1 Data and Computer Communications Lecture 4 Transmission Media

2 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

3 Design Factors zBandwidth yHigher bandwidth gives higher data rate zTransmission impairments yAttenuation yOrder of losses: Twisted pair, coaxial then fibre zInterference yOverlapping of frequencies in unguided medium yEmanations from adjacent cables in guided. (Use screening) zNumber of receivers yIn guided media yMore receivers (multi-point) introduce more attenuation

4 Electromagnetic Spectrum

5 Guided Transmission Media zTwisted Pair zCoaxial cable zOptical fiber

6 Twisted Pair

7 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

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

9 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) y1 Ghz at short distances & new encoding schemes zSusceptible to interference and noise

10 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)

11 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 (1 GHz using 4 pair & compression) yData grade cable yCommonly pre-installed in new office buildings yTwist length 0.6 cm to 0.85 cm zCat 6 y200 MHz to 250MHz y1 Ghz uncompressed: 4 x 250 Mhz yhttp://

12 Attenuation & Near End Crosstalk zAttenuation yGenerally logarithmic & expressed as dB per unit length zNear End Cross talk yCoupling of signal from one pair to another yCoupling takes place when transmit signal entering the link couples back to receiving pair yi.e. near transmitted signal is picked up by near receiving pair

13 Coaxial Cable

14 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

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

16 Optical Fiber

17 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

18 Optical Fiber - Applications zLong-haul trunks y1500km, 20 – 60k voice channels zMetropolitan trunks y12 km, 100k channels zRural exchange trunks y40 – 160Km, 5k voice channels zSubscriber loops yVoice data cables leased by corporate clients zLANs y100Mbps – 1 Ghz

19 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 yForm of FDM ySeparate coour, different channel y100 10Gbps = 1 trillion bps in Bell labs y80 10Gbps available

20 Optical Propagation modes zStep-index multimode yGood for short distance yProblem with multi propagation paths zSingle Mode ySingle path, no distortion yLong distance ySupports different wavelengths zGraded Index multimode yIntermediate of top two yTwo layers with different refractive index yUsed commonly in LANs ySupports different wavelengths

21 Optical Fiber Transmission Modes

22 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

23 Frequencies In term of data transmission, we are are interested in 3 ranges z2GHz to 40GHz yMicrowave yHighly directional yPoint to point ySatellite z30MHz to 1GHz yOmnidirectional yBroadcast radio z3 x to 2 x yInfrared yLocal

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

25 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

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

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

28 Required Reading zStallings Chapter 4