05-SignalEncodingTechniques.pptx

Data and Computer Communications Chapter 5 – Signal Encoding Techniques EIGHTH EDITION BY WILLIAM STALLINGS LECTURE SLIDES BY LAWRIE BROWN Signal Encoding Techniques continuous constant-frequency fc signal is carrier signal digital signaling analog signaling Digital Data, Digital Signal Digital sign...

Data and Computer Communications Chapter 5 – Signal Encoding Techniques EIGHTH EDITION BY WILLIAM STALLINGS LECTURE SLIDES BY LAWRIE BROWN Signal Encoding Techniques continuous constant-frequency fc signal is carrier signal digital signaling analog signaling Digital Data, Digital Signal Digital signal ◦ discrete, discontinuous voltage pulses ◦ each pulse is a signal element ◦ binary data encoded into signal elements Some Terms unipolar Unipolar - All signal elements have the same sign Polar - One logic state represented by positive voltage the other by negative voltage data rate Data rate - Rate of data (R) transmission in bits per duration or length of a bit second Duration or length of a bit - Time taken for modulation rate transmitter to emit the bit (1/R) Modulation rate -Rate at which the signal level mark and space changes, measured in baud = signal elements per second. Depends on type of digital encoding used. Mark and Space - Binary 1 and Binary 0 respectively polar Interpreting Signals The receiver need to know ◦ timing of bits - when they start and end ◦ signal levels factors affecting signal interpretation ◦ signal to noise ratio ◦ data rate ◦ bandwidth ◦ encoding scheme Comparison of Encoding Schemes signal spectrum clocking error detection signal interference and noise immunity cost and complexity Encoding Schemes 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 ◦ such as absence of voltage for zero, constant positive voltage for one ◦ more often, negative voltage for one value and positive for the other 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 ◦no transition denotes binary 0 example of differential encoding since have ◦data represented by changes rather than levels ◦more reliable detection of transition rather than level ◦easy to lose sense of polarity Multilevel Binary Bipolar-AMI Use more than two levels Bipolar-AMI ◦ 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 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 Multilevel Binary Issues synchronization with long runs of 0’s or 1’s ◦can insert additional bits, cf ISDN ◦scramble data (later) not as efficient as NRZ ◦each signal element only represents one bit ◦ receiver distinguishes between three levels: +A, -A, 0 ◦a 3 level system could represent log23 = 1.58 bits ◦requires approx. 3dB more signal power for same probability of bit error 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. 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 used by IEEE 802.5 Modulation Rate Scrambling use scrambling to replace sequences that would produce constant voltage these filling sequences must ◦produce enough transitions to sync ◦be recognized by receiver & replaced with original ◦be same length as original design goals ◦have no dc component ◦have no long sequences of zero level line signal ◦have no reduction in data rate ◦give error detection capability B8ZS and HDB3 bipolar with 8-zeros substitution (B8ZS) high-density bipolar-3 zeros (HDB3) Digital Data, Analog Signal main use is public telephone system ◦ has freq range of 300Hz to 3400Hz ◦ use modem (modulator-demodulator) encoding techniques ◦ Amplitude shift keying (ASK) ◦ Frequency shift keying (FSK) ◦ Phase shift keying (PK) Modulation Techniques Amplitude Shift Keying 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 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 ◦high frequency radio ◦even higher frequency on LANs using co-ax Multiple FSK each signalling element represents more than one bit more than two frequencies used more bandwidth efficient more prone to error Phase Shift Keying phase of carrier signal is shifted to represent data binary PSK ◦ two phases represent two binary digits differential PSK ◦ phase shifted relative to previous transmission rather than some reference signal