02_datatransmission_tvm

7/27/2019 02_DataTransmission_TVM

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TRANSMISSION TERMINOLOGY

TRANSMISSION SYSTEM/MEDIA Guided or Unguided

DIRECT LINK

Transmission path between two devices in which signals propagate

DIRECTLY from transmitter to receiver with NO INTERMEDIATE

DEVICES OTHER THAN REPEATERS OR AMPLIFIERS

Applicable to BOTH guided and unguided media


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POINT-TO-POINT

GUIDED transmission

medium

There exist a DIRECT

LINK between 2 devices

Only 2 devices share link

MULTIPOINT

GUIDED transmission

medium

There exist a DIRECTLINK between multiple

devices



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SIMPLEX

One direction

One station is the receiver and the other is the transmitter

e.g. Television

HALF-DUPLEX Either direction, but only one way at a time

e.g. police radio

FULL-DUPLEX

Both directions at the same time e.g. telephone



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Viewed as a function of time, and EM

signal can be either continuous or

discrete

Continuous Signal Analog Signal One in which the signal intensity varies in a

smooth fashion over time

No breaks or discontinuities in the signal

Discrete Signal - Digital signal Maintains a constant level & then changes to

another constant level

TYPES OF SIGNAL


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Periodic signal Pattern repeated over time

Sine wave, square wave

Satisfies the criteria s(t+T) = s(t)

Aperiodic signal Pattern not repeated over time

TYPES OF SIGNAL


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SINE WAVE

A sine wave is the fundamental continuous signal

The general formula for a sine wave is s(t) = A sin(2ft +) Peak Amplitude (A)

maximum strength of signal

Measure in volts

Frequency (f)

Rate of change of signal

Hertz (Hz) or cycles per second

Period = time for one repetition (T) T = 1/f

Phase ()

Relative position in time


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VARYING SINE WAVES

S(T) = A SIN(2FT +)


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Distance occupied by one cycle

Distance between two points of corresponding

phase in two consecutive cycles

Represented by

Assuming signal velocity v

= vT for a particular signal

f = v

c = 3*108 m/s(speed of light in free space)

WAVELENGTH


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FREQUENCY DOMAIN

CONCEPTS


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In general, signal is made up of many frequencies

Components are sine waves

Can be shown (Fourier analysis) that any signal is made up ofcomponent sine waves

Can plot frequency domain functions

FREQUENCY DOMAIN CONCEPTS


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ADDITION OF FREQUENCY COMPONENTS

Representation of one

individual frequency

component

Addition of

individual frequency

components gives


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ADDITION OF FREQUENCY COMPONENTS

FUNDAMENTAL FREQUENCY lowest frequency of the periodic

waveform to whom other

frequencies are integer

multiples (also called First

Harmonic)

PERIOD OF THE TOTAL SIGNAL isequal to the period of the

fundamental frequency


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TIME DOMAIN VS FREQUENCY DOMAIN

REPRESENTATION

Peak Amplitude isrepresented on Y-Axis

X-Axis represents

frequency components

of a sinusoid

DC Component (Component of

Zero frequency

For each signal , there is a TIME DOMAINFUNCTION s(t ) that speci f ies theAMPLITUDE of the s ignal at each instant int ime

Simi lar ly , there is a FREQUENCY DOMAINFUNCTION S(f ) that speci f ies theCONSTITUENT FREQUENCIES of the s ignal .


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Spectrum

range of frequencies contained in signal Absolute bandwidth

width of spectrum

Effective bandwidth

Often just bandwidth

Narrow band of frequencies containing most of the energy

DC Component

Component of zero frequency

SPECTRUM & BANDWIDTH


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The durat ion of each pulse below is

1/(2f1) o r T

Data Rate = 2f 1 or 2bps

RELATIONSHIP BETWEEN DATA RATE AND

BANDWIDTH

The durat i on o f each pu l se i n the l e f t s i de i s1 / ( 2 f 1) o r T

What a re the f requency component o f th i s

s i gna l ? THE SQUARE WAVE HAS UNL IMITEDFREQUENCY COMPONENT AND THUS UNL IMITEDBANDWIDTH EFFECT IVE BANDWIDTH! I f we w i l l l i mi t i s to on l y 3 f requency

component , i f f 1 = 1MHz , then BW = 4MHz anddata ra te i s 2Mbps (1 b i t fo r eve ry 0 .5us ec)

B W i s d i r e c t l y p r o p o r t i on a l t o D a t a R a t e


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ANALOG AND DIGITAL

DATA TRANSMISSION


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The term analog and digital is a term used frequently in at

least 3 context namely:

DATA

Entities that convey meaning

SIGNALS

Electric or electromagnetic representations of DATA Signaling is the act of propagating the signal along a suitable medium

TRANSMISSION

Communication of data by propagation and processing of signals

ANALOG AND DIGITAL DATA

TRANSMISSION


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ANALOG SIGNAL REPRESENTING ANALOG

AND DIGITAL DATA

Digital data can also be

represented by analog

signals by use of a MODEM(modulator/demodulator).

The MODEM converts aseries of binary (two-

valued) voltage pulses into

an analog signal by

encoding the digital data

onto a carrier frequency.


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CONVERSION OF VOICE SIGNAL INTO

ANALOG SIGNAL

voice frequencies becomes theinput of a conversion-device

Loudness of voice frequency is

the amplitude of the input signal


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DIGITAL SIGNAL REPRESENTING ANALOG

AND DIGITAL DATA

Analog data can be

represented by digital signals.

The device that performs this

function for voice data is a

CODEC (coder-decoder). In essence, the CODEC takes

an analog signal that directly

represents the voice data and

approximates that signal by a

bit stream. At the receiving

end, the bit stream is used to

reconstruct the analog data.


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CONVERSION OF BINARY INPUT TO

DIGITAL SIGNAL


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Both analog and digital signals may be transmitted on suitable

transmission media

ANALOG TRANSMISSION ANALOG MODULATION means of transmitting analog signals without regard to their content

the signals may represent analog data (e.g., voice) or digital data (e.g.,

binary data that pass through a modem)

the analog signal will become weaker (attenuated) after a certain

distance

AMPLIFIERS boost the energy of the signal in order to achieve longer distances

Also boosts the noise component

As more amplifiers are added, signal become more distorted

For analog data, such as voice, quite a bit of distortion can be tolerated

and the data remain intelligible. However, for digital data, cascaded

amplifiers will introduce errors.

ANALOG AND DIGITAL TRANSMISSION


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DIGITAL TRANSMISSION DIGITAL MODULATION Concern with the content of the signal

can be transmitted only a limited distance before attenuation

endangers the integrity of the data.

REPEATERS Used to achieve greater distances for digital transmission

repeater receives the digital signal, recovers the pattern of 1s and Os, and

retransmits a new signal, thereby overcoming the attenuation

The same technique may be used with an analog signal if it is assumed

that the signal carries digital data.

The repeater recovers the digital data from the analog signal andgenerates a new, clean analog signal. Thus, noise is not cumulative.

ANALOG AND DIGITAL TRANSMISSION


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Digital technology. The advent of large-scale integration (LSI) and verylarge scale integration (VLSI) technology has caused a continuing drop inthe cost and size of digital circuitry. Analog equipment has not shown asimilar drop.

Data integrity. With the use of repeaters rather than amplif iers, the effectsof noise and other signal impairments are not cumulative. I t is possible,then, to transmit data longer distances and over lesser qual ity l ines by

digital means while maintaining the integrity of t he data. Capacity uti l ization. I t has become economical to bui ld transmission l inks

of very high bandwidth, including satel l i te channels and connectionsinvolving optical f iber. A high degree of multiplexing is needed toeffectively uti l ize such capacity, and this is more easi ly and cheaplyachieved with digital (t ime division) rather than analog (frequency -division)techniques.

Security and privacy. Encryption techniques can be readi ly appl ied todigital data and to analog data that have been digit ized.

Integration. By treating both ana log and digital data digital ly, al l s ignalshave the same form and can be treated similarly. Thus, economies of scaleand convenience can be achieved by integrating voice, v ideo, and digitaldata.

ADVANTAGE OF DIGITAL TRANSMISSION


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TRANSMISSIONIMPAIRMENTS


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With any communications system, it must be recognized that

the received signal will dif fer from the transmitted signal due

to various transmission impairments.

Analog Signals Degradation of signal quality Digital Signals Bit errors Most Significant Impairments Attenuation Delay distortion Noise

TRANSMISSION IMPAIRMENTS


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The strength of a signa l falls off with distance over any

transmission medium.

For guided media, this reduction in strength, or attenuation,

is generally logarithmic and is thus typically expressed as a

constant number of decibels per unit distance.

For unguided media, attenuation is a more complex functionof distance and of the makeup of the atmosphere.

Attenuation introduces three considerations for the

transmission engineer.

DESIGNER NEEDS TO ADDRESS PROBLEMS: 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 frequency

Equalizer circuit

Amplifiers that amplify high frequencies more than low frequencies

ATTENUATION


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HOW DIGITAL SIGNALS ARE ATTENUATED

2 voltage levels to represent binary 0 and binary 1

Revived waveform is rounded and small


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Unique to GUIDED TRANSMISSION MEDIA

Propagation velocity through a guided medium varies with

frequency

Different frequency components experience different delays

but they eventually arrive BUT at different time Particularly critical for digital data consider that a sequence

of bits is being transmitted, using either analog or digital

signals

Because of delay distortion, some of the signal components of

one bit position will spill over into other bit positions, causingINTERSYMBOL INTERFERENCE which is a major limitation to

maximum bit rate over a transmission control

Equalizing techniques can also be used for delay distortion.

DELAY DISTORTION


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Additional unwanted signals that are inser ted somewhere

between transmission and reception

Noise may be divided into four categories :

Thermal

Intermodulation Crosstalk

Impulse

NOISE


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Due to thermal agitation of electrons Function of temperature thus it is distributed across the

frequency spectrum

Referred to as WHITE NOISE

It CANNOT be eliminated

Sets an upper bound on the performance of the

communication system

NOISE - THERMAL

N = kTW

N = noise power density, watts/hertz

k = Boltzmann's constant = 1.3803 X 10-23 Joules/deg Kelvin

(J/K)

T= temperature, degrees Kelvin

W= bandwidth, Hertz


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Crosstalk has been experienced by anyone who, while using

the telephone, has been able to hear another conversation

It is an unwanted coupling between signal paths

It can occur by ELECTRICAL COUPLING between nearby twistedpair or, rarely, coax cable lines carrying multiple signals.

It can also occur when unwanted signals are picked up by

microwave antennas; although h ighly directional, microwave

energy does spread during propagation

It is of the same order of magnitude (or less) as thermal

noise.

NOISE - CROSSTALK


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Noncountinuous, irregular pulses or spikes in short

duration but of RELATIVELY HIGH AMPLITUDE

External electromagnetic disturbance such as lightning andfaults and flaws in the communication system

Minor annoyance for analog signal but it is a MAJOR source

of error for digital data e

NOISE - IMPULSE


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We have seen that there are a variety of impairments that distort orcorrupt a signal.

For digital data, the question that then arises is to what extent theseimpairments l imit the data rate that can be achieved.

The rate at which data can be transmitted over a given communicationpath, or channel, under given conditions, is referred to as the channel

capacity. There are four concepts here that we are try ing to relate to each other:

Data Rate Rate at which data can be communicated, bps

Bandwidth This is the bandwidth of the transmitted signal as constrained by the transmitter

and by the nature of the transmission medium, expressed in cycles per second, orhertz.

Noise Average level of noise in the communication path

BER The rate at which errors occur, where an error is the reception of a 1 when a 0 was

transmitted, or the reception of a 0 when a 1 was transmitted.

CHANNEL CAPACITY


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The problem we are addressing is this:

Communications facilities are expensive, and, in general, the greaterthe bandwidth of a facility, the greater the cost.

The limitations arise from the physical properties of the

transmission medium or from deliberate limitations at thetransmitter on the bandwidth to prevent interference from

other sources.

Accordingly, we would like to make as efficient use as

possible of a given bandwidth.

For digital data, this means that we would like to get as higha data rate as possible at a particular limit of error rate for a

given bandwidth.

The main constraint on achieving this ef ficiency is NOISE .

CHANNEL CAPACITY


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In this envi ronment , the l imitat ion on data rate is s imply the bandwidth of thesignal .

A formulat ion of th is l imitat ion, due to Nyquist , states that i f the rate of s ignalt ransmission is 2W, then a s ignal wi th f requencies no gre ater than W is suff ic ientto carry the data rate.

The converse is a lso t rue: Given a bandwidth of W, the highest s ignal rate that canbe carr ied is 2W. This l imitat ion is due to the effect of intersymbol interference,such as is produced by delay d istort ion.

With mult i level s ignal ing, the Nyquist formulat ion becomes:

So, for a g iven bandwidth, the data rate can be increased by increasing the numberof d i f ferent s ignals . However, th is p laces an increased burden on the receiver :Instead of d ist inguishing one of two possib le s ignals during each signal t ime, i tmust d ist inguish one of M possib le s ignals .

Noise and other impairments on the t ransmission l ine wi l l l imit the pract ical valueof M.

Thus, a l l other things being equal , doubl ing the bandwidth doubles the data rate.

CASE 1: NOISEFREE CHANNEL

NYQUIST THEOREM

C = 2Wlog2 M

C = datarate, bps

W= bandwidth, Hertz

M = number of discrete signals or voltage levels


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The higher the dat a rate, the more damage that unwanted noise can do.

For a g iven level of noise, we would expect that a greater s ignal st rength wouldimprove the abi l i ty to correct ly receive data in the pr esence of noise.

The key parameter involved in this reasoning is the s ignal - to -noise rat io (SNR),which is the rat io of the power in a s ignal to the power co ntained in the noise thatis present at a part icular point in the t ransmission

Typical ly , th is rat io is measured at a receiver , as i t is at th is point that an at temptis made to process the s ignal and el iminate the unwanted nois e.

For convenience, th is rat io is often reported in decibels :

SNR db =10 log 10 (s ignal/noise) Shannon Capaci ty formula states:

This formula indicates the ERROR FREE CAPACITY. Thus, Shanno n proved that i f theactual informat ion rate on a channel is le ss than the error - f ree capac i ty , then i t istheoret ical ly possib le to use a sui table s ignal code to achieve error - f reetransmission through the channel .

CASE 2: WITH NOISE AND ERROR RATE

SHANNON CAPACITY FORMULA

C = Wlog2 (1+SNR) or C = 3.32Wlog (1+SNR)

C = datarate, bps

W= bandwidth, Hertz

SNR = Signal to Noise ratio (in ratio and not logarithmic)

O O S G


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Consider a signal, digital or analog, that contains binar y digital datatransmitted at a certain bit rate R. Recalling that 1 watt = 1 joule/s,the energy per bit in a si gnal is given by Eb = ST b, where S is the signalpower and Tb is the t ime requir ed to send one bit . The data rate R is

just R = l/Tb. Thus,

This ratio is i mportant because the bit error rate for digital data is a(decreasing) function of this ratio.

Given a value of Eb/Noneeded to achieve a desired error rate, theparameters in the preceding formula may be selected.

Note that as the bit rate R inc reases, the transmitted signal power,relative to noise, must increase to maintain the req uired Eb/No.

RATIO OF SIGNAL

ENERGY PER BIT TO NOISE-POWER DENSITY

PER HERTZ EB/NO

Eb/No = (S/R)/No = S/kTR

S = bps signal power, watts

No= noise power density in 1Hertz, watts/hertz

R = datarate, Hertz

k = Boltzmann's constant = 1.3803 X 10-23 Joules/deg Kelvin (J/K)

T= temperature, degrees Kelvin


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END

02_datatransmission_tvm

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