion caused a drop in cost and size of digital circuitry)

Data integrity

Repeaters rather than amplifiers are used so the effects of noise and other impairments are not cumulative. Signals can be propagated for longer distances and over lower quality lines while maintainingintegrity.

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Capacity utilization High bandwidth links - economical

High degree of multiplexing needed to utilize capacity is easily achieved with digital techniques

Security & Privacy

Encryption

Integration Can treat analog and digital data similarly and e

conomies of scale and convenience can be achieved by integrating voice, video and digital data.

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Transmission Impairments

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Transmission Impairments

Signal received may differ from signal transmitted

Transmission impairments like:

Attenuation and attenuation distortion

Delay distortion

Noise

Cause: Degradation of signal quality (Analog)

Bit errors (Digital) 39


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Attenuation

Signal strength falls off with distance Depends on medium

Received signal strength: must be enough to be detected

must be sufficiently higher than noise to be

received without error Attenuation is an increasing function of

frequency40


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Delay Distortion

Only in guided media

Propagation velocity varies with frequency

Signal components propagate at different speeds

in the wire. This speed difference leads to

distortion of the signal received at the other end.

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Noise (1)

Defined as additional signals inserted betweentransmitter and receiver

1. Thermal

Due to thermal agitation of electrons

Uniformly distributed over the whole frequency bandwidth Also called White noise

2. Intermodulation

Signals that are the sum and difference of originalfrequencies sharing a medium

Eg. Signal moving at F1 combines with that at F2 and the

resultant F3 affects what is moving at F3 42


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Noise (2)

3. Crosstalk

A signal from one line is picked up by another.

Crosstalk is caused by inductive coupling between two

wires that are close to each other. Sometimes when

talking on the telephone, you can hear another conversationin the background. That is crosstalk

Electrical coupling between nearby twisted pairs

4. Impulse Irregular pulses or spikes

e.g. External electromagnetic interference, lightening

Take a short duration and at a High amplitude43


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Channel Capacity

We have seen that there are a variety of

impairments that distort or corrupt a signal.

For digital data, the question that then arises is

to what extent these impairments limit the data

rate that can be achieved. The maximum rate a

t which data can be transmitted over a given

communication path, or channel, under givenconditions, is referred to as the channel capa

city.44


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Four important concepts in defining capacity Data rate: The rate, in bits per second (bps), at which

data can be communicated

Bandwidth: The bandwidth of the transmitted signal as con

strained by the transmitter and the nature of thetransmission medium, expressed in cycles per second, or

Hertz

Noise: The average level of noise over the communication

path Bit Error Rate (BER) The rate at which errors occur, where

an error is the reception of a1 when a 0 was transmitted or

the reception of a 0 when a 1 was transmitted 45


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BER is the number of bit errors divided by the

total number of transferred bits during a studied

time interval often expressed as a percentage.

As an example, assume this transmitted bit sequence: 0 1 1 0 0 0 1 0 1 1,

and the following received bit sequence:

0 0 1 0 1 0 1 0 0 1,

The number of bit errors (the underlined bits) is in this

case 3. The BER is 3 incorrect bits divided by 10

transferred bits, resulting in a BER of 0.3 or 30%.46


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Nyquist Bandwidth

Two theoretical formulas were developed to calculate the

data rate: one by Nyquistfor a noiselesschannel.

Another by Shannonfor a noisychannel. For a noiseless

channel, Nyquist formula defines the theoretical

maximum bit rate.

If rate of signal transmission is 2B then signal with

frequencies no greater than B is sufficient to carry signal

rate. Given bandwidth B, highest signal rate is 2B

Given a binary signal, the data rate supported by B Hz is2B bps

The data rate can be increased by using M signal levels

C= 2B log2M47


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Example

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Consider a voice channel being used, via modem, to

transmit digital data. Assume a bandwidth of 3100 Hz.

Then the Nyquist capacity, C, of the channel is

2B=2X3100 = 6200bps.For M=8, a value used with some modems, C

becomes 18,600 bps for a bandwidth of 3100 Hz.

C= 2B log2MC=2X3100 log28C=2X3100X3log22C=6200X3 = 18 600bps


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According to this formula, there are two means to

enhance the data rate. First, if we use a large number

of signaling levels M, the capacity increases.

Thus, the receiver will decide much easier if it has todistinguish between only two signaling levels (M=2) than

in the case of a much larger number of levels (e.g. M=64).

By the other hand, another way to increase the data

rate is by increasing the bandwidth. As already seen,bandwidth is always a scarce resource, limited by physical

constraints and regulations.49


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Another example: We need to send information at a

data rate of 256 kbps, over a noiseless channel with

16KHz of bandwidth. How many signaling levels do

we need?

Answer: log2M=C/2B=256*103/2*16*103=8. It

follows that M=28=256 levels.

OR.

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C=256kbps = 256000bps

B=16KHz = 16000Hz

M?

C=2Blog2M

256000 = 2x16000log2M

256000 = 32000log2M

256000 = 32000log2M

32000 32000

256/32=log2M

M=2 (256/32)

M=2 8

M= 256 bit levels51

Comment: This shows quite

a large number of levels

which are required, and the

receiver task is difficult. The

reason behind is that,

through quite a lowbandwidth we try to send a

fairly high data rate.


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Parameters as subjects of the equation

C= 2B X logM

log2

B= C

logM 2

log2

C = 2B log2M C = log2M

2B

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C= 2B log2M

M = 2 (C/2B)


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Logarithm Principle

If we call the three numbers in each case a, bandc, so that

ab= c

then we can say that the logarithm of cto the base

ais b, and we write

logac = b

That is to say, the logarithm of the number cto the

base a, is the power to which amust be raised toget c.

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Shannon Capacity Formula

In reality, we cannot have a noiseless channel; thechannel is always noisy. In 1944,Claude Shannon

Introduced a formula, called the Shannon capacity, to

determine the theoretical highest data rate for a noisy

channel:

Capacity (C) =Bandwidth (B) X log2(1 +SNR)

Where: Bandwidth (B) is the bandwidth of the channel,

SNR is the signal-to noise ratio,

Capacity is the capacity of the channel in bits per second.54


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This formula defines a characteristic of the

channel, not the method of transmission, it is

because of this that signal levels are not

indicated. And given a noisy channel , this

implies that we cannot achieve the maximum

data transfer rate even when signal levels are in

creased.

Considers data rate, noise and error rate 55

Sh C it F l


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With faster data rate, in event of bursts of noise,

more bits are affected.

At given noise level, high data rate means

higher error rate

Signal to noise ration (in decibels)

SNRdb=10 log10 (signal/noise)

Capacity C=B log2(1+SNR)

Note: the decibel is used to measure the changes

in the strength of a signal. It can also be used to

measure the changes in the strength of a signal)56


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Try-Out Questions

1. A digital signaling system is required to operateat 9600 bps

a) if a signal element encodes a 4 bit word, whatis the minimum required bandwidth of thechannel?

b) Repeat part a) for the case of an 8 bit word.

2. Given a channel with an intended capacity of 20

Mbps, the bandwidth of the channel is 3MHz.2. What signal to noise ratio is required to achieve

3. this capacity? 57


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Try-Out Questions

b) What is the channel capacity for the tele-printer

channel with a 300Hz bandwidth and a SNR of

3db?

3. Given a signal power of 15 mill watts(mW)

moving through a channel with capacity of 20

bps and noise power of 2mW, find the BW ofthe channel.

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Required Reading

Chapter 3 of

Behrouz A. Forouzan & Sophia Chung F

egans book on Data Communications an

d Networking.

And

Chapter 3 of William Stallings book on

Data and Computer Communications59


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4. Transmission Media (Gu

ided and Unguided)

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