William Stallings Data and Computer Communications

1. William StallingsData and Computer Communications Chapter 5 Data Encoding

2. Data Communication Basics • Analog or Digital • Three Components • Data • Signal • Transmission

3. Analog Data Choices

4. Digital Data Choices

5. Encoding Techniques • Digital data, digital signal • Analog data, digital signal • Digital data, analog signal • Analog data, analog signal

6. Transmission Choices • Analog transmission • only transmits analog signals, without regard for data content • attenuation overcome with amplifiers • Digital transmission • transmits analog or digital signals • uses repeaters rather than amplifiers

7. Advantages of Digital Transmission • The signal is exact • Signals can be checked for errors • Noise/interference are easily filtered out • A variety of services can be offered over one line • Higher bandwidth is possible with data compression

8. Encoding schemes Analog data, Digital signal Analog data, Analog signal digital analog analog voice Telephone CODEC Digitaldata, Digital signal Digitaldata, Analogsignal analog digital digital digital Modem Digital transmitter

9. Encoding and Modulation x(t) x(t) g(t) g(t) Encoder Decoder digital or analog digital t s(f) s(t) m(t) Modulator Demodulator m(t) digital or analog analog f fc fc

10. Why encoding? • Three factors determine successfulness of receiving a signal • S/N (Signal to Noise Ratio) • data rate • bandwidth

11. Encoding Schemes' evaluation factors • Signal spectrum • Clocking • Error detection • Signal interference & noise immunity • Cost and complexity

12. Digital Data, Digital Signal / Characteristics • Digital signal • Uses discrete, discontinuous, voltage pulses • Each pulse is a signal element • Binary data is encoded into signal elements

13. Terms (1) • Unipolar • All signal elements have 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 • Duration or length of a bit • Time taken for transmitter to emit the bit

14. Terms (2) • 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

15. Interpreting Signals • Need to know • Timing of bits - when they start and end • Signal levels • Factors affecting successful interpretation of signals: • Signal to noise ratio • Data rate • Bandwidth

16. Comparison of Encoding Schemes (1) • Signal Spectrum • Lack of high frequencies reduces required bandwidth • Lack of dc component allows ac coupling via transformer, providing isolation • It is important to concentrate power in the middle of the bandwidth • Clocking issues • Synchronizing transmitter and receiver is essential • External clock is one way used for synchronization • Synchronizing mechanism based on signal is also used & preferred (over using an external clock)

17. Spectral density 1.5 B8ZS,HDB3 NRZ-L, NRZI 1 AMI, Pseudoternary 0.5 Mean square voltage per unit bandwidth Manchester, Differential Manchester 0 0 1 0.5 1.5 -0.5 Normalizedfrequency (f/r)

18. Comparison of Encoding Schemes (2) • Error detection • Can be built into signal encoding • Signal interference and noise immunity • Some codes are better than others • Cost and complexity • Higher signal rate (& thus data rate) lead to higher costs • Some codes require signal rate greater than data rate

19. Encoding Schemes • Nonreturn to Zero-Level (NRZ-L) • Nonreturn to Zero Inverted (NRZI) • Bipolar -AMI (Alternate Mark Inversion) • Pseudoternary • Manchester • Differential Manchester • B8ZS • HDB3

20. Digital data, Digital signal 0 1 0 0 1 1 0 0 0 1 1 NRZ NRZI Bipolar-AMI Pseudoternary Manchester Differential Manchester

21. Nonreturn to Zero-Level (NRZ-L) • Two different voltages for 0 and 1 bits • Voltage constant during bit interval • Most often, negative voltage for one value and positive for the other

22. 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 a binary 1 • No transition denotes binary 0 • An example of differential encoding (Data represented by changes rather than levels)

23. NRZ

24. NRZ pros and cons • Pros • Easy to engineer • Makes good use of bandwidth • Cons • dc component • Lack of synchronization capability • Used for magnetic recording • Not often used for signal transmission

25. Multilevel Binary • 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 happens (zeros still a problem) • No net dc component  Can use a transformer for isolating transmission line • Lower bandwidth • Easy error detection

26. Pseudoternary • One represented by absence of line signal • Zero represented by alternating positive and negative • No advantage or disadvantage over bipolar-AMI

27. Bipolar-AMI and Pseudoternary

28. Trade Off for Multilevel Binary • Not as efficient as NRZ • With multi-level binary coding, the line signal may take on one of 3 levels, but each signal element, which could represent log23 = 1.58 bits of information, bears only one bit of information • Receiver must distinguish between three levels (+A, -A, 0) • Requires approx. 3dB more signal power for same probability of bit error

29. Biphase • Manchester • 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 (Ethernet) • Differential Manchester • Midbit transition is for clocking only • Transition at start of a bit period represents zero • No transition at start of a bit period represents one • Note: this is a differential encoding scheme • Used by IEEE 802.5 (Token Ring)

30. 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) • No dc component • Error detection • Absence of expected transition points to error in transmission

31. Modulation Rate R=Data Rate=bits/sec=1 Mbps for both cases Modulation Rate=Baud Rate=Rate at which signal elements are generated=R for NRZI=2R for Manchester

32. Scrambling Techniques • Used to reduce signaling rate relative to the data rate by replacing sequences that would produce constant voltage for a priod of time with a filling sequence that accomplishes the following goals: • Must produce enough transitions to maintain syncchronization • Must be recognized by receiver and replaced with original data sequence • is same length as original sequence • No dc component • No long sequences of zero level line signal • No reduction in data rate • Error detection capability • As an example, fax machines use the modified Huffman code to accomplish this.

33. 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 • This is unlikely to occur as a result of noise • Receiver detects and interprets the sequence as octet of all zeros

34. HDB3 • High Density Bipolar 3 Zeros • Based on bipolar-AMI • String of four zeros replaced with one or two pulses Note: The following is the explanation for the HDB3 code example on the next slide (see rules in Table 5.4, page 142): Assuming that an odd number of 1's have occurred since the last substitution, since the polarity of the preceding pulse is "-", then the first 4 zeros are replaced by "000-". For the next 4 zeros, since there have been no Bipolar pulses since the 1st substitution, then they are replaced by"+00+" since the preceding pulse is a "-". For the 3rd case where 4 zeros happen, 2 (even) Bipolar pulses have happened since the last substitution and the polarity of the preceding pulse is "+", so "-00-" is substituted for the zeros.

35. B8ZS and HDB3 (Assume odd number of 1s since last substitution) See Table 5.4 for HDB3 Substitution Rules

36. Digital Data, Analog Signal • Transmitting digital data through PSTN (Public telephone system) • 300Hz to 3400Hz bandwidth • modem (modulator-demodulator) is used to convert digital data to analog signal and vice versa • Three basic modulation techniques are used: • Amplitude shift keying (ASK) • Frequency shift keying (FSK) • Phase shift keying (PSK)

37. Modulation Techniques

38. Amplitude Shift Keying • Values represented by different amplitudes of carrier • Usually, one amplitude is zero • i.e. presence and absence of carrier is used • Susceptible to sudden gain changes • Inefficient • Up to 1200bps on voice grade lines • Used over optical fiber

39. ASK Vd(t) Vc(t) VASK(t) Signal power frequencyspectrum Frequency fc fc+f0 fc+3f0 fc-3f0 fc-f0

40. Frequency Shift Keying • Values represented by different frequencies (near carrier) • Less susceptible to error than ASK • Up to 1200bps on voice grade lines • High frequency radio (3-30 MHz) • Higher frequency on LANs using co-ax

41. FSK Data signal vd(t) v1(t) Carrier 1 v2(t) Carrier 2 vFSK(t) Signal power frequencyspectrum Frequency f1 f2

42. Amplitude PSTN bandwidth Frequency(Hz) 1650 (2025) 3400 1180 (1270) 400 1850 (2225) 980 (1070) FSK in modem (on Voice Grade Line) frequency spectrum

43. Phase Shift Keying • Phase of carrier signal is shifted to represent data • Differential PSK • Phase shifted relative to previous transmission rather than some reference signal

44. PSK • bit rate = signaling rate Data Signal vc(t) Carrier vc(t) Phase coherent vPSK(t) Differential v’PSK(t) 180=0 0=1 Differential example: for every logic 1, 180 degree phase shift phasediagram

45. Quadrature PSK • More efficient use by each signal element representing more than one bit • e.g. shifts 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

46. Multilevel modulation method • bit rate = n x signaling rate 10 11 00 01 0° +90° +180° +270°

47. Multilevel modulation method +90°=01 +180°=10 0°=00 +270°=11 4-PSK phase diagram 16-QAM phase diagram

48. Performance of Digital to Analog Modulation Schemes • Bandwidth • ASK and PSK bandwidth directly related to bit rate • FSK bandwidth related to data rate for lower frequencies • requires more analog bandwidth than ASK • (See Stallings for math) • In the presence of noise, bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK

49. Analog Data, Digital Signal • Digitization • Conversion of analog data into digital data • Digital data can then be transmitted using NRZ-L or using other codes • Digital data can then be converted to analog signal • Analog to digital conversion done using a codec • Pulse code modulation • Delta modulation

50. Analog data, Digital signal • Two principle techniques used • PCM (Pulse Code Modulation) • DM (Delta Modulation) Sampling clock PAMsignal PCMsignal Sampling Circuit Quantizer and compander Analog voicesignal Digitized voicesignal