Digital transmission fundamentals (chap.3 & 4)What is digital transmission?Slide 3Slide 4Analog Digital ? --PCMSlide 6Why digital?Slide 8Slide 9Slide 10Slide 11Digital representations for different informationSlide 13Basic properties of digital transmission systemsBasic properties of digital transmission systems (cont.)Shannon Channel CapacitySlide 17Slide 18Telephone Modem exampleBit Rates of Digital Transmission SystemsLine CodingVarious line codingSlide 23mBnB encoding (n>m)Transmission MediaTransmission DelayError Detection and CorrectionOdd error detection using parity bitTwo-dimension parity checksInternet ChecksumCRC (Cyclic Redundancy Check) (chapter 3.9.4)Slide 32How to compute CRCSlide 34Slide 35Typical standard CRC polynomialsSlide 37The error detection capabilities of CRCsStatistical multiplexingSlide 40Slide 41Slide 42Slide 43Slide 44Slide 45CIS, IUPUI 1Digital transmission fundamentals (chap.3 & 4)1. What is digital transmission?2. Why digital transmission?3. How to represent different information in digital format? 4. What is Bit rate/bandwidth?5. What are the properties of different media?6. How to do error detection and correction?7. Multiplexing?CIS, IUPUI 2What is digital transmission?•Analog transmission –Continuous waveform•Digital representation and transmission–Discrete binary sequence/pulses–1: a rectangular pulse of amplitude 1 and of duration 0.125 milliseconds–0: a rectangular pulse of amplitude -1 and of duration of 0.125 milliseconds.CIS, IUPUI 3Figure 3.13CIS, IUPUI 41 0 1 0 1 0 1 0. . . . . .t1 ms(a)1 ms1 1 1 1 0 0 0 0. . . . . .t(b)Figure 3.14CIS, IUPUI 5Analog Digital ? --PCM•Analog signal such as voice/music: –continuous waveform, i.e, variations in air pressure. –Bandwidth: a measure of how fast the signal varies, i.e., cycles/second, or Hertz.•Two stages:–Sampling: for bandwidth W, minimum sampling rate is 2W. –Quantizing: how many levels to represent a sample.•Example: W=4 kHz, sampling rate=8K samples/second. Sample period is T=1/8000=125 microseconds. Suppose 8 bits/sample (256 levels), then PCM bit rate is 8000*8=64 kbps.CIS, IUPUI 6(a)(b)Figure 3.2CIS, IUPUI 7Why digital?•For analog: –output should reproduce the input exactly. No distortion.–Repeater amplifies noise, difficult task.–Too much repeaters may make noise too large, so limited distance–Cost is high.–Basic voice/telephone service.•For digital:–Not exact, as long as can distinguish 1 or o. –Digital regenerator generates pure digital numbers, easy.–No limitation on digital regenerators, no distance limitation.–Cost is low.–More other services, easily multiplexing, more functions.CIS, IUPUI 8(a) Analog transmission: all details must be reproduced accuratelySentSentReceivedReceived• e.g digital telephone, CD Audio(b) Digital transmission: only discrete levels need to be reproduced• e.g. AM, FM, TV transmissionFigure 3.6CIS, IUPUI 9Source RepeaterDestinationRepeaterTransmission segmentFigure 3.7CIS, IUPUI 10Attenuated & distorted signal + noiseEqualizerRecovered signal+residual noiseRepeaterAmp.Figure 3.8CIS, IUPUI 11AmplifierEqualizerTimingRecoveryDecision Circuit.& SignalRegeneratorFigure 3.9Distorted Digital signal is easy to restore by regenerator.CIS, IUPUI 12Digital representations for different information•Text: ASCII•Scanned WB documents: –A4 paper, 200 X 100 pixels/inch. 256KB.•Color pictures/images:–8 X 10 inch photo, 400 X400 pixels/inch. 38.4MB.•Voice: PCM/ADPCM, 4kHz, 64kbps (this as well the followings called stream)•Music/Audio: –MPEG/MP3, 16-24 kHz, 512-748 kbps.•Video: a sequence of pictures (moving pictures)–H.261, 176 X 144 pixels/frame, 10-30 frames/second, 2 Mbps.–MPEG-2, 720 X 480 pixels/frame, 30 frames/second, 249 Mbps. •Compression:–249Mbps  2 – 6 Mbps.–Compression cost, but reduce the transmission cost.CIS, IUPUI 13ReceiverCommunication channelTransmitterFigure 3.5ReceiverCommunication channelTransmitter(Digital) Transmission System0110101…0110101…CIS, IUPUI 14Basic properties of digital transmission systems•Bit rate or transmission speed R: bits/second.•Can be viewed as cross-section of the channel: the higher R is, the larger the volume of the channel.•Bandwidth of a signal: Ws–The range of frequencies contained in the signal. •Bandwidth of channel: Wc–The range of input frequencies passed by the channel.•Wc limits Ws that can pass through the channel.CIS, IUPUI 15Basic properties of digital transmission systems (cont.)•Theory: if Wc=W, then the narrowest pulse has duration =1/2W. Thus, the maximum rate for pulses is rmax=2W pulses/second.•If transmitting binary information by sending two kinds of pulses: +A for 1 and _A for 0, then the system bite rate is R=2W pulses/second * 1bit/pluse =2W bits/second.•If pulses can be multiple levels (M=4): -A, -A/3, +A/3, +A for (00, 01, 10,11), then each pulse can represent 2 bits. So R=4W bps.•If multiple levels M=2m, then R=2W *m=2Wm bps.•Theoretically, the bit rate R can be increased without limits as long as we increase M.CIS, IUPUI 16Shannon Channel Capacity•Unfortunately, in practice, R is greatly limited: –Levels can not be measured accurately if too many levels.–there exists noise in real world.•Signal –to-noise ratio SNR= average signal power/average noise power. •SNR(dB)=10log10SNR (decibels).•So Shannon Channel Capacity:–C (i.e., maximum reliable bite rate R ) – = W log 2(1+SNR) bits/second.CIS, IUPUI 17Figure 3.11SNR = Average signal powerAverage noise powerSNR (dB) = 10 log10 SNRSignalNoiseSignal + noiseHighSNRtttSignalNoiseSignal + noiseLowSNRtttCIS, IUPUI 18Four signal levels Eight signal levelsTypical noiseFigure 3.32When too many levels: 1. difficult to measure 2. noise will easily affect the value.CIS, IUPUI 19Telephone Modem example•W=3.4 kHz, SNR=10,000, SNR(dB)=40 dB.•C=3400log2(1+10000) =45200 bits/second.•So telephone channel is at 45.2kbps. •Interesting point: V.90 model’s rate: 56kbps.–Inbound to network: 33.6kbps •Analog to digital, quantization noise, SNR=39 dB.–Outbound to user from ISP: already in digital.CIS, IUPUI 20Bit Rates of Digital Transmission SystemsSystem Bit Rate ObservationsTelephone