Data and Computer Communications

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

2. Signal Encoding Techniques Even the natives have difficulty mastering this peculiar vocabulary —The Golden Bough, Sir James George Frazer

3. Digital Signal • Analog Signal • Digital Signal • Analog Signal Encoding Techniques • Digital data • Analog data

4. Signal Encoding Techniques

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

6. Signal Encoding Techniques • Modulation: • The process of encoding source data onto a carrier signal with frequency f • Carrier signal • A continuous constant-frequency that is chosen to be compatible with the transmission media. • Baseband signal (modulating signal) • The input signal (analog or digital) • The result of modulating the carrier signal is called the modulated signal s(t)

7. Some Terms • Unipolar • All signal elements have the same sign. • Polar • One logic state represented by positive voltage the other by negative voltage. • data rate • Rate of data transmission in bits per second

8. Some Terms • duration or length of a bit • Time taken for a transmitter to emit the bit (1/R) • modulation rate • Rate at which the signal level changes. • Measured in baud = signal elements per second. • mark and space • Binary 1 and Binary 0 respectively

9. Interpreting Signals • 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

10. Data Encoding Criteria • An increase in DR increases BER • An increase in SNR decreases BER • An increase in BW allows an increase in DR • The other factor that improves performance is the encoding scheme • The encoding scheme is simply the mapping from data bits to signal elements

11. Encoding Schemes • Non-return to Zero-Level (NRZ-L) • Non-return to Zero Inverted (NRZI) • Bipolar -AMI • Pseudoternary • Manchester • Differential Manchester • B8ZS • HDB3

12. Comparing Encoding Schemes • Signal spectrum • With lack of high-frequency components, less bandwidth required • With no DC component, AC coupling via transformer possible • Concentrate power in the middle of the bandwidth • Clocking • Ease of determining beginning and end of each bit position • Not easy task. • Separate clock for synchronization .(expensive) • Synchronization based on the transmitted signal

13. Comparing Encoding Schemes • Error detection • Can be built into signal encoding • Signal interference and noise immunity • Performance in the presence of noise • Cost and complexity • The higher the signal rate to achieve a given data rate, the greater the cost • Some codes require signal rate greater than data rate

14. Encoding Schemes

15. 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

16. 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

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

18. Spectral Density of Various Signal Encoding Schemes • Most of the energy in NRZ and NRZI signals is between dc and half the bit rate. • Data rate of 9600 bps, most of the energy in the signal is concentrated between dc and 4800 Hz.

19. RZ encoding

20. 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

21. Multilevel Binary Bipolar-AMI

22. Multilevel BinaryPseudoternary • 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

23. Multilevel BinaryPseudoternary

24. 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

25. Theoretical Bit Error Rate for Various Encoding

26. 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 802.3

27. 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

28. Biphase Pros and Cons • Con • at least one transition per bit time and possibly two • maximum modulation rate is twice NRZ • requires more bandwidth • Pros • synchronization on mid bit transition (self clocking) • has no dc component • has error detection • Absence of expected transition • Noise would have to invert both before and after expected transition.

29. Modulation Rate

30. Modulation/Baud Rate • Baud rate, also known as signaling rate or modulation rate: • Signal elements per second (baud). • The rate at which signal elements are transmitted. • In general, D = R/L = R/(log2M) • D = modulation rate, baud • R = data rate, bps • M = number of different signal elements = 2L • L = number of bits per signal element • For two-level signaling (binary), bit rate is equal to the baud rate.

31. Modulation/Baud Rate • Example: a stream of binary ones at 1 Mbps. NRZI is 1 MBaud. Manchester has 0.5 bits/signal element: Baud rate = Bit rate/Nb = 1 Mbps/0.5 = 2 MBaud

32. Signal element versus data element

33. 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

34. B8ZS • Bipolar With 8 Zeros Substitution • Based on bipolar-AMI • If octet of all zeros and last voltage pulse preceding was positive encode as 000+-0-+ • If octet of all zeros and last voltage pulse preceding was negative encode as 000-+0+- • Causes two violations of AMI code • Unlikely to occur as a result of noise • Receiver detects and interprets as octet of all zeros

35. B8ZS V: violation voltage that breaks AMI rule of encoding (opposite polarity from the previous) B: Bipolar a nonzero level voltage in accordance with the AMI rule

36. Scrambling Techniques • HDB3: • High Density Bipolar 3 zeros. • String of four zeros replaced with one or two pulses. • 4 zeros are encoded as either 000-, 000+, +00+, or -00- • Number of the non-zero pulses after last substitution is odd use (000V) • Number of the non-zero pulses after last substitution is even use (B00V) • Substitution rule is s.t. the 4th bit is always a code violation, and successive violations are of alternate polarity (not to introduce dc component)

37. B8ZS and HDB3

38. 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)

39. Modulation Techniques

40. Amplitude Shift Keying • One binary digit represented by presence of carrier, at constant amplitude • Other binary digit represented by absence of carrier • where the carrier signal is A cos(2πfct)

41. ASK Characteristics • Susceptible to sudden gain changes • Inefficient modulation technique • used for • On voice-grade lines, used up to 1200 bps • Used to transmit digital data over optical fiber

42. ASK Implementation

43. Amplitude Shift Keying The carrier is only one simple sine wave, the process of modulation produce s a nonperiodic composite signal

44. Binary Frequency-Shift Keying (BFSK) • Two binary digits represented by two different frequencies near the carrier frequency • where f1 and f2 are offset from carrier frequency fc by equal but opposite amounts

45. BFSK Characteristics • Less susceptible to error than ASK • On voice-grade lines, used up to 1200bps • Used for high-frequency (3 to 30 MHz) radio transmission • Can be used at higher frequencies on LANs that use coaxial cable

46. Relationship between baud rate and bandwidth in FSK

47. FSK on Voice Grade Line

48. Multiple FSK • each signalling element represents more than one bit • more than two frequencies used • more bandwidth efficient • more susceptible to error

49. Multiple FSK • fi= fc+ (2i – 1 – M) fd • fc = the carrier frequency • fd = the difference frequency • M = number of different signal element = 2L • L = number of bits per signal element

50. Multiple FSK • To mach the data rate of the input, each signal element is hold for a period of Ts = LT • T is a bit period • One signal element encodes L bits • total bandwidth required = 2Mfd • minimum frequency separation = 2fd = 1/Ts • modulator requires a bandwidth of Wd = 2Mfd = M/Ts