William Stallings Data and Computer Communications Chapter 4 Transmission Media

Overview zGuided - wire zUnguided - wireless zCharacteristics and quality determined by medium and signal zFor guided, the medium is more important zFor unguided, the bandwidth produced by the antenna is more important zKey concerns are data rate and distance

Design Factors zBandwidth yHigher bandwidth gives higher data rate zTransmission impairments yAttenuation zInterference zNumber of receivers yIn guided media yMore receivers (multi-point) introduce more attenuation

Electromagnetic Spectrum

Guided Transmission Media zTwisted Pair zCoaxial cable zOptical fiber

Twisted Pair

Twisted Pair - Applications zMost common medium zTelephone network yBetween house and local exchange (subscriber loop) zWithin buildings yTo private branch exchange (PBX) zFor local area networks (LAN) y10Mbps or 100Mbps

Twisted Pair - Pros and Cons zCheap zEasy to work with zLow data rate zShort range

Twisted Pair - Transmission Characteristics zAnalog yAmplifiers every 5km to 6km zDigital yUse either analog or digital signals yrepeater every 2km or 3km zLimited distance zLimited bandwidth (1MHz) zLimited data rate (100MHz) zSusceptible to interference and noise

Unshielded and Shielded TP zUnshielded Twisted Pair (UTP) yOrdinary telephone wire yCheapest yEasiest to install ySuffers from external EM interference zShielded Twisted Pair (STP) yMetal braid or sheathing that reduces interference yMore expensive yHarder to handle (thick, heavy)

UTP Categories zCat 3 yup to 16MHz yVoice grade found in most offices yTwist length of 7.5 cm to 10 cm zCat 4 yup to 20 MHz zCat 5 yup to 100MHz yCommonly pre-installed in new office buildings yTwist length 0.6 cm to 0.85 cm

Near End Crosstalk zCoupling of signal from one pair to another zCoupling takes place when transmit signal entering the link couples back to receiving pair zi.e. near transmitted signal is picked up by near receiving pair

Coaxial Cable

Coaxial Cable Applications zMost versatile medium zTelevision distribution yAriel to TV yCable TV zLong distance telephone transmission yCan carry 10,000 voice calls simultaneously yBeing replaced by fiber optic zShort distance computer systems links zLocal area networks

Coaxial Cable - Transmission Characteristics zAnalog yAmplifiers every few km yCloser if higher frequency yUp to 500MHz zDigital yRepeater every 1km yCloser for higher data rates

Optical Fiber

Optical Fiber - Benefits zGreater capacity yData rates of hundreds of Gbps zSmaller size & weight zLower attenuation zElectromagnetic isolation zGreater repeater spacing y10s of km at least

Optical Fiber - Applications zLong-haul trunks zMetropolitan trunks zRural exchange trunks zSubscriber loops zLANs

Optical Fiber - Transmission Characteristics zAct as wave guide for to Hz yPortions of infrared and visible spectrum zLight Emitting Diode (LED) yCheaper yWider operating temp range yLast longer zInjection Laser Diode (ILD) yMore efficient yGreater data rate zWavelength Division Multiplexing

Optical Fiber Transmission Modes

Wireless Transmission zUnguided media zTransmission and reception via antenna zDirectional yFocused beam yCareful alignment required z Omnidirectional ySignal spreads in all directions yCan be received by many antennae

Frequencies z2GHz to 40GHz yMicrowave yHighly directional yPoint to point ySatellite z30MHz to 1GHz yOmnidirectional yBroadcast radio z3 x to 2 x yInfrared yLocal

Terrestrial Microwave zParabolic dish zFocused beam zLine of sight zLong haul telecommunications zHigher frequencies give higher data rates

Satellite Microwave zSatellite is relay station zSatellite receives on one frequency, amplifies or repeats signal and transmits on another frequency zRequires geo-stationary orbit yHeight of 35,784km zTelevision zLong distance telephone zPrivate business networks

Broadcast Radio zOmnidirectional zFM radio zUHF and VHF television zLine of sight zSuffers from multipath interference yReflections

Infrared zModulate noncoherent infrared light zLine of sight (or reflection) zBlocked by walls ze.g. TV remote control, IRD port