Data Data Transmission and Transmission and Communication Communication

TechnologyTechnology

Terminology (1)Terminology (1)

• TransmitterTransmitter• ReceiverReceiver• MediumMedium

– Guided mediumGuided medium• e.g. twisted pair, optical fibere.g. twisted pair, optical fiber

– Unguided mediumUnguided medium• e.g. air, water, vacuume.g. air, water, vacuum

Terminology (2)Terminology (2)

• Direct linkDirect link– No intermediate devicesNo intermediate devices

• Point-to-pointPoint-to-point– Direct link Direct link – Only 2 devices share linkOnly 2 devices share link

• Multi-pointMulti-point– More than two devices share the linkMore than two devices share the link

Terminology (3)Terminology (3)

• SimplexSimplex– One directionOne direction

• e.g. Televisione.g. Television

• Half duplexHalf duplex– Either direction, but only one way at a timeEither direction, but only one way at a time

• e.g. police radioe.g. police radio

• Full duplexFull duplex– Both directions at the same timeBoth directions at the same time

• e.g. telephonee.g. telephone

Frequency, Spectrum Frequency, Spectrum and Bandwidthand Bandwidth

• Time domain conceptsTime domain concepts– Continuous signalContinuous signal

• Various in a smooth way over timeVarious in a smooth way over time

– Discrete signalDiscrete signal• Maintains a constant level then changes to Maintains a constant level then changes to

another constant levelanother constant level

– Periodic signalPeriodic signal• Pattern repeated over timePattern repeated over time

– Aperiodic signalAperiodic signal• Pattern not repeated over timePattern not repeated over time

Continuous & Discrete Continuous & Discrete SignalsSignals

PeriodicPeriodicSignalsSignals

Sine WaveSine Wave• Peak Amplitude (A)Peak Amplitude (A)

– maximum strength of signalmaximum strength of signal– voltsvolts

• Frequency (f)Frequency (f)– Rate of change of signalRate of change of signal– Hertz (Hz) or cycles per secondHertz (Hz) or cycles per second– Period = time for one repetition (T)Period = time for one repetition (T)– T = 1/fT = 1/f

• Phase (Phase ())– Relative position in timeRelative position in time

Varying Sine WavesVarying Sine Waves

WavelengthWavelength

• Distance occupied by one cycleDistance occupied by one cycle• Distance between two points of Distance between two points of

corresponding phase in two consecutive corresponding phase in two consecutive cyclescycles

• Assuming signal velocity Assuming signal velocity vv

= vT= vT f = vf = v– c = 3*10c = 3*108 8 msms-1 -1 (speed of light in free space)(speed of light in free space)

Frequency Domain Frequency Domain ConceptsConcepts

• Signal usually made up of many Signal usually made up of many frequenciesfrequencies

• Components are sine wavesComponents are sine waves• Can be shown (Fourier analysis) that Can be shown (Fourier analysis) that

any signal is made up of component any signal is made up of component sine wavessine waves

• Can plot frequency domain functionsCan plot frequency domain functions

Addition of Addition of FrequencyFrequency

ComponentsComponents

FrequencyFrequencyDomain Domain

Spectrum & BandwidthSpectrum & Bandwidth

• SpectrumSpectrum– range of frequencies contained in signalrange of frequencies contained in signal

• Absolute bandwidthAbsolute bandwidth– width of spectrumwidth of spectrum

• Effective bandwidthEffective bandwidth– Often just Often just bandwidthbandwidth– Narrow band of frequencies containing most of Narrow band of frequencies containing most of

the energythe energy

• DC ComponentDC Component– Component of zero frequencyComponent of zero frequency

Signal with DC Signal with DC ComponentComponent

Data Rate and BandwidthData Rate and Bandwidth

• Any transmission system has a Any transmission system has a limited band of frequencieslimited band of frequencies

• This limits the data rate that can be This limits the data rate that can be carriedcarried

Analog and Digital Data Analog and Digital Data TransmissionTransmission

• Data Data – Entities that convey meaningEntities that convey meaning

• SignalsSignals– Electric or electromagnetic Electric or electromagnetic

representations of datarepresentations of data

• TransmissionTransmission– Communication of data by propagation Communication of data by propagation

and processing of signalsand processing of signals

DataData

• AnalogAnalog– Continuous values within some intervalContinuous values within some interval– e.g. sound, videoe.g. sound, video

• DigitalDigital– Discrete valuesDiscrete values– e.g. text, integerse.g. text, integers

Acoustic Spectrum Acoustic Spectrum (Analog)(Analog)

SignalsSignals

• Means by which data are propagatedMeans by which data are propagated• AnalogAnalog

– Continuously variableContinuously variable– Various mediaVarious media

• wire, fiber optic, spacewire, fiber optic, space

– Speech bandwidth 100Hz to 7kHzSpeech bandwidth 100Hz to 7kHz– Telephone bandwidth 300Hz to 3400HzTelephone bandwidth 300Hz to 3400Hz– Video bandwidth 4MHzVideo bandwidth 4MHz

• DigitalDigital– Use two DC componentsUse two DC components

Data and SignalsData and Signals

• Usually use digital signals for digital Usually use digital signals for digital data and analog signals for analog data and analog signals for analog datadata

• Can use analog signal to carry digital Can use analog signal to carry digital datadata– ModemModem

• Can use digital signal to carry analog Can use digital signal to carry analog data data – Compact Disc audioCompact Disc audio

Analog Signals Carrying Analog Signals Carrying Analog and Digital DataAnalog and Digital Data

Digital Signals Carrying Digital Signals Carrying Analog and Digital DataAnalog and Digital Data

Analog TransmissionAnalog Transmission

• Analog signal transmitted without Analog signal transmitted without regard to contentregard to content

• May be analog or digital dataMay be analog or digital data• Attenuated over distance Attenuated over distance • Use amplifiers to boost signalUse amplifiers to boost signal• Also amplifies noiseAlso amplifies noise

Digital TransmissionDigital Transmission

• Concerned with contentConcerned with content• Integrity endangered by noise, Integrity endangered by noise,

attenuation etc.attenuation etc.• Repeaters usedRepeaters used• Repeater receives signalRepeater receives signal• Extracts bit patternExtracts bit pattern• RetransmitsRetransmits• Attenuation is overcomeAttenuation is overcome• Noise is not amplifiedNoise is not amplified

Advantages of Digital Advantages of Digital TransmissionTransmission• Digital technologyDigital technology

– Low cost LSI/VLSI technologyLow cost LSI/VLSI technology

• Data integrityData integrity– Longer distances over lower quality linesLonger distances over lower quality lines

• Capacity utilizationCapacity utilization– High bandwidth links economicalHigh bandwidth links economical– High degree of multiplexing easier with digital techniquesHigh degree of multiplexing easier with digital techniques

• Security & PrivacySecurity & Privacy– EncryptionEncryption

• IntegrationIntegration– Can treat analog and digital data similarlyCan treat analog and digital data similarly

Transmission Transmission ImpairmentsImpairments

• Signal received may differ from Signal received may differ from signal transmittedsignal transmitted

• Analog - degradation of signal qualityAnalog - degradation of signal quality• Digital - bit errorsDigital - bit errors• Caused byCaused by

– Attenuation and attenuation distortionAttenuation and attenuation distortion– Delay distortionDelay distortion– NoiseNoise

AttenuationAttenuation

• Signal strength falls off with distanceSignal strength falls off with distance• Depends on mediumDepends on medium• Received signal strength:Received signal strength:

– must be enough to be detectedmust be enough to be detected– must be sufficiently higher than noise to must be sufficiently higher than noise to

be received without errorbe received without error

• Attenuation is an increasing function Attenuation is an increasing function of frequencyof frequency

Delay DistortionDelay Distortion

• Only in guided mediaOnly in guided media• Propagation velocity varies with Propagation velocity varies with

frequencyfrequency

Noise (1)Noise (1)

• Additional signals inserted between Additional signals inserted between transmitter and receivertransmitter and receiver

• ThermalThermal– Due to thermal agitation of electronsDue to thermal agitation of electrons– Uniformly distributedUniformly distributed– White noiseWhite noise

• IntermodulationIntermodulation– Signals that are the sum and difference of Signals that are the sum and difference of

original frequencies sharing a mediumoriginal frequencies sharing a medium

Noise (2)Noise (2)

• CrosstalkCrosstalk– A signal from one line is picked up by A signal from one line is picked up by

anotheranother

• ImpulseImpulse– Irregular pulses or spikesIrregular pulses or spikes– e.g. External electromagnetic e.g. External electromagnetic

interferenceinterference– Short durationShort duration– High amplitudeHigh amplitude

Channel CapacityChannel Capacity

• Data rateData rate– In bits per secondIn bits per second– Rate at which data can be Rate at which data can be

communicatedcommunicated

• BandwidthBandwidth– In cycles per second of HertzIn cycles per second of Hertz– Constrained by transmitter and mediumConstrained by transmitter and medium

Required ReadingRequired Reading

• Stallings chapter 3Stallings chapter 3