data transmission ch 2

8/2/2019 Data Transmission Ch 2

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Data Communications and Networking

Chapter 2 Data Transmission

Prepared by:

A. A. Waseem &

Waleej Haider


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TransmissionTerminology

Transmission communication of data by propagation and processing of signals

Data transmission occurs between a transmitter & receivervia some medium

Transmission media is classified as Guided or Unguided Data must be transformed to electromagnetic waves, in bot

h cases

Guided medium The waves are guided along a physical path

eg. twisted pair, coaxial cable, optical fiber

Unguided / wireless medium The waves are not guided

eg. air, seawater, vacuum, outer space


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Data

Data can be analog or digital

Analog data are continuous and take continuous values

Analog data can be converted to an analog or modulated into a digital signal

Digital data have discrete states and take discrete values (0s and 1s)

Digital data can be converted to a digital signal ormodulated into an analog signal for transmission across a medium


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Signals

Signal is the electric or electromagnetic representation of data

An electromagnetic signal is generated by the transmitter and then transmitted over a medium

Signals can also be analog or digital

Analog signal

Signal intensity varies in a smooth way over time

It has infinitely many levels of intensity (values) along its path over a period of time

There are no breaks or discontinuities in the signal e.g; speech

Digital signal

Signal intensity maintains a constant level for some period of time and th

en changes to another constant level e.g; binary 1s and 0s A digital signal can have only a limited number of defined values (often

0 and 1)

A Signal is a function of time, but it can also be expressed as a function of frequency

There are two concepts of data transmission Time domain view andfrequency domain view of a signal


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Analog Data Analog data take on continuous values in some interval

E.g. voice, video, temperature, and pressure are continuously varying patterns of intensity

freq range of sound wave is 20Hz-20kHz

human speech spectrum range is 100Hz-7kHz

Audio signals are easily and directly converted into electromagnetic signals

All sound freqs, whose amplitude is measured in terms of loudness, are converted into electromagnetic freqs, whose amplitude is measured in volts

The standard spectrum for a voice channel is 300-3400Hz


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Digital Data

Digital data take on discrete values e.g. text, integers, etc

Textual data cannot be easily stored or transmitted by data processing and communication systems

Communication systems are designed for binary data

Therefore some text codes have been devised by which characters a

re represented by a sequence of bits Commonly used text code: IRA (international reference alphabet)

IRA-encoded characters are using 8 bits per character

8th bit is a parity bit used for error detection

Thus binary data is generated by terminals and computers etc and then converted into digital voltage pulses for transmission

The signal uses two constant dc components (voltage levels) 0 or 1


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Analog Signals


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Digital Signals


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Time Domain Concepts

Time Domain Concepts:

Electromagnetic signals are viewed as a function of tim

e

The time-domain plot shows changes in signal amplitude with respect to time

It is an amplitude-versus-time plot


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Analogue & Digital Signals

Analog and Digital Waveforms

(continuous)

(discrete)


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Periodic and Non-periodic Signals

Periodic signal

A signal completes a pattern within a measurable time frame c

alled a Period

one full pattern is called a cycle

If same signal pattern repeats over subsequent identical period

s called periodic signal

Non-periodic signal

pattern not repeated over time

Both analog and digital signals can be periodic or non-periodic (a) shows periodic continuous signal (sine wave)

(b) shows periodic discrete signal (square wave)

In data comm., we commonly use periodic analog signals and no

n-periodic digital signals


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Cont.. example


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Sine Wave

Sine wave is the fundamental periodic analog signal Its change over the course of a cycle is smooth and consistent

It is a continuous rolling flow

Each cycle consists of a single are above the time axis followed by asingle are below it (shown in previous fig.)

It is represented by three parameters:1) Peak amplitude (A)

Highest intensity (value) of a signal over time

Proportional to the energy it carries

Measured in volts

2) frequency (f) Number of cycles per second

Rate of change of signal (rate at which signal repeats)

Measured in Hertz (Hz)

period = time required for one repetition (one cycle) =T

T = 1/f or f = 1/T

Period and frequency are the inverse of each other


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Cont..

Frequency is the rate of change with respect to time

Change in a short span of time means high freq

Change over a long span of time means low freq

If signal does not change at all, its freq is zero

Any electromagnetic signal consists of a collection

of periodic analog signals (sine waves) at different

amplitude, frequencies, and phases


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Cont..

3) Phase ()

Phase describes the position of the waveform relative to time 0

Measure of the relative position in time within a single period of signal

Phase is the fractional part t/T of the period T through which t has advanced relative to an arbitrary origin

If we think of the wave as something that can be shifted backward or f

orward along the time axis, phase describes the amount of that shift The origin is usually taken as the last previous passage through zero

is sometimes referred to as a phase-shift, because it represents a "shift" from zero phase.

Phase shift is any change that occurs in the phase of one signal, or in t

he phase difference between two or more signals.


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Cont..

Phase is measured in degrees or radians

360 is 2 rad; 1 is 2/360rad; and 1 rad is 360/(

2 )

A phase shift of 360 corresponds to a shift of a co

mplete period

a phase shift of 180 corresponds to a shift of one-

half of a period; and a phase shift of 90 corresponds to a shift of one-q

uarter of a period (see Figure).


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Varying Sine Wavess(t) = A sin(2ft +)


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Wavelength ()

Wavelength of a signal is the distance traveled by one cycle

Distance between two points of corresponding phase of two

consecutive cycles

Wavelength can be calculated if one is given the propagation speed and the period of the signal.

Assume signal is traveling with velocity v then

= vT

or equivalently = v/ f wavelength is normally measured in micrometers (m)


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Frequency Domain Concept

Time-domain plot shows changes in signal amplitude with

respect to time

A frequency-domain plot is concerned with only the peak

value and the frequency of the signal as a whole. Changes of amplitude during one period are not shown.

A complete sine wave in the time domain can be represent

ed by one single spike in the frequency domain. (see fig)

The position of the spike shows the frequency and its height shows the peak amplitude


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Cont..

The frequency domain is more compact and useful when we are dealingwith more than one sine wave.

Figure below shows three sine waves, each with different amplitude and frequency.

All can be represented by three spikes in the frequency domain


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Composite Signals A single frequency sine wave is not useful in data communications

If we had only one single sine wave to convey a conversation over the phone, itwould make no sense and carry no information. We would just hear a buzz

we need to send a composite signal to communicate data.

Any composite signal is a combination of simple sine waves with different frequencies, amplitudes, and phases as shown in fig (c)

The fig (a) fig (b) shows the components of fig (c) which are just simple sine w

aves of frequencies f and 3f Their sine waves are:

Fig (a): s(t) = sin(2ft)

Fig (b): s(t) = (1/3) sin(2(3f)t)

The second freq is the integer multiple of the first freq.

Thus first freq component is called Fundamental freq.

The period of the composite signal is equal to the period of the fundamental freq as in fig (c)

The composite signal generated from fig (a) & (b) will be:

Fig (c): s(t) = (4/) [sin(2ft) + (1/3) sin(2(3f)t)]


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Decomposition of a composite signal in time domain


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Frequency Domain Representations

Fig (d): Frequency-domain decomposition of the comp

osite signal of Fig (c)


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Periodic/ Non- Periodic Composite signals

A composite signal can be periodic or non-periodic

A periodic composite signal can be decomposed into a series of simple sine waves with discrete frequencies.

Discrete Frequencies have integer values (1, 2, 3, )

A non-periodic composite signal can be decomposed into acombination of an infinite number of simple sine waves with continuous frequencies

Continuous Frequencies have real values.

For composite periodic signal see previous fig (c)

Fig (a) & (b) shows decomposition of fig (c) For its freq-domain decomposition see fig (d), it shows dis

crete frequencies


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Example: Non-periodic Composite Signal


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Example (Cont..)

There are an infinite number of simple sine frequencies (waves) in a non-periodic composite signal created by a microphone

A normal human being can create a continuous range of freq

uencies between 0 and 4 kHz Frequency decomposition of this signal produces a continuo

us curve.

There are an infinite number of frequencies between 0.0 and

4000.0 (real values). The height of the vertical line is the amplitude of the corresp

onding frequency


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Spectrum & Bandwidth

Spectrum of a signal range of frequencies contained in a composite signal

For the signal of fig (c), the spectrum extends from f to 3f

Bandwidth of a signal

Width of the spectrum (diff. b/w max & min freq of the spectrum)

In case of fig (c), the bandwidth is 2f

Effective bandwidth

Most of the composite signal energy is contained in a relatively nar

row band of frequencies

This band is called effective bandwidth

often called bandwidth


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Fig. shows the spectrum and bandwidth

(a) All integer frequencies between 100

0 and 5000(b) frequencies arecontinuous between1000 and 5000

High amplitude atthe center of both figs shows effective bandwidth


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Digital Signal An electronic signal transmitted as binary code that can be either the presence

or absence of current, high and low voltages or short pulses at a particular frequency

An arbitrary bit stream; 1 can be encoded as a high (positive) voltage and 0 as low (non-positive) voltage

Digital format is ideal for electronic communication as the string of 1s and 0scan be transmitted by a series of "on/off" signals represented by pulses of ele

ctricity or light. A pulse "on" can represent a 1, and the lack of a pulse "off" can represent a 0

Most digital signals are non periodic

A digital signal can have more than two levels

In this case, we can send more than 1 bit for each level (see fig)

(a) shows 1 bit per level

(b) shows 2 bits per level

If a signal has L levels then no. of bits per level = log2 L


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Signal Element Versus Data Element

In data communications, our goal is to send data elements

A data element is the smallest entity that can represent a piece of information: this is the bit.

In digital data communications, a signal element carries dat

a elements.

A signal element is the shortest unit (time wise) of a digital signal.

data elements are what we need to send; signal elements ar

e what we can send. Data elements are being carried; signal elements are the car

riers.


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Data Rate or Bit Rate

The data rate defines the number of data elements

(bits) sent in 1 sec.

The unit is bits per second (bps)

The data rate is sometimes called the bit rate

Frequency is not appropriate characteristic for digi

tal signal

The term-bit rate isused to describe digital signals


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Bit Length

The concept of wavelength is for an analog signal:

that is the distance one cycle occupies on the trans

mission medium.

For a digital signal Bit length is used instead of wavelength.

Bit length is the distance one bit occupies on the tr

ansmission medium.


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Digital Signal as a Composite Analog Signal

Based on Fourier analysis, a digital signal is a composite analog signal. Having only infinite bandwidth

A digital signal, in the time domain (see fig), comprises connected vertical and horizontal line segments.

A vertical line in the time domain means a frequency of infinity (sudden change in time)

A horizontal line in the time domain means a frequency of zero (no change in time).

Going from a frequency of zero to a frequency of infinity (and vice versa) implies all frequencies in between are part of the domain. Hence infinite bandwidth

If the digital signal is periodic, which is rare in data communications, t

he decomposed signal has a frequency domain representation with an infinite bandwidth and discrete frequencies (see fig).

If the digital signal is non-periodic, the decomposed signal still has an infinite bandwidth, but the frequencies are continuous (see fig)


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Transmission of Digital Signals

Thus a digital signal, periodic or non-periodic, is a

composite analog signal with frequencies between

zero and infinity.

In data communications, we consider the case of anon-periodic digital signal.

A digital signal can be transmitted by using one of

two different approaches:

baseband transmission or

broadband transmission (using modulation).


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Baseband Transmission Means sending a digital signal over a channel without changing to an

analog signal. See Fig below Baseband transmission requires a low-pass channel

It is a channel with a bandwidth that starts from zero freq..

This is, the entire bandwidth of a cable connecting two computers isone single channel (a dedicated medium)

Baseband transmission of a digital signal that preserves the entire shape (bandwidth) of the digital signal is possible only if we have a low-pass channel with an infinite or very wide bandwidth e.g; coaxial cable or fiber optic

It will also be needed to send bits faster


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Broadband Transmission (Using Modulation)

Broadband transmission means changing the digital signalto an analog signal for transmission.

Modulation allows us to use a Bandpass channel

It is a channel with a bandwidth that does not start from zero freq.

we cannot send the digital signal directly to this channel; we need to convert the digital signal to an analog signal before transmission


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Example


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Relationship b/w Data Rate and Bandwidth

Any transmission system ( transmitter + medium + receiver) accommodates only a limited band of frequencies

This limits the data rate that can be carried on the medium

A square waveform (digital) has an infinite no. of frequency c

omponents and hence an infinite bandwidth

Furthermore, the greater the bandwidth of a channel, the greate

r the cost

On the other hand limiting the bandwidth of a channel increase

s distortion

The higher the data rate of a signal, the greater is its required e

ffective bandwidth

There is a direct relationship between data rate & bandwidth


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Bandwidth Used in two context

1) Bandwidth in hertz, refers to the range of frequencies in a composite signal or the range of frequencies that a channel can pass.

If a telephone channel can transmit frequencies from 300

Hz to 3400Hz, it has a BW of 3100 Hz.2) Bandwidth in bits per second, refers to the speed of bit trans

mission in a channel or link

The bandwidth of a Fast Ethernet network is a maximumof 100 Mbps. This means that this network can send 100Mbps.

An increase in bandwidth in hertz means an increase in bandwidth in bits per second


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Bandwidth categories of a Channel

Narrowband:

This is for the channels with BW less than 4000 Hz.

E.g. a telegraph channel has a BW of 200 Hz.

Voiceband: This is the range of frequencies transmitted over a norm

al telephone network channel i.e. 4000 Hz.

Wideband:

Channels which have a BW exceeding 4000 Hz are usually placed under this category.


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Other Terms

Pass Band: a particular range of frequencies which can be passed through the

transmission equipment.

e.g a telegraph circuit could have a pass band between 1200 to 14

00Hz, and a BW of 200Hz.

Cut-off frequencies:

The cut-off frequencies of the telegraph channel above are 1200 H

z and 1400 Hz.

A frequency at which the attenuation of a device begins to increas

e sharply, such as the limiting frequency below which a travelingwave in a given mode cannot be maintained in a waveguide, or the

frequency above which an electron tube loses efficiency rapidly.

Also known as critical frequency or corner frequency.


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Throughput

It is a measure of how fast we can actually send data t

hrough a network.

Although, bandwidth in bits per second and throughpu

t seem the same, but they are different.

In other words, the bandwidth is a potential measurement of a link; the throughput is an actual measurement

of how fast we can send data.

E.g; we may have a link with a bandwidth of 1 Mbps,

but the devices connected to the end of the link may handle only 256 kbps. This means that we cannot send

more than 256 kbps through this link.


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Transmission Impairments

Signals travel through transmission media, which are not perfect

Thus signal received may differ from signal transmitted causing:

For analog signals - degradation of signal quality For digital signals - bit errors (1becomes 0 or 0 becomes1)

Called Transmission Impairments

Most significant impairments are

Attenuation (weak signals) Delay distortion (delay due to distortion)

Jitters (variation in delay)

Noise (unwanted signals)


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Attenuation

When signal strength falls off with distance due to medium imp

erfection is called Attenuation received signal strength must be:

strong enough to be detected

sufficiently higher than noise to be received without error

Solution: Strength can be increased using amplifiers/repeaters

To show that a signal has lost or gained strength, engineers use

the unit of the decibel.

The decibel (dB) measures the relative strengths of two signals

or one signal at two different points (P1 & P2).

Note that the decibel is negative if a signal is attenuated and po

sitive if a signal is amplified.


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Delay Distortion or Distortion

Distortion means the signal changes its form or shape Distortion can occur in a composite signal made of differ

ent frequencies.

Each signal component has its own propagation speed through a medium

Hence various components arrive at the receiver at

different times

Resulting in phase shifts between the different frequencies if the received signal is distorted due to varying delays

experienced at its component frequencies Differences in delay may create a difference in phase if t

he delay is not exactly the same as the period duration.


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Cont.. The shape of the composite signal is therefore not the same. (See Figure)

It only occurs in guided media

It is particularly critical for digital data

Because some signal components of one bit position will run over into oth

er bit positions, causing inter-symbol interference


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Jitters

Due to variation in phase delay

We can roughly say that jitter is a problem if different packets of data encounter different delays and the application using the data at the receiver site is time-sensitive (audio and

video data) For example, Ifthe delay for the first packet is 20 ms, for t

he second is 45 ms, and for the third is 40 ms, then the real-time application that uses the packets suffers jitter

Attenuation, distortion and the modulation of the telephonechannel causes variation in the propagation delay at any single signal

This is known as Jitter.


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Noise

Additional undesired signals added between transmitterand receiver

Thermal noise

Due to random motion of electrons in a wire which creates anextra signal

It is a function of temperature

Uniformly distributed across the bandwidth

Present in all electronic devices and transmission media

Also called white noise

Particularly significant for satellite communication


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Noise

Intermodulation noise When signal at different frequencies share the same transmis

sion medium

Produce new signals at a freq that is the sum, difference, ormultiple of the two original frequencies sharing the same me

dium

Crosstalk noise

Crosstalk is the interference of one wire on the other.

A signal from one line is picked up by another

Unwanted coupling between signal paths

Due to electrical coupling between nearby twisted pairs


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Noise

Impulse noise It is an irregular noise spike (a signal with high energy in a ve

ry short time)

Such as short clicks and crackles with no loss of intelligibility

Due to the fault in system, external electromagnetic interference e.g; power lines, lightning, and so on

It is of short duration and high amplitude

A minor annoyance for analog data

But a major source of error in digital data

a noise spike could corrupt many bits


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Noise

Echo noise On some very long circuits, mismatching of the lines ca

uses the signals to be reflected back to the speaker after

a slight delay.

To overcome this, echo suppressors or echo cancellers are fitted to the line so that speech is being transmitted o

nly in one direction at a time.


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BIT and BAUD RATES Transmission speed is measured in terms of bits per second (BPS)

The higher the BW, the greater the data carrying capacity of a channel.

The Baud rate is the no. of distinct signals (signal elements) sent in one sec. It indicates the no. of signal elements per second or no. of pulses per second

A signal element is represented by a change in a particular characteristic (Amplitude , frequency & phase) of a Sine wave form. Therefore , the Baud rate indicates how many changes of phase, frequency or amplitude there will be inone second.

The baud rate is sometimes called the pulse rate, the modulation rate, or the signal rate

The term ModulationRate is used in preference to Baud rate when talkingabout modems.

Nyquists law states that maximum theoretical Baud or Bit rate of the telephone channel is twice the BW:

Max. Baud Rate = 2 x Bandwidth ; or

Max. Bit Rate = (Max. Bd) x log2 L

where L is the number of signal levels


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Noiseless Channel: Nyquist Capacity

Usually bit rate (bps) and baud rate (Bd) are the same exce

pt when a baud represents more than one bit of information

For a Noise free channel the limitation on bit rate is only th

e BW of the signal

Nyquists theorem states that given a BW (W) the highest

data rate that can be carried is 2W. Which means if you are

using a voice grade telephone of a BW of 3000 Hz to trans

mit your data, then the capacity of the channel is 2W=6000

bps. (when log2 L= 1).


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However if multilevel coding (modulation) is used,

the capacity of the link becomes:

C= 2W log2L

Where L= no. of discrete signals or voltage levels.

If we use 3-bit encoding scheme , 23=8=L;and

W=3000; then C = 2 (3000) (log28)

= 6000(3) = 18000bps But we know that increasing the levels of a signal

may reduce the reliability of the system.

Cont..


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Example:

Data is to be transmitted over the PSTN (public s

witched telephone network)using a transmission s

cheme with 16 levels per signaling element. If the

BW of the PSTN is 3000 Hz, work out the Nyquist maximum data transfer rate (C)

Solution: L=16 means we use 4-bit encoding sche

me and W=3000; then

C = 2 (3000) (log2 16)

= 6000(4) = 24000bps


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Signal-to-Noise Ratio (SNR) But communication channels are always affected by noise a

nd distortions. To find the theoretical bit rate limit, we need to know the rat

io of the signal power to the noise power

If signal-to-noise ratio is:

S/N = Average signal power / Average noise power S/N is actually the ratio of what is wanted (signal) to what is

not wanted (noise).

A high S/N means the signal is less corrupted by noise; a lo

w S/N means the signal is more corrupted by noise. Because S/N is the ratio of two powers, it is often described

in decibel units and is called Signal-to-Noise ratio (SNR) as:

SNR = l0logl0 (S/N ) dB


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For a noisy channel, the data rate is related to the s

ignal to noise ratio, and we use the formula given

by Shannon and Hartley:

C = W x log2(1+ S/N) bpsC = data rate in bps; W is the BW of the line in Hz

; S is the average signal power in Watts; N is the a

verage noise power in Watts.

Noisy Channel: Shannon Capacity


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Example:

Assuming that a PSTN has a BW of 3000Hz and a

typical signal to noise power ratio of 20 dB, deter

mine the maximum data rate that can be achieved.


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Solution:

SNR = signal-to-noise-ratio=10 log10(S/N)

therefore:

20 = 10 log10(S/N)

2 = log10(S/N)102 = S/N; Hence: S/N = 100;

now : C = W log2(1+ S/N) bps

Therefore: C = 3000 x log2

(1+ 100) bps

C = 3000 [ log2 (101)] bps

C = 19,976 bps


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Summary

Looked at:

Analog vs Digital signals

Simple vs Composite signals

Periodic vs Non-Periodic signals

Frequency, wavelength, spectrum & bandwidth

Transmission of digital signals

Bit rate, bit length, Baud rate

Channel Capacity

Transmission impairments

Noiseless and Noisy channels


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A sine wave is offset 1/6 cycle with r

espect to time 0. What is its phase in

degrees and radians?


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If a periodic signal is decomposed into five sine wave

s with frequencies of 100, 300, 500, 700, and 900 Hz,

what is its bandwidth? Draw the spectrum, assuming

all components have a maximum amplitude of 10 V


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Solution

Let fh be the highest frequency, fl the

lowest frequency, and B the bandwidth

. Then B =fh - it = 900 - 100 =800 Hz

data transmission ch 2

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