DATA COMMUNICATION (ELA…)

1. DATA COMMUNICATION(ELA…) SIGNAL ENCODING TECHNIQUES

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

3. Encoding onto a Digital Signal

4. Modulation onto an Analog Signal

5. Data Encoding Criteria • An increase in DR increases BER • An increase in SNR decreases BER • An increase in BW allows an increase in DR

6. Data Encoding Criteria (cont.) • The other factor that improves performance is the encoding scheme • The encoding scheme is simply the mapping from data bits to signal elements

7. Digital Data  Digital Signal

8. Digital Data  Digital Signal • Receiver needs to know • Timing of bits • Signal levels • Factors affecting successful interpretation of signals

9. Encoding Schemes • Non-return to Zero-Level (NRZ-L) • Non-return to Zero Inverted (NRZI) • Bipolar-Alternate Mark Inversion (AMI) • Pseudoternary • Manchester • Differential Manchester • Bipolar with 8-Zeros Substitution (B8ZS) • High-Density Bipolar 3-zeros (HDB3)

10. 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

11. 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

12. NRZ-L • Two different voltages for 0 and 1 bits • Voltage constant during bit interval • No transition (i.e., no return to zero voltage) • Options: • Absence of voltage for zero, constant positive voltage for one • More often, negative voltage for one value and positive for the other

13. NRZI • 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 a binary 1 • No transition denotes binary 0 • An example of differential encoding

14. Differential Encoding • In complex transmission layouts, it is easy to lose sense of polarity • Therefore • Data represented by changes (i.e., transitions) rather than levels • More reliable detection of transition rather than level

15. 0 1 0 0 1 1 0 0 0 1 1 Nonreturn to Zero (NRZ) NRZ-L NRZI

16. NRZ – Pros and 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

17. Bipolar-AMI • Uses more than two levels • “0”  represented by no line signal • “1”  represented by positive or negative pulse, pulses alternate in polarity • No loss of sync if a long string of 1s (0s still a problem) • No net DC component • Because the “1” signals alternate in voltage from + to - • Lower bandwidth • Easy error detection • Because pulses alternate in polarity

18. Pseudoternary • Uses more than two levels • “1”  represented by absence of line signal • “0”  represented by alternating positive and negative levels • No advantage or disadvantage over bipolar-AMI

19. Trade Off for Multilevel Binary • Not as efficient as NRZ • Technically, a 3 signal level system • Log2 3 = 1.58 bits • However, each signal element only represents one bit • Receiver must distinguish between three levels (+A, -A, 0) • Requires ≈ 3dB more signal power for same probability of bit error

20. 0 1 0 0 1 1 0 0 0 1 1 Bipolar-AMI & Pseudoternary B-AMI PT

21. Manchester • Transition in middle of each bit period • Transition serves as clock AND data • Low-to-high represents “1” • High-to-low represents “0” • Used in IEEE 802.3 (Ethernet LAN)

22. Differential Manchester • Mid-bit transition is clocking only • Transition at start of a bit period represents “0” • No transition at start of a bit period represents “1” • This is a differential encoding scheme • Used in IEEE 802.5 (Token Ring LAN)

23. 0 1 0 0 1 1 0 0 0 1 1 Biphase (Manchester and D-Manchester) Man D-Man

24. Biphase – Pros and Cons • Pros • Synchronization on mid bit transition (self clocking) • No DC component • Error detection • Absence of expected transition • Cons • At least one transition per bit time and possibly two • Maximum modulation rate is twice NRZ • Requires more bandwidth

25. Transmission Rates

26. Transmission Rates

27. Modulation Rate

28. Scrambling • Use scrambling to replace sequences that would produce constant voltage • Filling sequence • Must produce enough transitions to sync • Must be recognized by receiver and replaced with original • Same length as original • Design Goals • No DC component • No long sequences of zero level line signal • No reduction in data rate • Error detection capability

29. Bipolar With 8 Zeros Substitution (B8ZS) • 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

30. High Density Bipolar 3 Zeros (HDB3) • Based on bipolar-AMI • String of four zeros replaced with one or two pulses

31. B8ZS and HDB3

32. Digital Data  Analog Signal

33. Digital Data  Analog Signal • Public telephone system • 300Hz to 3400Hz • Use modem (modulator-demodulator) • Basic Encoding Techniques • Amplitude-shift keying (ASK) • Amplitude difference of carrier frequency • Frequency-shift keying (FSK) • Frequency difference near carrier frequency • Phase-shift keying (PSK) • Phase of carrier signal shifted

34. Amplitude-Shift Keying (ASK) • 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)

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

36. ASK – Principle of Operation

37. ASK – Principle of Operation

38. ASK Bandwidth Requirements

39. ASK – Example Assuming ASK modulation is to be used, estimate the BW required of a channel to transmit at the following bit rates: 300bps, 1200bps and 4800bps, assuming a) the fo of the sequence 101010… is to be received b) the fo and 3fo are to the received Comment on your results in relation to the PSTN Source: Halsall, F., Data Communications, Computer Networks and Open Systems, (USA: Addison-Wiley, 1996), pg. 61

40. 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

41. 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

42. BFSK – Principle of Operation

43. BFSK – Principle of Operation

44. BFSK Bandwidth Requirements

45. BFSK – Example

46. Phase-Shift Keying (PSK) • Two-level PSK (BPSK) • Uses two phases to represent binary digits • Differential PSK

47. Quadrature PSK • More efficient use by each signal element representing more than one bit • Uses shifts separated by multiples of /2 (90o) • Each element represents two bits • Can use 8 phase angles and have more than one amplitude • 9600bps modems use 12 angles , four of which have two amplitudes

48. Quadrature PSK • QuadraturePSK (QPSK) • Each element represents more than one bit

49. Phase-Shift Keying (PSK) • Multilevel PSK • Using multiple phase angles with each angle having more than one amplitude, multiple signals elements can be achieved • D = modulation rate, baud • R = data rate, bps • M = number of different signal elements = 2L • L = number of bits per signal element

50. PSK Bandwidth Requirements