Materi 2 Data Transmission

Chap. 2 Data Transmission 1

1. Concepts and Terminology

• Guided media - twisted pair, coaxial cable, optical fiber• Unguided media - air, vacuum, sea water

Transmission media

Transmitter/receiver


Medium Amp orrepeater

Medium

0 or more

Point-to-point vs. Multipoint



... Transmitter/receiver


...

Amp orrepeater

Medium Medium

0 or more

Simplex vs. Duplex

A BSend or receive

A BSend and receive

Only one way at a time

A BSend receivesimultaneously

Simplex

Half-duplex

Full-Duplex


• Signals can be described:– in the time domain– in the frequency domain

• Time-Domain CharacterizationAmplitude

Time, t

• Continuous• Discrete• Periodic• Aperiodic

• Periodic Signal


• Sinusoidal signal

s(t) = A sin(2πft + θ)phase

Frequency = 1 / period (T)amplitude


• Frequency Domain Concepts– Any periodic signal can be decomposed into a

sum of sinusoidal signals using a Fourierseries expansion

– The component sinusoids are at frequenciesthat are multiples of the basic frequency ofperiodicity

TftnfCCtx n

nn

1 ),2cos()( 00

10 =++= ∑

∞

=

θπ

harmonics Fundamental frequency


– Even non-periodic signals can be characterizedin the frequency domain using a continuousspectrum of frequency components

dtetsfS ftj∫∞

∞−−=

2)()( π

S(t)

t1/2-1/2

f�

f�sin

– Spectrum of a signal - the range of frequencies it contains

– Absolute bandwidth - the width of the spectrum– Effective bandwidth or just bandwidth - the band

of frequencies which contains most of the energy of the signal - half-power bandwidth

– dc component - when the signal contains zero frequency


Signal with dc component


• Relationship Between Data Rate andBandwidth

– Consider the case binary data is encodedinto digital signal, and to be transmitted by atransmission medium

– Digital signal contains an infinite bandwidth,but a real transmission medium has a finitebandwidth, which can limit the data rate thatcan be carried on the transmission medium

( ) .1,3,5,..k ,2sin1

11

== ∑∞

=

tf��k

s(t)k


– Limited bandwidth creates distortions of the inputsignal,which makes the task of interpreting the receivedsignal more difficult

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

– The high the data rate of a signal, the greater is itseffective bandwidth

– The grater the bandwidth of a transmission system, thehigher is the data rate that can be transmitted


• Signal Strength– Signal amplification / attenuation are expresses in

logarithmic unit, decibel

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

• e.g. Pin = 10 watts, Pout = 100 watts,NdB = 10 log (100/10) = 10 dB

Pin = 100 watts, Pout = 10 wattsNdB = 10 log (10/100) = -10 dB

– The decibel is also used to measure thedifference in voltage

Comm.systemPin Pout

=

PowerPowerlog10N

in

out10dB

Amp+10dB

Medium-7dB

Amp+10dB

Medium-7dB

PoutPin

NdB = 10 log (Pout/Pin) = +10 -7 +10 -3 = +10 dB Pout = 10 Pin

VV20log

PP10logN

in

out

in

outdB ==


• Data: Entity that conveys meaning• Signal: Electric/Electromagnetic encoding

(representation) of data• Signaling: Act of propagating the signal

along a suitable medium• Transmission: Communication of data by

the propagation and processing ofsignals

2. Analog and Digital Data Transmission


• Transmission techniques can be analog or digital• With analog transmission, signals are transmitted

without regard to content; with digitaltransmission, the content of message could beinterpreted to aid in faithful transmission

• Important distinction is in the manner signalattenuation is handled at repeater / amplifiers

• Analog - Attenuated signal is amplified andretransmitted

• Digital - Data encoded in attenuated signal isrecovered, a new signal is generated encodingthat data, and retransmitted

• Digital signals always digitally transmitted, butanalog signals can be transmitted either way(assuming the signal carries digital data)

Analog Data Digital Data

AnalogSignal

DigitalSignal

Modem (ASK, FSK, PSK)

Usually binary (NRZ, Manchester)

e.g. telephone

CODEC


• Attenuation– the strength of a signal falls off with distance– varies as a function of frequency

• Delay distortion– the velocity of propagation of a signal

through a guided medium varies withfrequency

• Noise– Thermal noise

• white noise

– Intermodulation noise• when two signals at different frequencies are

mixed in the same medium, sum or differenceof original frequencies or multiples of thosefrequencies can be produced, which caninterfere with the intended signal

• occurs when there is some nonlinearity in thesystem

– Crosstalk• when there is an unwanted coupling between

signal paths

– Impulse noise

3. Transmission Impairments(Signal corruption during transmission)


Attenuation

Delay

Noise

Regular pulse

Attenuation and delayas a function of frequency


• Effect of noise on a digital signal


• Channel Capacity– The rate at which digital data can be transmitted

over a given communication channel– Nyquist limit (In a noise-free environment)

– Ex: Transmitted sequence

C = 2 W log2M

Channel capacityin bits/second

Bandwidth of physical channel (medium)

# of levels used in signaling

1 0 0 1 1 0 1 0 0 0 1 1 0 1 0 0 1 0T

2-level encoding, W = 1/2T, C= 1/T

10 01 10 10 00 11 01 00 10

111110101100011010001000

C= 2/T

11100100

100 110 100 011 010 010

C= 3/T


• Channel Capacity– Shannon’s law

• considers the noise

• key parameter is signal-to-noise ratio (S/N, orSNR), which is the ratio of the power in a signal tothe power contained in the noise, typicallymeasured at the receiver

• often expressed in decibels

+=

NS

1logWC 2

( )power noise

power signallog10/ =NS

dB

Materi 2 Data Transmission

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