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Data and Computer Communications Chapter 5 – Signal Encoding Techniques

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Signal Encoding Techniques

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Digital Data, Digital Signal – Equipment less costly Analog Data, Digital Signal – Use of modern digital transmission and switching equipment Digital Data, Analog Signal – Optical Fiber Analog Data, Analog Signal – Voice transmitted and modulation to shift bandwidth of a baseband signal.

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Digital Data, Digital Signal Digital signal discrete, discontinuous voltage pulses discrete, discontinuous voltage pulses each pulse is a signal element each pulse is a signal element binary data encoded into signal elements binary data encoded into signal elements

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Some Terms unipolar polar data rate duration or length of a bit modulation rate mark and space

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Interpreting Signals need to know timing of bits - when they start and end timing of bits - when they start and end signal levels signal levels factors affecting signal interpretation signal to noise ratio signal to noise ratio data rate data rate bandwidth bandwidth encoding scheme encoding scheme

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Comparison of Encoding Schemes signal spectrum lack-of-DC is desirable lack-of-DC is desirable Good signal design transmitted power in the middle of the transmission bandwidth Good signal design transmitted power in the middle of the transmission bandwidth Clocking - expensive error detection signal interference and noise immunity cost and complexity – signal rate should not be higher

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Encoding Schemes

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Nonreturn to Zero-Level (NRZ-L) two different voltages for 0 and 1 bits voltage constant during bit interval no transition I.e. no return to zero voltage no transition I.e. no return to zero voltage such as absence of voltage for zero, constant positive voltage for one such as absence of voltage for zero, constant positive voltage for one more often, negative voltage for one value and positive for the other more often, negative voltage for one value and positive for the other

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Nonreturn to Zero Inverted nonreturn to zero inverted on ones constant voltage pulse for duration of bit data encoded as presence or absence of signal transition at beginning of bit time transition (low to high or high to low) denotes binary 1 transition (low to high or high to low) denotes binary 1 no transition denotes binary 0 no transition denotes binary 0 example of differential encoding since have data represented by changes rather than levels data represented by changes rather than levels more reliable detection of transition rather than level more reliable detection of transition rather than level easy to lose sense of polarity – (accidental inversion of twisted pair) easy to lose sense of polarity – (accidental inversion of twisted pair)

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NRZ Pros & Cons Pros easy to engineer easy to engineer make good use of bandwidth make good use of bandwidth Cons dc component dc component lack of synchronization capability lack of synchronization capability used for magnetic recording not often used for signal transmission

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Encoding Schemes

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Multilevel Binary Bipolar-AMI Use more than two levels Bipolar-AMI zero represented by no line signal zero represented by no line signal one represented by positive or negative pulse one represented by positive or negative pulse one pulses alternate in polarity one pulses alternate in polarity no loss of sync if a long string of ones no loss of sync if a long string of ones long runs of zeros still a problem long runs of zeros still a problem no net dc component no net dc component lower bandwidth lower bandwidth easy error detection easy error detection

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Multilevel Binary Pseudoternary one represented by absence of line signal zero represented by alternating positive and negative no advantage or disadvantage over bipolar-AMI each used in some applications

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Multilevel Binary Issues synchronization with long runs of 0’s or 1’s can insert additional bits, cf ISDN can insert additional bits, cf ISDN scramble data (later) scramble data (later) not as efficient as NRZ each signal element only represents one bit each signal element only represents one bit receiver distinguishes between three levels: +A, -A, 0receiver distinguishes between three levels: +A, -A, 0 a 3 level system could represent log 2 3 = 1.58 bits a 3 level system could represent log 2 3 = 1.58 bits requires approx. 3dB more signal power for same probability of bit error requires approx. 3dB more signal power for same probability of bit error Bit error rate for NRZ at an SNR is higher Bit error rate for NRZ at an SNR is higher

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Manchester Encoding has transition in middle of each bit period transition serves as clock and data low to high represents one high to low represents zero used by IEEE 802.

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Differential Manchester Encoding midbit transition is clocking only transition at start of bit period representing 0 no transition at start of bit period representing 1 this is a differential encoding scheme this is a differential encoding scheme used by IEEE 802.5

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Biphase Pros and Cons Con at least one transition per bit time and possibly two at least one transition per bit time and possibly two maximum modulation rate is twice NRZ maximum modulation rate is twice NRZ requires more bandwidth requires more bandwidth Pros synchronization on mid bit transition (self clocking) synchronization on mid bit transition (self clocking) has no dc component has no dc component has error detection has error detection

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Modulation Rate When signal-encoding techniques are used, a distinction needs to be made between data rate (expressed in bits per second) and modulation rate (expressed in baud). The data rate, or bit rate, is 1/T b, where T b = bit duration

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Scrambling use scrambling to replace sequences that would produce constant voltage these filling sequences must produce enough transitions to sync produce enough transitions to sync be recognized by receiver & replaced with original be recognized by receiver & replaced with original be same length as original be same length as original design goals have no dc component have no dc component have no long sequences of zero level line signal have no long sequences of zero level line signal have no reduction in data rate have no reduction in data rate give error detection capability give error detection capability

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B8ZS and HDB3

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Digital Data, Analog Signal main use is public telephone system has freq range of 300Hz to 3400Hz has freq range of 300Hz to 3400Hz use modem (modulator-demodulator) use modem (modulator-demodulator) encoding techniques Amplitude shift keying (ASK) Amplitude shift keying (ASK) Frequency shift keying (FSK) Frequency shift keying (FSK) Phase shift keying (PK) Phase shift keying (PK)

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Modulation Techniques

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Amplitude Shift Keying encode 0/1 by different carrier amplitudes usually have one amplitude zero usually have one amplitude zero susceptible to sudden gain changes inefficient used for up to 1200bps on voice grade lines up to 1200bps on voice grade lines very high speeds over optical fiber very high speeds over optical fiber

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Binary Frequency Shift Keying most common is binary FSK (BFSK) two binary values represented by two different frequencies (near carrier) less susceptible to error than ASK used for up to 1200bps on voice grade lines up to 1200bps on voice grade lines high frequency radio high frequency radio even higher frequency on LANs using co-ax even higher frequency on LANs using co-ax

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Multiple FSK each signalling element represents more than one bit more than two frequencies used more bandwidth efficient more prone to error

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Multiple FSK In this case each signaling element represents more than one bit. To match the data rate of the input bit stream, each output signal element is held for a period of T s = LT seconds, where T is the bit period (data rate = 1/T). Thus, one signal element, which is a constant-frequency tone, encodes L bits. S_i(t) = A Cos 2 \pi f_i t The total bandwidth required is 2Mf d. It can be shown that the minimum frequency separation required is 2f d = 1/T s. Therefore, the modulator requires a bandwidth of W d = 2Mf d = M/T s.

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Sum With f_c = 250 KHz, f_d = 25 KHz, M = 8 bits, find out all the frequencies f_i = f_c + (2i – 1 – M) f_d

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Phase Shift Keying phase of carrier signal is shifted to represent data binary PSK two phases represent two binary digits two phases represent two binary digits differential PSK phase shifted relative to previous transmission rather than some reference signal phase shifted relative to previous transmission rather than some reference signal

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Performance of Digital to Analog Modulation Schemes bandwidth ASK/PSK bandwidth directly relates to bit rate ASK/PSK bandwidth directly relates to bit rate multilevel PSK gives significant improvements multilevel PSK gives significant improvements in presence of noise: bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK for MFSK & MPSK have tradeoff between bandwidth efficiency and error performance for MFSK & MPSK have tradeoff between bandwidth efficiency and error performance

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Analog Data, Digital Signal digitization is conversion of analog data into digital data which can then: be transmitted using NRZ-L be transmitted using NRZ-L be transmitted using code other than NRZ-L be transmitted using code other than NRZ-L be converted to analog signal be converted to analog signal analog to digital conversion done using a codec pulse code modulation pulse code modulation delta modulation delta modulation

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Digitizing Analog Data

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Pulse Code Modulation (PCM) sampling theorem: “If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all information in original signal” “If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all information in original signal” eg. 4000Hz voice data, requires 8000 sample per sec eg. 4000Hz voice data, requires 8000 sample per sec strictly have analog samples Pulse Amplitude Modulation (PAM) Pulse Amplitude Modulation (PAM) so assign each a digital value

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PCM Example

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PCM Block Diagram

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Voice – 4000 Hz Sample – 8000 samples per second Use 8-bit sample, allows 256 quantization level 8000 samples per second x 8 bits per sample = 64kbps SNR_DB = 20 log 2^2 + 1.76 dB

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Non-Linear Coding

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Delta Modulation analog input is approximated by a staircase function can move up or down one level ( ) at each sample interval can move up or down one level ( ) at each sample interval has binary behavior since function only moves up or down at each sample interval since function only moves up or down at each sample interval hence can encode each sample as single bit hence can encode each sample as single bit 1 for up or 0 for down 1 for up or 0 for down

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Delta Modulation Example

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Delta Modulation Operation

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PCM verses Delta Modulation DM has simplicity compared to PCM but has worse SNR issue of bandwidth used eg. for good voice reproduction with PCM eg. for good voice reproduction with PCM want 128 levels (7 bit) & voice bandwidth 4khzwant 128 levels (7 bit) & voice bandwidth 4khz need 8000 x 7 = 56kbpsneed 8000 x 7 = 56kbps data compression can improve on this still growing demand for digital signals use of repeaters, TDM, efficient switching use of repeaters, TDM, efficient switching PCM preferred to DM for analog signals

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Analog Data, Analog Signals modulate carrier frequency with analog data why modulate analog signals? higher frequency can give more efficient transmission higher frequency can give more efficient transmission permits frequency division multiplexing (chapter 8) permits frequency division multiplexing (chapter 8) types of modulation Amplitude Amplitude Frequency Frequency Phase Phase

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Analog Modulation Techniques Amplitude Modulation Frequency Modulation Phase Modulation

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s(t) = [1 + n_d x(t)] Cos 2\pi f_c t n_d = modulation index ratio of the amplitude of the input signal to carrier s(t) = [1 + n_dCos2\pi f_m t] cos 2\pi f_c t Expand using trigonometric identity

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Summary looked at signal encoding techniques digital data, digital signal digital data, digital signal analog data, digital signal analog data, digital signal digital data, analog signal digital data, analog signal analog data, analog signal analog data, analog signal

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Quadrature PSK get more efficient use if each signal element represents more than one bit eg. shifts of /2 (90 o ) eg. shifts of /2 (90 o ) each element represents two bits each element represents two bits split input data stream in two & modulate onto carrier & phase shifted carrier split input data stream in two & modulate onto carrier & phase shifted carrier can use 8 phase angles & more than one amplitude 9600bps modem uses 12 angles, four of which have two amplitudes 9600bps modem uses 12 angles, four of which have two amplitudes

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QPSK and OQPSK Modulators

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Quadrature Amplitude Modulation QAM used on asymmetric digital subscriber line (ADSL) and some wireless combination of ASK and PSK logical extension of QPSK send two different signals simultaneously on same carrier frequency use two copies of carrier, one shifted 90 ° use two copies of carrier, one shifted 90 ° each carrier is ASK modulated each carrier is ASK modulated two independent signals over same medium two independent signals over same medium demodulate and combine for original binary output demodulate and combine for original binary output

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QAM Modulator

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QAM Variants two level ASK each of two streams in one of two states each of two streams in one of two states four state system four state system essentially QPSK essentially QPSK four level ASK combined stream in one of 16 states combined stream in one of 16 states have 64 and 256 state systems improved data rate for given bandwidth but increased potential error rate but increased potential error rate

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Companding

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Data and Computer Communications Eighth Edition by William Stallings Lecture slides by Lawrie Brown Chapter 5 – Signal Encoding Techniques.

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