1 William Stallings Data and Computer Communications Chapter 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 zInterference zNumber of receivers yMajor factor in guided media yMore receivers (multi-point) introduce more attenuation

4 Electromagnetic Spectrum

5 Guided Transmission Media zthe transmission capacity depends on the distance and on whether the medium is point- to-point or multipoint ze.g., zTwisted Pair zCoaxial cable zOptical fiber

6 Twisted Pair zconsists of two insulated copper wires arranged in a regular spiral pattern to minimize the electromagnetic interference between adjacent pairs zoften used at customer facilities and also over distances to carry voice as well as data communications zlow frequency transmission medium

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) using different modulation & signaling techniques zSusceptible to interference and noise

10 Unshielded and Shielded TP zUnshielded Twisted Pair (UTP) yOrdinary telephone wire yCheapest yEasiest to install ySuffers from external electromagnetic interference (EM) zShielded Twisted Pair (STP) ythe pair is wrapped with metallic foil or braid to insulate the pair from electromagnetic 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 (least common) 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

12 Twisted Pair Advantages zinexpensive and readily available zflexible and light weight zeasy to work with and install

13 Twisted Pair Disadvantages zsusceptibility to interference and noise zattenuation problem yFor analog, repeaters needed every 5-6km yFor digital, repeaters needed every 2-3km zrelatively low bandwidth

14 Coaxial Cable

15 Coaxial Cable Applications zMost versatile medium zTelevision distribution yAerial 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

16 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

17 Coax Advantages zhigher bandwidth y400 to 600Mhz yup to 10,800 voice conversations zcan be tapped easily (pros and cons) zmuch less susceptible to interference than twisted pair

18 Coax Disadvantages zhigh attenuation rate makes it expensive over long distance zbulky

19 Optical Fiber

20 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

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

22 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

23 Fiber Optic Types zmultimode step-index fiber ythe reflective walls of the fiber move the light pulses to the receiver zmultimode graded-index fiber yacts to refract the light toward the center of the fiber by variations in the density zsingle mode fiber ythe light is guided down the center of an extremely narrow core

24 Optical Fiber Transmission Modes

25 fiber optic multimode step-index fiber optic multimode graded-index fiber optic single mode Fiber Optic Signals

26 Fiber Optic Advantages zgreater capacity (bandwidth of up to 2 Gbps) zsmaller size and lighter weight zlower attenuation zimmunity to environmental interference zhighly secure due to tap difficulty and lack of signal radiation

27 Fiber Optic Disadvantages zexpensive over short distance zrequires highly skilled installers zadding additional nodes is difficult

28 Wireless Transmission zUnguided media zTransmission and reception via antenna zTwo techniques are used: zDirectional yFocused beam yCareful alignment required z Omnidirectional ySignal spreads in all directions yCan be received by many antennas

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

30 Wireless Examples zterrestrial microwave transmission zsatellite transmission zbroadcast radio zinfrared

31 Terrestrial Microwave zuses the radio frequency spectrum, commonly from 2 to 40 Ghz ztransmitter is a parabolic dish, mounted as high as possible zused by common carriers as well as by private networks zrequires unobstructed line of sight between source and receiver zcurvature of the earth requires stations (called repeaters) to be ~30 miles apart

32 Microwave Transmission Applications zlong-haul telecommunications service for both voice and television transmission zshort point-to-point links between buildings for closed-circuit TV or a data link between LANs

33 Microwave Transmission Advantages zno cabling needed between sites zwide bandwidth zmultichannel transmissions

34 Microwave Transmission Disadvantages zline of sight requirement zexpensive towers and repeaters zsubject to interference such as passing airplanes and rain

35 Satellite Microwave za microwave relay station in space zSatellite receives on one frequency, amplifies or repeats signal and transmits on another frequency zgeostationary satellites yremain above the equator at a height of 22,300 miles (geosynchronous orbit) ytravel around the earth in exactly the time the earth takes to rotate

36 Satellite Transmission Links zearth stations communicate by sending signals to the satellite on an uplink zthe satellite then repeats those signals on a downlink zthe broadcast nature of the downlink makes it attractive for services such as the distribution of television programming

37 dish uplink stationdownlink station satellite transponder 22,300 miles Satellite Transmission Process

38 Satellite Transmission Applications ztelevision distribution ya network provides programming from a central location using direct broadcast satellites (DBS) zlong-distance telephone transmission yhigh-usage international trunks zprivate business networks

39 Principal Satellite Transmission Bands zC band: 4(downlink) - 6(uplink) GHz ythe first to be designated zKu band: 12(downlink) -14(uplink) GHz yrain interference is the major problem zKa band: 19(downlink) - 29(uplink) GHz yequipment needed to use the band is still very expensive

40 Satellite Advantages zcan reach a large geographical area zhigh bandwidth zcheaper over long distances

41 Satellite Disadvantages zhigh initial cost zsusceptible to noise and interference zpropagation delay

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

43 Infrared zAchieved using tranceivers that modulate noncoherent infrared light zRequires line of sight (or reflection) zBlocked by walls ze.g. TV remote control, Infrared port

44 Common Carriers za government-regulated private company zinvolved in the sale of infrastructure services in transportation and communications zrequired to serve all clients indiscriminately zservices and prices from common carriers are described in tariffs

45 Leased (or Dedicated) Lines zpermanently or semi-permanently connect between two points zeconomical in high volume calls between two points zno delay associated with switching times zcan assure consistently high-quality connections

46 Leased (or Dedicated) Lines zvoice grade channels ynormal telephone lines yin the range of 300 Hertz to 3300 Hertz zconditioning or equalizing yreduces the amount of noise on the line, providing lower error rates and increased speed for data communications

47 T-1 Carrier zalso referred to as DS-1 signaling zprovides digital full-duplex transmission rates of 1.544Mbps zusually created by multiplexing Kbps voice or 56-Kbps data lines zhigher speeds are available with T-3 (45Mbps) [sometimes referred to a DS-3 lines; can be multiplexed into 28 T-1 signals; T-3 consists of 672 individual channels, each of which supports 64-Kbps] and T-4 services (274Mbps) zin Europe, E-1 (2.048Mbps) is used instead of T-1

48 Integrated Services Digital Network (ISDN) zall-digital transmission facility that is designed to replace the analog PSTN zbasic ISDN (basic rate access) ytwo 64Kbps bearer channels + 16Kbps data channel (2B+D) = 144 Kbps zbroadband ISDN (primary rate access) ytwenty-three 64Kbps bearer channels + 64 data channels (23B+D) = Mbps

49 Past Criticism of ISDN z“Innovations Subscribers Don’t Need” z“It Still Doesn’t Network” z“It Still Does Nothing” zWhy so much criticism? yoverhyping of services before delivery yhigh price of equipment ydelay in implementing infrastructure yincompatibility between providers' equipment.

50 ISDN Channel Definitions zB (bearer) channels y64 kbps channels that may be used to carry voice, data, facsimile, or image zD (demand) channels ymainly intended for carrying signaling, billing and management information to control ISDN services (out-of-band control messages) ymay be either 16 or 64 kbps

51 Two Levels of ISDN Service zbasic rate interface (BRI) y2B (64 kbps) + D (16 kbps) = 144 kbps zprimary rate interface (PRI) y23B (64 kbps) + D (64 kbps) = Mbps xNorth American standard y30B (64 kbps) + D (64 kbps) = Mbps xEuropean standard