Unformatted text preview: Data and Computer Communications Communications
Chapter 5 – Signal Encoding Chapter Techniques
Eighth Edition by William Stallings Lecture slides by Lawrie Brown Signal Encoding Techniques Signal
Even the natives have difficulty mastering this peculiar vocabulary —The Golden Bough, Sir James George Frazer Signal Encoding Techniques Signal Digital Data, Digital Signal Digital Digital signal discrete, discontinuous voltage pulses each pulse is a signal element binary data encoded into signal elements Some Terms Some unipolar polar data rate duration or length of a bit modulation rate mark and space Interpreting Signals Interpreting need to know timing of bits  when they start and end signal levels signal to noise ratio data rate bandwidth encoding scheme factors affecting signal interpretation Comparison of Encoding Schemes Schemes signal spectrum clocking error detection signal interference and noise immunity cost and complexity Encoding Schemes Encoding Nonreturn to ZeroLevel Nonreturn (NRZL) two different voltages for 0 and 1 bits voltage constant during bit interval no transition I.e. no return to zero voltage such as absence of voltage for zero, constant such positive voltage for one positive more often, negative voltage for one value more and positive for the other and Nonreturn to Zero Inverted Nonreturn nonreturn to zero inverted on ones constant voltage pulse for duration of bit data encoded as presence or absence of signal data transition at beginning of bit time transition transition (low to high or high to low) denotes binary 1 no transition denotes binary 0 data represented by changes rather than levels more reliable detection of transition rather than level easy to lose sense of polarity example of differential encoding since have NRZ Pros & Cons NRZ Pros easy to engineer make good use of bandwidth dc component lack of synchronization capability Cons used for magnetic recording not often used for signal transmission Multilevel Binary Multilevel BipolarAMI Use more than two levels BipolarAMI zero represented by no line signal one represented by positive or negative pulse one pulses alternate in polarity no loss of sync if a long string of ones long runs of zeros still a problem no net dc component lower bandwidth easy error detection Multilevel Binary Multilevel Pseudoternary one represented by absence of line signal zero represented by alternating positive zero and negative and no advantage or disadvantage over no bipolarAMI bipolarAMI each used in some applications Multilevel Binary Issues Multilevel synchronization with long runs of 0’s or 1’s can insert additional bits, cf ISDN scramble data (later) each signal element only represents one bit
• receiver distinguishes between three levels: +A, A, 0 not as efficient as NRZ a 3 level system could represent log23 = 1.58 bits requires approx. 3dB more signal power for same requires probability of bit error probability Manchester Encoding Manchester 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. Differential Manchester Encoding 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 used by IEEE 802.5 used Biphase Pros and Cons Biphase Con at least one transition per bit time and possibly two maximum modulation rate is twice NRZ requires more bandwidth synchronization on mid bit transition (self clocking) has no dc component has error detection Pros Modulation Rate Modulation Scrambling Scrambling use scrambling to replace sequences that would use produce constant voltage produce these filling sequences must produce enough transitions to sync be recognized by receiver & replaced with original be same length as original have no dc component have no long sequences of zero level line signal have no reduction in data rate give error detection capability design goals B8ZS and HDB3 B8ZS Digital Data, Analog Signal Digital main use is public telephone system has freq range of 300Hz to 3400Hz use modem (modulatordemodulator) Amplitude shift keying (ASK) Frequency shift keying (FSK) Phase shift keying (PK) encoding techniques Modulation Techniques Modulation Amplitude Shift Keying Amplitude encode 0/1 by different carrier amplitudes usually have one amplitude zero susceptible to sudden gain changes inefficient used for up to 1200bps on voice grade lines very high speeds over optical fiber Binary Frequency Shift Keying Keying most common is binary FSK (BFSK) two binary values represented by two different two frequencies (near carrier) frequencies less susceptible to error than ASK used for up to 1200bps on voice grade lines high frequency radio even higher frequency on LANs using coax Multiple FSK Multiple each signalling element represents more each than one bit than more than two frequencies used more bandwidth efficient more prone to error Phase Shift Keying Phase phase of carrier signal is shifted to phase represent data represent binary PSK two phases represent two binary digits phase shifted relative to previous transmission phase rather than some reference signal rather differential PSK Quadrature PSK Quadrature get more efficient use if each signal get element represents more than one bit element
eg. shifts of π /2 (90o) eg. each element represents two bits split input data stream in two & modulate onto split carrier & phase shifted carrier carrier can use 8 phase angles & more than one can amplitude amplitude 9600bps modem uses 12 angles, four of 9600bps which have two amplitudes which QPSK and OQPSK Modulators Modulators Performance of Digital to Analog Modulation Schemes Analog bandwidth ASK/PSK bandwidth directly relates to bit rate multilevel PSK gives significant improvements bit error rate of PSK and QPSK are about 3dB bit superior to ASK and FSK superior for MFSK & MPSK have tradeoff between for bandwidth efficiency and error performance in presence of noise: Quadrature Amplitude Modulation Modulation QAM used on asymmetric digital subscriber line QAM (ADSL) and some wireless (ADSL) combination of ASK and PSK logical extension of QPSK send two different signals simultaneously on send same carrier frequency same use two copies of carrier, one shifted 90° each carrier is ASK modulated two independent signals over same medium demodulate and combine for original binary output QAM Modulator QAM QAM Variants QAM two level ASK each of two streams in one of two states four state system essentially QPSK combined stream in one of 16 states four level ASK have 64 and 256 state systems have improved data rate for given bandwidth but increased potential error rate Analog Data, Digital Signal Analog digitization is conversion of analog data digitization into digital data which can then: into be transmitted using NRZL be transmitted using code other than NRZL be converted to analog signal analog to digital conversion done using a analog codec codec pulse code modulation delta modulation Digitizing Analog Data Digitizing Pulse Code Modulation (PCM) Pulse sampling theorem: “If a signal is sampled at regular intervals at a If rate higher than twice the highest signal frequency, the samples contain all information in original signal” in eg. 4000Hz voice data, requires 8000 sample eg. per sec per Pulse Amplitude Modulation (PAM) strictly have analog samples strictly so assign each a digital value PCM Example PCM PCM Block Diagram PCM NonLinear Coding NonLinear Companding Companding Delta Modulation Delta analog input is approximated by a analog staircase function staircase can move up or down one level (δ ) at each at sample interval sample since function only moves up or down at each since sample interval sample hence can encode each sample as single bit 1 for up or 0 for down has binary behavior Delta Modulation Example Delta Delta Modulation Operation Delta PCM verses Delta Modulation PCM DM has simplicity compared to PCM but has worse SNR issue of bandwidth used eg. for good voice reproduction with PCM
• want 128 levels (7 bit) & voice bandwidth 4khz • need 8000 x 7 = 56kbps data compression can improve on this still growing demand for digital signals use of repeaters, TDM, efficient switching PCM preferred to DM for analog signals Analog Data, Analog Signals Analog modulate carrier frequency with analog data why modulate analog signals? higher frequency can give more efficient transmission permits frequency division multiplexing (chapter 8) Amplitude Frequency Phase types of modulation Analog Modulation Modulation Techniques Amplitude Modulation Frequency Modulation Phase Modulation Summary Summary looked at signal encoding techniques digital data, digital signal analog data, digital signal digital data, analog signal analog data, analog signal ...
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 Spring '09
 Kang
 error detection, The Golden Bough, Analog Modulation Modulation, digital signal analog, analog signal analog

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