Transmission FundamentalsChapter 2

Electromagnetic SignalFunction of timeCan also be expressed as a function of frequencySignal consists of components of different frequencies

Time-Domain ConceptsAnalog signal - signal intensity varies in a smooth fashion over timeNo breaks or discontinuities in the signalDigital signal - signal intensity maintains a constant level for some period of time and then changes to another constant levelPeriodic signal - analog or digital signal pattern that repeats over time

Time-Domain ConceptsAperiodic signal - analog or digital signal pattern that doesn't repeat over timePeak amplitude (A) - maximum value or strength of the signal over time; typically measured in voltsFrequency (f )Rate, in cycles per second, or Hertz (Hz) at which the signal repeats

Time-Domain ConceptsPeriod (T ) - amount of time it takes for one repetition of the signalT = 1/fPhase () - measure of the relative position in time within a single period of a signalWavelength () - distance occupied by a single cycle of the signalOr, the distance between two points of corresponding phase of two consecutive cycles

Sine Wave Parameters

Sine Wave ParametersFigure 2.3 shows the effect of varying each of the three parameters(a) A = 1, f = 1 Hz, = 0; thus T = 1s(b) Reduced peak amplitude; A=0.5(c) Increased frequency; f = 2, thus T = (d) Phase shift; = /4 radians (45 degrees) note: 2 radians = 360 = 1 period

Frequency-Domain ConceptsFundamental frequency - when all frequency components of a signal are integer multiples of one frequency, its referred to as the fundamental frequencySpectrum - range of frequencies that a signal containsAbsolute bandwidth - width of the spectrum of a signalEffective bandwidth (or just bandwidth) - narrow band of frequencies that most of the signals energy is contained in

Frequency-Domain ConceptsAny electromagnetic signal can be shown to consist of a collection of periodic analog signals (sine waves) at different amplitudes, frequencies, and phasesThe period of the total signal is equal to the period of the fundamental frequency

Figure 4 illustrates the idea of taking two waves and adding them together. In this case, waves from Figures 2 and 3 are added. There are primarily two ways of viewing any type of a wave; in the time domain, or in the frequency domain. The frequency domain of Figure 4 might look something like Figure 5.

Relationship between Data Rate and BandwidthThe greater the bandwidth, the higher the information-carrying capacityConclusionsAny digital waveform will have infinite bandwidthBUT the transmission system will limit the bandwidth that can be transmittedAND, for any given medium, the greater the bandwidth transmitted, the greater the costHOWEVER, limiting the bandwidth creates distortions

Data Communication TermsData - entities that convey meaning, or informationSignals - electric or electromagnetic representations of dataTransmission - communication of data by the propagation and processing of signals

Examples of Analog and Digital Data AnalogVideoAudioDigitalTextIntegers

Analog SignalsA continuously varying electromagnetic wave that may be propagated over a variety of media, depending on frequencyExamples of media:Copper wire media (twisted pair and coaxial cable)Fiber optic cableAtmosphere or space propagationAnalog signals can propagate analog and digital data

Digital SignalsA sequence of voltage pulses that may be transmitted over a copper wire mediumGenerally cheaper than analog signalingLess susceptible to noise interferenceSuffer more from attenuationDigital signals can propagate analog and digital data

Analog Signaling

Digital Signaling

Choosing Data and Signal CombinationsDigital data, digital signalEquipment for encoding is less expensive than digital-to-analog equipmentAnalog data, digital signalConversion permits use of modern digital transmission and switching equipmentDigital data, analog signalSome transmission media will only propagate analog signalsExamples include optical fiber and satelliteAnalog data, analog signalAnalog data easily converted to analog signal

Analog TransmissionTransmit analog signals without regard to content Attenuation limits length of transmission link Cascaded amplifiers boost signals energy for longer distances but cause distortionAnalog data can tolerate distortionIntroduces errors in digital data

Digital TransmissionConcerned with the content of the signalAttenuation endangers integrity of dataDigital SignalRepeaters achieve greater distanceRepeaters recover the signal and retransmitAnalog signal carrying digital dataRetransmission device recovers the digital data from analog signalGenerates new, clean analog signal

2.3 Channel CapacityImpairments, such as noise, limit data rate that can be achievedFor digital data, to what extent do impairments limit data rate?Channel Capacity the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions

Channel CapacityData rate - rate at which data can be communicated (bps)Bandwidth - the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium, expressed in cycles/secNoise - average level of noise over the communications pathError rate - rate at which errors occurError = transmit 1 and receive 0; transmit 0 and receive 1

2.4 Classifications of Transmission MediaTransmission MediumPhysical path between transmitter and receiverGuided MediaWaves are guided along a solid mediumE.g., copper twisted pair, copper coaxial cable, optical fiberUnguided MediaProvides means of transmission but does not guide electromagnetic signalsUsually referred to as wireless transmissionE.g., atmosphere, outer space

Unguided MediaTransmission and reception are achieved by means of an antennaConfigurations for wireless transmissionDirectional Omnidirectional

General Frequency RangesMicrowave frequency range1 GHz to 40 GHzDirectional beams possibleSuitable for point-to-point transmissionUsed for satellite communicationsRadio frequency range30 MHz to 1 GHz Suitable for omnidirectional applicationsInfrared frequency rangeRoughly, 3x1011 to 2x1014 HzUseful in local point-to-point multipoint applications within confined areas Speed: 2.4kbit/s to 1Gbit/s

Terrestrial MicrowaveDescription of common microwave antennaParabolic "dish", 3 m in diameterFixed rigidly and focuses a narrow beamAchieves line-of-sight transmission to receiving antennaLocated at substantial heights above ground level ApplicationsLong haul telecommunications serviceShort point-to-point links between buildings

Terrestrial Microwave

Band GHzBandwidth MHzData Rate Mbps271263090114013518220274

Satellite MicrowaveDescription of communication satelliteMicrowave relay stationUsed to link two or more ground-based microwave transmitter/receiversReceives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink)ApplicationsTelevision distributionLong-distance telephone transmissionPrivate business networks

Broadcast RadioDescription of broadcast radio antennasOmnidirectionalAntennas not required to be dish-shapedAntennas need not be rigidly mounted to a precise alignmentApplicationsBroadcast radioVHF and part of the UHF band; 30 MHZ to 1GHzCovers FM radio and UHF and VHF television

MultiplexingCapacity of transmission medium usually exceeds capacity required for transmission of a single signalMultiplexing - carrying multiple signals on a single mediumMore efficient use of transmission medium

Multiplexing

Widespread Use of MultiplexingCost per kbps of transmission facility declines with an increase in the data rateCost of transmission and receiving equipment declines with increased data rateMost individual data communicating devices require relatively modest data rate support

Multiplexing TechniquesFrequency-division multiplexing (FDM)Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signalTime-division multiplexing (TDM)Takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal

Frequency-division Multiplexing

Time-division Multiplexing