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Chapter 2Data Transmission and MediaTransmission TerminologyAnalog and Digital TransmissionTransmission ImpairmentsChannel CapacityTransmission MediaReference: William Stallings, Data and Computer Communications, 9th Edition#TRANSMISSION TERMINOLOGYData transmission occurs between a transmitter & a receiver via some mediumGuided mediumtwisted pair, coaxial cable, optical fiberUnguided / wireless mediumair, water, vacuumDirect linkno intermediate devicesPoint-to-Pointdirect link only 2 devices share linkMulti-Pointmore than two devices share the link#2TRANSMISSION TERMINOLOGYSimplexone directioneg. televisionHalf Duplexeither direction, but only one way at a timeeg. police radioFull Duplexboth directions at the same timeeg. telephone#3FREQUENCY, SPECTRUM AND BANDWIDTHTime domain conceptsAnalog signalvarious in a smooth way over timeDigital signalmaintains a constant level then changes to another constant levelPeriodic signalpattern repeated over timeAperiodic signalpattern not repeated over time#4SINE WAVE, s(t) = A sin(2ft +)Peak amplitude (A)Frequency (f)Phase ()Wavelength ()Velocity ()#5FREQUENCY DOMAIN CONCEPTSSignal are made up of many frequenciesComponents are sine wavesFourier analysis can show that any signal is made up of component sine wavesCan plot frequency domain functions

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FREQUENCY DOMAIN REPRESENTATION#7SPECTRUM & BANDWIDTHSpectrumrange of frequencies contained in signalAbsolute Bandwidthwidth of spectrumEffective Bandwidthoften just bandwidthnarrow band of frequencies containing most energyDC Componentcomponent of zero frequency#8DATA RATE AND BANDWIDTHAny transmission system has a limited band of frequenciesThis limits the data rate that can be carriedSquare have infinite components and hence bandwidth Most energy in first few componentsLimited bandwidth increases distortionA direct relationship between data rate & bandwidth

#9ANALOG AND DIGITAL DATA TRANSMISSIONData entities that convey meaningSignals & Signalingelectric or electromagnetic representations of data, physically propagates along mediumTransmissioncommunication of data by propagation and processing of signals#10ACOUSTIC SPECTRUM (ANALOG)

#11AUDIO SIGNALSFrequency range 20 Hz 20 kHz (speech 100 Hz 7 kHz)Easily converted into electromagnetic signalsVarying volume converted to varying voltageCan limit frequency range for voice channel to 300-3400Hz

#12VIDEO SIGNALSUSA - 483 lines per frame, at 30 frames per sechave 525 lines but 42 lost during vertical retrace525 lines x 30 scans = 15750 lines per sec63.5s per line11s for retrace, so 52.5 s per video lineMax Frequency if line alternates black and whiteHorizontal resolution is about 450 lines giving 225 cycles of wave in 52.5 sMax Frequency of 4.2MHz#13

National Television System Committee (NTSC)Phase Alternate Line (PAL)Sequential Color with Memory (SECAM)#DIGITAL DATAas generated by computers etc.has two dc componentsbandwidth depends on data rate

#15ANALOG SIGNALS

#16DIGITAL SIGNALS

#17TRANSMISSION IMPAIRMENTSSignal received may differ from signal transmitted causing:(Analog) degradation of signal quality(Digital) bit errorsMost significant impairments areattenuation and attenuation distortiondelay distortionnoise

#18ATTENUATIONwhere signal strength falls off with distancedepends on mediumreceived signal strength must be:strong enough to be detectedsufficiently higher than noise to receive without errorso increase strength using amplifiers/ repeatersis also an increasing function of frequencyso equalize attenuation across band of frequencies usedeg. using loading coils or amplifiers#19DELAY DISTORTIONonly occurs in guided mediapropagation velocity varies with frequencyhence various frequency components arrive at different timesparticularly critical for digital datasince parts of one bit spill over into otherscausing intersymbol interference (ISI)Equalization can be used to overcome delay distortion.#20NOISEadditional signals inserted between transmitter and receiverthermaldue to thermal agitation of electronsuniformly distributedwhite noiseintermodulationsignals that are the sum and difference of original frequencies sharing a medium#21NOISEcrosstalka signal from one line is picked up by anotherimpulseirregular pulses or spikeseg. external electromagnetic interferenceshort durationhigh amplitudea minor annoyance for analog signalsbut a major source of error in digital dataa noise spike could corrupt many bits#22CHANNEL CAPACITYmax possible data rate on comms channel is a function ofdata rate - in bits per secondbandwidth - in cycles per second or Hertznoise - on comms linkerror rate - of corrupted bitslimitations due to physical propertieswant most efficient use of capacity#23NYQUIST BANDWIDTHConsidering noise free channels, if rate of signal transmission is 2B then can carry signal with frequencies no greater than B ie. given bandwidth B, highest signal rate is 2BFor binary signals, 2B bps needs bandwidth B HzIncrease rate by using M signal levelsNyquists Formula: C = 2B log2MIncrease rate by increasing signalsat cost of receiver complexitylimited by noise & other impairments#24SHANNON CAPACITY FORMULAconsider relation of data rate, noise & error ratefaster data rate shortens each bit so bursts of noise affects more bitsgiven noise level, higher rates means higher errorsShannon developed formula relating these to signal to noise ratio (in decibels)SNRdb=10 log10 (signal/noise)Capacity C=B log2(1+SNR)theoretical maximum capacityget lower in practise#25TRANSMISSION MEDIA: DESIGN FACTORSBandwidthTransmission ImpairmentsInterferenceNumber of Receivers#ELECTROMAGNETIC SPECTRUM

#27GUIDED MEDIAFrequency Range

Typical AttenuationTypical DelayRepeater SpacingTwisted pair (with loading)0 3.5 kHz0.2 dB/km @ 1 kHz50 s/km2 kmTwisted pairs (multi-pair cables)0 1 MHz0.7 dB/km @ 1 kHz5 s/km2 kmCoaxial cable0 500 MHz 7 dB/km @ 10 MHz4 s/km1 9 km Optical fiber186 to 370 THz0.2 to 0.5 dB/km5 s/km40 km#

TWISTED PAIRanalog needs amplifiers every 5km to 6kmdigitalcan use either analog or digital signalsneeds a repeater every 2-3kmlimited distancelimited bandwidth (1MHz)limited data rate (100MHz)susceptible to interference and noise#29UNSHIELDED VS SHIELDED TPUnshielded Twisted Pair (UTP)ordinary telephone wirecheapesteasiest to installsuffers from external EM interferenceShielded Twisted Pair (STP)metal braid or sheathing that reduces interferencemore expensiveharder to handle (thick, heavy)#30Cat 3Class CCat 5Class DCat 5ECat 6Class ECat 7Class FBandwidth16 MHz100 MHz100 MHz200 MHz600 MHzCable TypeUTPUTP/FTPUTP/FTPUTP/FTPSSTPLink Cost (cat 5 = 1)0.711.21.52.2UTP : Unshielded Twister Pair; FTP : Foil Twisted Pair; SSTP : Shielded Screen Twisted Pair Frequency(MHz)Attenuation (db/100 m)Near-End Crosstalk (dB)Cat 3UTPCat 5UTP150-ohm STPCat 3UTPCat 5UTP150-ohm STP12.52.01.141625845.64.12.23253581613.18.24.4234450.42510.46.24147.510022.012.33238.530021.431.3#NEAR END CROSSTALKcoupling of signal from one pair to anotheroccurs when transmit signal entering the link couples back to receiving pairie. near transmitted signal is picked up by near receiving pair#32

COAXIAL CABLEsuperior frequency characteristics to TPperformance limited by attenuation & noiseanalog signalsamplifiers every few kmcloser if higher frequencyup to 500MHzdigital signalsrepeater every 1kmcloser for higher data rates#33OPTICAL FIBER

greater capacitydata rates of hundreds of Gbpssmaller size & weightlower attenuationelectromagnetic isolationgreater repeater spacing10s of km#34Uses total internal reflection to transmit lighteffectively acts as wave guide for 1014 to 1015 Hz Can use several different light sourcesLight Emitting Diode (LED)cheaper, wider operating temp range, lasts longerInjection Laser Diode (ILD)more efficient, has greater data rateRelation of wavelength, type & data rateOPTICAL FIBER#35FREQUENCY UTILIZATION FOR FIBER APPLICATIONS Wavelength (vacuum) (nm)Frequency Range (THz)Band LabelFiber TypeApplication820 to 900366 to 333MultimodeLAN1280 to 1350234 to 222SSingle modeVarious1528 to 1561196 to 192CSingle modeWDM1561 to 1620192 to 185LSingle modeWDM#ATTENUATION IN GUIDED MEDIA

#37WIRELESS TRANSMISSION FREQUENCIES2GHz to 40GHzmicrowavehighly directionalpoint to pointsatellite30MHz to 1GHzomnidirectionalbroadcast radio3 x 1011 to 2 x 1014infraredlocal#38TUTORIAL(a) Suppose that a digitized TV picture is to be transmitted from a source that uses a matrix of 480*500 picture elements (pixels), where each pixel can take on one of 32 intensive values. Assume that 30 pictures are sent per second. (This digital source is roughly equivalent to broadcast TV standards that have been adopted.) Find the source rate R (bps). (b) Assume that the TV picture is to be transmitted over a channel with 4.5-MHz bandwidth and a 35 dB signal-to-noise ratio. Find the capacity of the channel (bps).(c) Discuss how the parameters given in part (a) could be modified to allow transmission of color TV signals without increasing the required value for R. Given an amplifier with an effective noise temperature of 10,000 K and a 10-MHz bandwidth, what thermal noise level, in dBW, may we expect at its output? What is the channel capacity for a teleprint channel with a 300-Hz bandwidth and a signal-to-noise ratio of 3 dB, where the noise is white thermal noise? A digital signaling system is required to operate at 9600 b/s. (a) If a signal element encodes a 4-bit word, what is the minimum required bandwidth of the channel?(b) Repeat part (a) for the case of 8-bit words. #Consider a channel with a 1-MHz capacity and an SNR of 63. (a) What is the upper limit to the data rate that the channel can carry?(b) The result of part (a) is the upper limit. However, as a practical matter, better error performance will be achieved at a lower data rate. Assume we choose a data rate of 2/3 the maximum theoretical limit. How many signal levels are needed to achieve this data rate. Given a channel with an intended capacity of 20 Mbps, the bandwidth of the channel is 3 MHz. Assuming white thermal noise, what signal-to-noise ratio is required to achieve this capacity? Suppose that data are stored on 1.4-Mbyte floppy diskettes that weigh 30 g each. Suppose that an airliner carries 10,000 kg of these floppies at a speed of 1000 km/h over a distance of 5000 km. What is the data transmission rate in bits per second of this system? A telephone line is known to have a loss of 20 dB. The input signal power is measured as 0.5W, and the output noise level is measured as 4.5 uW. Calculate the output SNR in dB. Given a 100 W power source, what is the maximum allowed length for the following transmission media if a signal of 1 W is to be received? (a) 24-gauge (0.5 mm) twisted pair operating at 300 Hz. (b) 24-gauge (0.5 mm) twisted pair operating at 1MHz. (c) 0.375-in (9.5 mm) coaxial cable operating at 1 MHz. (d) 0.375-in (9.5 mm) coaxial cable operating at 25 MHz. (e) optical fiber operating at its optimal frequency. (attenuation graph in slide 41) TUTORIAL#