Data and Computer Communications

by William StallingsEighth Edition

Data Transmission Data Transmission

Click to edit Master subtitle styleNetworks and Communication Department

Chapter 3

1

Transmission Terminology

Data transmission occurs between a transmitter & receiver via some medium

Types of communications

direct link : no intermediate devices.

Types of communications:

1) point-to-point : direct link , only 2 devices share link

2) Point –to- multipoint : more than two devices share the link, one sender and multiple recipients. eg: voice conferencing, one person will be talking but many others can listen.

3) Simplex : only in one direction , there is one sender and one receiver; the sender and receiver cannot change roles. eg. Television ,.

4) half duplex : either direction, but only one way at a time, eg. police radio, A walkie-talkie

5) full duplex : both directions at the same time , eg. telephone

Frequency, Spectrum and BandwidthTime domain concepts

analog signal various 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

Analogue & Digital Signals

Periodic Signals

Sine Wave

peak amplitude (A) maximum strength of signal 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

Varying Sine Waves s(t) = A sin(2ft +)

Wavelength ()•is distance occupied by one cycle b, or, put another way, the distance between two points of corresponding phase of two consecutive cycles. •assuming signal velocity v have = vT•or equivalently f = vespecially when v=c

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

Wavelength ()

Frequency Domain Conceptssignal are made up of many frequencies

components are sine waves

Fourier analysis can shown that any signal is made up of component sine waves

can plot frequency domain functions

Addition of Frequency Components (T=1/f)

c is sum of f & 3f

Spectrum & Bandwidth

spectrum range of frequencies contained in signal , eg: Fig

3.4c, it extends from f to 3f.

absolute bandwidth width of spectrum , eg : 2f in Fig 3.4c

DC Component component of zero frequency

DC Component

DC component =1Nonzero average amplitude

No DC component ,Average amplitude of zero

Frequency Domain Representations

freq domain function of Fig 3.4c

Data Rate and Bandwidth

Data rate: is the amount of data that is moved from one place to another in a given time. (bps) , Data rate=2*fEffective bandwidth(or bandwidth ): is the actual speed

at which data can be transmitted on a connection.any transmission system has a limited band of frequenciesthis limits the data rate that can be carriedsquare have infinite bandwidth.limited bandwidth increases distortionhave a direct relationship between data rate & bandwidth

Sine Waves s(t) = A sin(2ft +)

Figure 3.7 (a) & (b)

Data Rate Calculation

Suppose that we are using a digital transmission system that is capableof transmitting signals with a bandwidth of 4 MHz. Let us attempt to transmit a sequence of alternating 1s and 0s as the square wave of Figure.What data rate can be achieved? We look at three cases

Data Rate Calculation

Case 1 Bandwidth 4MHz, use the sine wave of Fig. 3-7 (a) 4MHz = 5f – f f = 1MHz Data rate = 2 Mbps

Case 2 Bandwidth 8MHz, use the sine wave of Fig. 3-7 (a) 8MHz = 5f – f f = 2MHz Data rate = 4 Mbps

Case 3 Bandwidth 4MHz, use the sine wave of Fig. 3-4 (c) 4MHz = 3f – f f = 2MHz Data rate = 4 Mbps

Data Rate vs. BandwidthBandwidth ↑

Data rate ↑ (compare case 1 & 2) Same signal quality

Same bandwidth Higher signal quality lower data rate Compare case 1 & 3

Same data rate Bandwidth ↑ better signal quality Compare case 2 & 3

Data Rate vs. Bandwidth

In general , any digital waveform will have infinite bandwidth. If

we attempt to transmit this waveform as a signal over any medium, the transmission system will limit the bandwidth that can be transmitted.

greater the bandwidth transmitted, the greater the cost.

limiting the bandwidth creates distortions, and the greater the potential for error by the receiver.

Effect of bandwidth on a digital signal

greater the bandwidth transmitted, the greater the quality and accuracy .

Transmission Impairments

signal received may differ from signal transmitted causing: analog - degradation of signal quality digital - bit errors

most significant impairments are attenuation and attenuation distortion delay distortion noise

Attenuationwhere signal strength falls off with distancedepends on mediumAttenuation introduces three considerations for the transmission engineer:1- strong enough to be detected2-sufficiently higher than noise to receive without error3-attenuation varies with frequency , for analog signals

Db=10 log10 (Ps/Pd)Ps is the signal power at the transmitting end (source) of a communications circuit and Pd is the signal power at the receiving end (destination), Ps > Pd.

Attenuation oThe first and second problems are dealt with by attention to signal strength and the use of amplifiers or repeatersoTo overcome the third problem , techniques are available for equalizing attenuation across a band of frequencies.

Delay Distortion

•only occurs in guided media•propagation velocity varies with frequency•hence various frequency components arrive at different times•particularly critical for digital signals•Equalizing techniques can also be used for delay distortion

Delay Distortion

NoiseNoise :additional signals inserted between transmitter and receiver.

1- Thermal noise due to thermal agitation of electrons uniformly distributed white noise N0=KT(W/Hz)

N0 = noise power density in watts per 1 Hz of bandwidth k = Boltzmann’s constant = 1.38 * 10-23 J/K T = temperature, in Kelvin

Noise

2- Intermodulation noise

When signals at different frequencies share the same transmission medium, the result may be intermodulation noise. The effect of intermodulation noise is to produce signals at a frequency that is the sum or difference of the two original frequencies or multiples of those frequencies, thus possibly interfering with services at these frequencies. It is produced by nonlinearities in the transmitter, receiver, and/or intervening transmission medium.

Noise3-crosstalkan unwanted coupling between signal paths. It can occur by electrical coupling between nearby twisted pairs or, rarely, coax cable lines carrying multiple signals. It can also occur when microwave antennas pick up unwanted signals4-Impulse noise

irregular pulses or spikes eg. external electromagnetic interference

short duration high amplitude a minor annoyance for analog signals but a major source of error in digital data

a noise spike could corrupt many bits

Channel Capacitymax possible data rate on comms channel

is a function of: Data rate, in bits per second (bps), at which data can be

communicated Bandwidth, as constrained by the transmitter and the nature of the

transmission medium, expressed in cycles per second, or Hertz Noise, average level of noise over the communications path Error 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

limitations due to physical properties

want most efficient use of capacity

Nyquist Bandwidth

consider noise free channels

if rate of signal transmission is 2B then can carry signal with frequencies no greater than B

ie. given bandwidth B, highest signal rate is 2B

for binary signals, 2B bps needs bandwidth B Hz

For multilevel ,can increase rate by using M signal levels

Nyquist Formula is: C = 2B log2M

so increase rate by increasing signals at cost of receiver complexity limited by noise & other impairments

Shannon Capacity Formula

consider relation of data rate, noise & error rate faster data rate shortens each bit so bursts of noise affects

more bits given noise level, higher rates means higher errors

Shannon developed formula relating these to signal to noise ratio (in decibels)

SNRdb=10 log10 (signal/noise)

Capacity C=B log2(1+SNR) theoretical maximum capacity get lower in practise

Shannon Capacity Formula

Summary

•data transmission•Time domain and Frequency domain •transmission impairments