1

Data and Computer Communications

Lecture 3Data Transmission

DecibelsDigital DataAnaologue DataTransmission Losses

2

Decibels & Signal Strength As a signal propagates along a transmission media,

there will be loss or attenuation of signal strength.

Alexander Graham Bell discovered that the human ear responded logarithmically to power difference and invented a unit called the Bel. The most common form is the decibel which is 1/10 of a Bel.

The decibel is a measure of the ratio between the power of two signal levels.

Ndb=10 log10 P2/P1

Ndb = Number of decibels P1 = Input power P2 = Output power level

3

dB’sDecibel values refer to relative

magnitudes or changes in relative magnitude, not to an absolute level.

A loss from 1000mW to 500mW is a loss of 3dB. A loss of 3 dB halves the power level A gain of 3 dB doubles the power

4

Data Rate and Bandwidth The greater the bandwidth, the higher the information-

carrying capacity Any transmission system has a limited band of

frequencies This limits the data rate that can be carried Conclusions

Any digital waveform will have infinite bandwidth BUT the transmission system will limit the bandwidth that can be

transmitted AND, for any given medium, the greater the bandwidth transmitted,

the greater the cost HOWEVER, limiting the bandwidth creates distortions

5

Finding the BandwidthA square wave represents a a binary

stream 1010101The duration of each pulse is 1/fDate rate is 2f bits per second (bps)Bandwidth of signal:

Upper freq – fundamental frequency

6

Analog and Digital Data TransmissionData

Entities that convey meaning

Signals Electric or electromagnetic representations of

data

Transmission Communication of data by propagation and

processing of signals

7

DataAnalog

Continuous values within some interval e.g. sound, video

Digital Discrete values e.g. text, integers

8

Acoustic Spectrum (Analog)

9

SignalsMeans by which data are propagatedAnalog

Continuously variable Various media

wire, fiber optic, space

Speech bandwidth 100Hz to 7kHz Telephone bandwidth 300Hz to 3400Hz Video bandwidth 4MHz

Digital Use two DC components

10

Data and SignalsUsually use digital signals for digital data

and analog signals for analog dataCan use analog signal to carry digital data

Modem

Can use digital signal to carry analog data Compact Disc audio

11

Analog Signals Carrying Analog and Digital Data

12

Digital Signals Carrying Analog and Digital Data

13

Analog TransmissionAnalog signal transmitted without regard

to contentMay be analog or digital dataAttenuated over distance Use amplifiers to boost signalAmplifiers also amplifies noise (SNR)

14

Digital TransmissionConcerned with contentIntegrity endangered by noise,

attenuation etc.Repeaters used

Repeater receives signal Extracts bit pattern Retransmits

Attenuation is overcomeNoise is not amplified

15

Advantages of Digital Transmission Digital technology

Low cost LSI/VLSI technology Data integrity

Longer distances over lower quality lines Uses repeaters as opposed to amplifiers

Capacity utilization High bandwidth links now economical High degree of multiplexing easier with digital techniques

Security & Privacy Encryption can be readily applied

Integration By digitising analog data, analog and digital data can be

treated similarly

16

Transmission ImpairmentsSignal received may differ from signal

transmitted. Why?Analog - degradation of signal qualityDigital - bit errorsCaused by

Attenuation and attenuation distortion Delay distortion Noise

17

Attenuation Signal strength falls off with distance Reduction tends to be logarithmic and is

expressed in dB’s per unit distance Depends on medium Received signal strength:

1. must be strong enough to be detected2. must be sufficiently higher than noise to be

received without error3. Attenuation is an increasing function of

frequency

18

Attenuation SolutionsProblems 1 & 2 can be solved by ensuring

the signal strength is maintained using amplifiers.

Problem 3 requires one of two solutions Equalization of attenuation across a band of

frequencies ( Coils used in telephone circuits) Variation in amplification of signal at different

frequencies

19

Delay DistortionOnly in guided mediaPropagation velocity varies with frequencyLight passing through a prism caused the

separation of white lightVelocity will tend to be faster at center of

bandwidthIntersymbol interference is where

component frequencies of one bit position overlap with the next bit position

20

NoiseAdditional signals inserted between

transmitter and receiverMay be divided into four categories

Thermal Intermodulation Crosstalk Impulse noise

21

Thermal Noise Due to thermal agitation of electronsUniformly distributedWhite noiseNo = kT (W/Hz)

No Noise power density per 1 Hz K Boltzmann’s constant 1.3803 x 10-23J/oK T Temperature in degrees Kelvin

22

Intermodulation NoiseWhen signals of different frequecies share

the same medium, intermodulation noise may occur

Intermodulation noise are signals that are the sum and difference of original frequencies sharing the medium

Intermodulation occurs when there is some form of nonlinearality occurs in the transmitter, medium or receiver.

23

Crosstalk Crosstalk

A signal from one line is picked up by another Electrical coupling from adjacent twisted pair Same order of magnitude as thermal noise

All noise so far is reasonabley predictable and of a constant amplitude. This allows for some form of compensation circuit / layout

24

Impulse NoiseNoncontinuos noise consisting of irregular

pulses or spikese.g. External electromagnetic interference

Short durationHigh amplitudeThere are a variety of causes

Electrical switchgear Lightning Circuit faults

25

Channel CapacityChannel capacity is the maximum rate at which

data can be transmitted over a certain communication path.

Data rate In bits per second (bps) Rate at which data can be communicated

Bandwidth In cycles per second of Hertz As constrained by transmitter and medium

Noise Average level of noise over the communications path

Error rate Rate at which errors occur

26

Nyquist Bandwidth

For binary signals (two voltage levels) C = 2B

C = capacityB = Bandwidth

A bandwidth of 3100hz gives a capacity of 6200 bps

We would like to make the most efficient use of available bandwidth.

For digital data we would like the highest data rate possible at a particular error rate for a given bandwidth.

Nyquist formulated this limation as follows:

27

Multilevel SignallingWith multilevel signaling, each level representing

more than one bitNyquists formula becomes

C = 2B log2 MM = number of discrete signal or voltage levelsC = Capacity

Assuming M = 8, this gives a capacity of 18600 bps for the previous example.

28

Signal-to-Noise Ratio Ratio of the power in a signal to the power contained in the

noise that’s present at a particular point in the transmission Typically measured at a receiver Signal-to-noise ratio (SNR, or S/N)

A high SNR means a high-quality signal, low number of required intermediate repeaters

SNR sets upper bound on achievable data rate

power noise

power signallog10)( 10dB SNR

29

Shannon Capacity FormulaEquation:

Represents theoretical maximum that can be achieved In practice, only much lower rates achieved

Formula assumes white noise (thermal noise) Impulse noise is not accounted for Attenuation distortion or delay distortion not accounted for

SNR1log2 BC

30

Example of Nyquist and Shannon FormulationsSpectrum of a channel between 3 MHz and 4 MHz ;

SNRdB = 24 dB

Using Shannon’s formula

251SNR

SNRlog10dB 24SNR

MHz 1MHz 3MHz 4

10dB

B

Mbps88102511log10 62

6 C

31

Example of Nyquist and Shannon FormulationsHow many signaling levels are required?

16

log4

log102108

log2

2

266

2

M

M

M

MBC

32

Required ReadingStallings chapter 3