data communication - DataTransmission

8/3/2019 data communication - DataTransmission

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Dat a Com m unic a t ion

Chapt er 3

Dat a Transm iss ion


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Transm iss ionTerm inology (1) Transmitter

Receiver

Medium

Guided medium

e.g. twisted pair, coaxial cable, optical fiberUnguided medium

e.g. air, water, vacuum


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Transm iss ionTerm inology (2) Direct link

No intermediate devices between Tx and Rx.

Point-to-point

Direct link from Tx to Rx.

Only 2 devices share link Multi-point

More than two devices share the link


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Point -t o-poin t vs. Mul t ipoint


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Transm iss ion Term inology (3) Simplex transmission

One direction

e.g. Radio and television broadcast.

Half duplex transmission

Either direction, but only one way at a time

e.g. police radio(walki-talki)

Full duplex transmission

Both directions at the same time e.g. telephone, satellite.


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Sim plex vs. Duplex


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Signal represent at ionSignals can be described:

in the time domain

in the frequency domain


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Frequenc y, Spec t rum and

Bandw id th 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 time


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Analogue & Dig i t a l Signals


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Periodic

Signals


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Sine Wave Peak Amplitude (A)

maximum strength of signal

volts

Frequency (f)

Rate of change of signalHertz (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 s in (2f t +)


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Wavelength Distance occupied by one cycle

Distance between two points of correspondingphase in two consecutive cycles

Assuming signal velocity v = vT

f = v

c = 3*108 ms-1 (speed of light in free space)


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Frequenc y Dom ain Conc ept s

Any periodic signal can be decomposed into a

sum of sinusoidal signals using a Fourier seriesexpansion.

The component sinusoids are at frequencies that

are multiples of the basic frequency ofperiodicity

Harmonics Fundamental frequency


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Even non-periodic signals can be characterized in the frequency domain usina continuous spectrum of frequency components


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Spec t rum & Bandw idt h

Spectrum

range of frequencies contained in signal

Absolute bandwidth

width of spectrum

Effective bandwidthNarrow band of frequencies containing most of the

signal energy.

DC ComponentComponent of zero frequency


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Signal w i t h DC Com ponent


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Dat a Rat e and Bandw id t h

Any transmission system has a limited band of frequencies This limits the data rate that can be carried.

Consider the case binary data is encoded into digital signal,and to be transmitted by a transmission medium Digital signal contains an infinite bandwidth, but a real

transmission medium has a finite bandwidth, which can limit

the data rate that can be carried on the transmission medium. Limited bandwidth creates distortions of the input signal,

which makes the task of interpreting the received signal moredifficult .

The more limited bandwidth, the greater the distortion, andthe greater the potential for error by the receiver

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

The grater the bandwidth of a transmission system, the higheris the data rate that can be transmitted


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Signal St rengt h

Signal amplification / attenuation are expressesin logarithmic unit, decibel.

Gain (amplification) / loss (attenuation) of asystem is expressed as


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Analog and Dig i t a l Dat a

Transmiss ion

Data

Entities that convey meaning

Signals

Electric or electromagnetic representations of data

TransmissionCommunication of data by propagation and

processing of signals


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Analog Dat a and Dig i t a l Dat a

Analog

Continuous values within some interval

e.g. sound, video

Digital

Discrete valuese.g. text, integers


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Ac oust ic Spec t rum (Ana log)


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Analog and Dig i t a l Signals

Means by which data are propagated

Analog

Continuously varying EM wave. propagate in various media

wire, fiber optic, space

Speech bandwidth 100Hz to 7kHzTelephone bandwidth 300Hz to 3400Hz

Video bandwidth 4MHz

Tv channel bandwidth 6.2 MHz. Digital

Use two DC components


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At t enuat ion o f Dig i t a l Signals


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Com ponent s o f Speec h

Frequency range (of hearing) 20Hz-20kHz

Speech 100Hz-7kHz

Easily converted into electromagnetic signal fortransmission

Sound frequencies with varying volumeconverted into electromagnetic frequencies withvarying voltage

Limit frequency range for voice channel300-3400Hz


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Convers ion o f Vo ic e Input in t o

Analog Signal


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Binary Dig i t a l Dat a

From computer terminals etc.

Two dc components

Bandwidth depends on data rate, the higher thedata rate the higher the bandwidth of the

signal.


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Convers ion of PC Input t o

Digi t a l Signa l


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Dat a and Signa ls

Usually use digital signals for digital data andanalog signals for analog data

Can use analog signal to carry digital data

Modem

Can use digital signal to carry analog datacodec

A l Si l C i A l


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Analog Signa ls Carry ing Analog

and Dig i t a l Dat a

Di i t l Si l C i A l


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Digi t a l Signa ls Carry ing Analog

and Dig i t a l Dat a


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Analog Transm iss ion

Analog signal transmitted without regard tocontent (May be analog or digital data)

Attenuated over distance

Use amplifiers to boost signal

Also amplifies noise, thus received signal will bedistorted.

If digital data is encoded then amplifiers will

increase BER (bit error rate).


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Dig i t a l Transm iss ion

Concerned with content of the signal.

Integrity endangered by noise, attenuation etc.

Repeaters used to achieve grater distance.

Repeater receives signal

-Extracts bit pattern-Retransmits new signal

-Attenuation is overcome-Noise is not amplified

Ad t f Di i t l


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Advant ages of Dig i t a l

Transmiss ion

Digital technology Low cost LSI/VLSI technology

Data integrity Longer distances over lower quality lines

Capacity utilization High bandwidth links economical

High degree of multiplexing easier with digital techniques

Security & Privacy Encryption

Integration Can treat analog and digital data similarly


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Transm ission Im pa irm ent s

Signal received may differ from signaltransmitted

Analog - degradation of signal quality

Digital - bit errors

Caused byAttenuation and attenuation distortion

Delay distortion

Noise


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At tenua t ion

Signal strength falls off with distance

Depends on medium

-guided: attenuation is logarithmic.-unguided: attenuation depends on atmospheric structure.

Received signal strength:

must be enough to be detected

must be sufficiently higher than noise to be received withouterror

Attenuation is an increasing function of frequency-attenuation distortion affects analog signals much more

than digital signals.


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Delay Dis t or t ion

Only in guided media

Caused by: Propagation velocity varies with

frequency.

-diffirent frequency components arrive at the

receiver at different times causing phase shifts. for digital data delay distortion introduces ISI.

Equalizers : reduce delay distortion.


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Noise (1)

Additional signals inserted between transmitterand receiver

ThermalDue to thermal agitation of electrons

Uniformly distributed

White noise Intermodulation

produce signals at frequency that is the sum and

difference of original frequencies sharing a medium.Caused by nonlinearity in Tx, Rx, or channel because

of signal strength.


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Noise (2)

Crosstalk

A signal from one line is picked up by another

ImpulseIrregular pulses or spikes

e.g. External electromagnetic interference

Short duration

High amplitude

Severe effect on digital signal of high data rate.


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Channel Capac i t y

Data rate

In bits per second

Rate at which data can be communicated

Bandwidth of transmitted signal

Constrained by transmitter and medium


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Nyquist c apac i t y form u la

C: channel capacity(maximum error free data rate)

M: # of signaling levels.

22 logC B M=


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Shannon Capac i t y Form ula

Consider data rate, noise and error rate

Faster data rate shortens each bit so burst of noise

affects more bitsAt given noise level, high data rate means higher error rate

Signal to noise ration (in decibels)

C=B log2(1+SNR)

This is error free capacity (AWGN)

Ex.: SNR=24dB, B=1MHzC=8Mbps and M=16

10signal power10lognoise power

dBSNR =

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