1. Topic 4 –Data Encoding and Modulation1 FIT1005 FIT – Monash University Topic - Data Encoding And Modulation Reference: Chapter 5 -Stallings

2. Topic 4 –Data Encoding and Modulation2 Encoding The electromagnetic signal is generated at the physical layer This analog or digital signal must carry the data – 0,1 The data must be encoded into the signal –Modulation of analog signal –Data Encoding on digital signal

3. Topic 4 –Data Encoding and Modulation3 Modulation The process of encoding source data – 0,1 onto an analog carrier signal, centred at frequency f c Modulation techniques involve an operation on one or more of the following characteristics of the analog signal: –amplitude –frequency –phase

4. Topic 4 –Data Encoding and Modulation4 Digital Data, Digital Signals A digital signal is a sequence of discrete, discontinuous voltage pulses Each pulse is a signal element Binary data are transmitted by encoding each data bit into signal elements Simplest data encoding, there is a one-to-one correspondence between bits and signal elements

5. Topic 4 –Data Encoding and Modulation5 Digital Data, Digital Signals Data rate –the rate R, in bits per second, that data are transmitted, eg 10 bps Bit duration –is the amount of time it takes for the transmitter to emit the bit –for a data rate R, the bit duration is 1/R eg 1/10 = 0.10 secs

6. Topic 4 –Data Encoding and Modulation6 Digital Data, Digital Signals To interpret digital signals at the receiver –The receiver must know the timing of each bit, that is, must know the start and end times of a bit, this is called bit level synchronisation –The receiver must determine whether the signal level for each bit position is high(0) or low(1) This can be done sampling each bit position in the middle of the interval and comparing the value to a threshold

7. Topic 4 –Data Encoding and Modulation7 Digital Data, Digital Signals A Data Encoding scheme is the mapping of data bits to signal elements Data Encoding techniques: –NRZ-L –NRZI –Manchester –Differential Manchester

8. Topic 4 –Data Encoding and Modulation8 Data Encoding Techniques Non return to Zero – Level (NRZ-L) Uses two different voltage levels for the two binary digits Voltage level is constant during a bit interval A (-) voltage represents a 1 and a (+) voltage represents a 0

9. Topic 4 –Data Encoding and Modulation9 Data Encoding Techniques

10. Topic 4 –Data Encoding and Modulation10 Data Encoding Techniques Non return to Zero, invert on ones (NRZI) Data encoded as the presence or absence of a signal transition at the beginning of the bit time A transition at the beginning of a bit time denotes a 1 no transition indicates a 0 WAN links

11. Topic 4 –Data Encoding and Modulation11 Data Encoding Techniques Non return to Zero, invert on ones (NRZI) Is an example of differential encoding, the information to be transmitted is represented in terms of the changes between successive signal elements rather than the signal elements themselves One benefit of differential encoding is, more reliable to detect a transition in the presence of noise than to compare a value to a threshold

12. Topic 4 –Data Encoding and Modulation12 Data Encoding Techniques

13. Topic 4 –Data Encoding and Modulation13 Data Encoding Techniques Manchester There is a transition at the middle of each bit period The mid-bit transition serves as a clocking mechanism and also as data Low-to-high transition represents a 1, and a high-to-low transition represents a 0 Ethernet LANs

14. Topic 4 –Data Encoding and Modulation14 Data Encoding Techniques

15. Topic 4 –Data Encoding and Modulation15 Data Encoding Techniques Differential Manchester the mid-bit transition is used only to provide clocking A 0 is represented by the presence of a transition at the begging of a bit period, and a 1 is represented by the absence of a transition at the beginning of a bit period Token Ring LANs

16. Topic 4 –Data Encoding and Modulation16 Data Encoding Techniques

17. Topic 4 –Data Encoding and Modulation17 Digital Data, Analog Signals Common use of this transformation is for transmitting digital data through the PSTN The PSTN is designed to receive, switch, and transmit analog signals in the voice frequency range of about 300 – 3400 Hz The digital devices are attached to the network via a 56K modem, which converts digital data to analog signals, and vice versa 56K Modems produce signals in the voice-frequency range

18. Topic 4 –Data Encoding and Modulation18 Digital Data, Analog Signals Modulation involves operation on one or more of the characteristics of a carrier signal: amplitude, frequency, and phase Modulation techniques for transforming digital data onto analog signals –Amplitude Shift Keying (ASK) –Frequency Shift Keying (FSK) –Phase Shift Keying (PSK) In all these cases, the resulting signal occupies a bandwidth centred on the carrier frequency f c

19. Topic 4 –Data Encoding and Modulation19 Digital Data, Analog Signals Amplitude Shift Keying (ASK) 0,1 are represented by two different amplitudes of the carrier frequency Is susceptible to impulse noise Is used to transmit digital data over optical fibre

20. Topic 4 –Data Encoding and Modulation20 Digital Data, Analog Signals

21. Topic 4 –Data Encoding and Modulation21 Digital Data, Analog Signals Frequency Shift Keying 0, 1 are represented by different frequencies near the carrier frequency Is less susceptible to impulse noise than ASK It is also commonly used for high-frequency (3 to 30 MHz) radio transmission

22. Topic 4 –Data Encoding and Modulation22 Digital Data, Analog Signals

23. Topic 4 –Data Encoding and Modulation23 Digital Data, Analog Signals Phase Shift Keying The phase of the carrier signal is shifted to represent data The simplest scheme uses two phases (Two-Level PSK) to represent 0, 1 digits

24. Topic 4 –Data Encoding and Modulation24 Digital Data, Analog Signals

25. Topic 4 –Data Encoding and Modulation25 Digitisation The process of transforming analog data into digital (data) signals CODEC –The device used for converting analog data into digital form for transmission, and subsequently recovering the original data from the digital –Codec (coder-decoder) –Pulse code modulation (PCM) is a technique used in a codec

26. Topic 4 –Data Encoding and Modulation26 Pulse Code Modulation (PCM) Is based on the sampling theorem: “If a signal is sampled at regular intervals of time and at a rate higher than twice the highest signal frequency, then the samples contain all the information of the original signal.”

27. Topic 4 –Data Encoding and Modulation27 Pulse Code Modulation (PCM) If voice data are limited to frequencies below 4000 Hz, 8000 samples per second would be sufficient to characterise the voice signal completely –However, these are analog samples, called pulse amplitude modulation (PAM) samples –To convert to digital, each of these samples must be assigned a binary code PCM starts with a continuous-time, continuous-amplitude (analog) signal, from which a digital signal is produced

28. Topic 4 –Data Encoding and Modulation28 Pulse Code Modulation (PCM)

29. Topic 4 –Data Encoding and Modulation29 Pulse Code Modulation (PCM) The digital signal consists of blocks of n bits, where each n-bit number is the amplitude of a PCM pulse On reception, the process is reversed to reproduce the analog signal By quantising the PAM pulse, the original signal is now only approximated and cannot be recovered exactly, this effect is known as quantising error

30. Topic 4 –Data Encoding and Modulation30 The slides after this are for your interest ONLY refer soft copy of topic notes on MUSO

31. Topic 4 –Data Encoding and Modulation31 Introduction In data communications, a distinction is made between analog and digital data and analog and digital signals For digital signalling, which may be either digital or analog, is encoded into a digital signal –The actual form of the converted signal depends on the encoding technique and is chosen to optimise the use of transmission medium For example, the encoding may be chosen to conserve bandwidth or to minimise errors

32. Topic 4 –Data Encoding and Modulation32 Introduction The input signal may be analog or digital and is called the modulation signal or baseband signal The result of modulating the carrier signal is called the modulated signal –The modulated signal is a bandlimited (bandpass) signal –The location of the bandwidth on the spectrum is related to f c and is often centred on f c –The actual form of the encoding is chosen to optimise some characteristic of the transmission

33. Topic 4 –Data Encoding and Modulation33 Introduction Digital data to digital signal encoding may be used as the equipment for encoding digital data into a digital signal is less complex and less expensive than digital-to-analog modulation equipment Analog data to digital signal conversion permits the use of modern digital transmission and switching equpment Digital data to anlog signal may be used as some transmission media, such as optical fibre and unguided media, will only propagate analog signals

34. Topic 4 –Data Encoding and Modulation34 Introduction Analog data and anlog signal combination will be used as analog data in electrical form can be transmitted as baseband signals easily and cheaply –This is done with voice transmission over voice-grade lines –One common use of modulation is to shift the bandwidth of a baseband signal to another portion of the spectrum –In this way multiple signals, each at a different position on the spectrum, can share the same medium This is known as frequency division multiplexing

35. Topic 4 –Data Encoding and Modulation35 Digital Data, Digital Signals If all the signal elements have the same algebraic sign, positive or negative, then the signal is unipolar; otherwise polar Data signalling rate, or just data rate, of a signal is the rate, in bits per second, that data are transmitted The duration or length of a bit is the amount of time it takes for the transmitter to emit the bit –For a data rate R, the bit duration is 1/R

36. Topic 4 –Data Encoding and Modulation36 Digital Data, Digital Signals An encoding scheme is the mapping of data bits to signal elements Data Encoding techniques can be evaluated or compared in the following ways: –Signal spectrum –Clocking –Error detection –Signal interference and noise immunity –Cost and complexity

37. Topic 4 –Data Encoding and Modulation37 Data Encoding Techniques NRZI is an example of differential encoding: –the information to be transmitted is represented in terms of the changes between successive signal elements rather than the signal elements themselves One benefit of differential encoding is that it may be more reliable to detect a transition in the presence of noise than to compare a value to a threshold Another benefit is that it is not possible to lose the sense of polarity of the signal

38. Topic 4 –Data Encoding and Modulation38 Data Encoding Techniques The NRZ codes in general are easiest to engineer and, in addition, make efficient use of bandwidth –Most of the energy in NRZ and NRZI signals is between dc and half the bit rate The main limitation s of NRZ signals are the presence of a dc component and lack of synchronisation capability

39. Topic 4 –Data Encoding and Modulation39 Data Encoding Techniques Multilevel Binary: –These codes use more than two signal levels –addresses some of the deficiencies of the NRZ codes –In the case of bipolar-AMI scheme, a binary 0 is represented by no line signal, and a binary 1 is represented by a positive or negative pulse The binary 1 pulses must alternate in polarity

40. Topic 4 –Data Encoding and Modulation40 Data Encoding Techniques One advantage of this approach is that each one introduces a transition, and the receiver can resynchronise on that transition The second advantage would be that as the 1 signals alternate in voltage from positive to negative, there is no net dc component Another advantage is that the bandwidth of the resulting signal is considerably less than that of NRZ –The pseudoternary technique represents binary 1 with absence of a line signal and, the binary 0 by alternating positive and negative pulses –Although a degree of synchronisation is provided with these codes, a long string of 0s or 1s (depending on polarity) still presents a problem

41. Topic 4 –Data Encoding and Modulation41 Data Encoding Techniques –With suitable modification, multilevel binary scheme overcome the problems of NRZ codes –However, as with any engineering design decision there is a trade off: The receiver of multilevel binary signals has to distinguish between three levels (+A, -A, 0) instead of just two levels in in NRZ coding –Because of this, multilevel binary signal requires more signal power than a two-valued signal for the same probability of bit error

42. Topic 4 –Data Encoding and Modulation42 Data Encoding Techniques

43. Topic 4 –Data Encoding and Modulation43 Data Encoding Techniques –Thus, the maximum modulation is twice that for NRZ This means that the bandwidth required is correspondingly greater –The bandwidth for biphase codes is reasonably narrow and contains no dc component However, it is wider than the bandwidth for the multilevel binary codes –Biphase codes are popular techniques for data transmission

44. Topic 4 –Data Encoding and Modulation44 Digital Data, Digital Signals When signal encoding techniques are used, a distinction needs to be made between data rate and modulation rate –The data rate, or bit rate, is 1/T b where T b is bit duration –The modulation rate is the rate at which signal elements are generated –In general,

45. Topic 4 –Data Encoding and Modulation45 Digital Data, Digital Signals Where D = modulation rate (in baud) R = data rate (in bps) L = number of bits per signal element M = 2 L = number of different signal elements One way of characterising the modulation rate is to determine the average number of transitions that occur per bit time –In general, this will depend on the exact sequence of bits being transmitted

46. Topic 4 –Data Encoding and Modulation46 Digital Data, Digital Signals

47. Topic 4 –Data Encoding and Modulation47 Digital Data, Digital Signals The biphase techniques have achieved widespread use in local area network applications at relatively high data rates –However, they have not been widely used in long-distance applications –The principal reason for this is that they require a high signalling rate relative to the data rate –Another approach is to make use of some sort of scrambling scheme: Sequences that would result in a constant voltage level on the line are replaced by filling sequences that will provide sufficient transitions for the receiver’s clock to maintain synchronisation

48. Topic 4 –Data Encoding and Modulation48 Digital Data, Digital Signals The filling sequence must be recognised by the receiver and replaced with original data sequence The filling sequence is of the same length as the original sequence, so there is no data rate penalty The design goals for this approach are: –No dc component –No long sequences of zero-level line signals –No reduction in data rate –Error detection capability

49. Topic 4 –Data Encoding and Modulation49 Digital Data, Digital Signals Two scrambling techniques are commonly used in long-distance transmission services: –Bipolar with 8-zeros substitution (B8ZS) in North America –High-density bipolar-3 zeros (HDB3) in Japan and Europe –Both of these coding schemes use bipolar-AMI encoding –Neither of them have a dc component and most of the energy is concentrated in a relatively sharp spectrum around a frequency equal to one half of the data rate

50. Topic 4 –Data Encoding and Modulation50 Digital Data, Digital Signals

51. Topic 4 –Data Encoding and Modulation51 Digital Data, Analog Signals –The resulting transmitted signal for one bit time is: Where f 1 and f 2 are typically offset from the carrier frequency f c by equal but opposite amounts –BFSK is less susceptible to error than ASK On voice-grade lines, it is typically used up to 1200 bps It is also commonly used for high-frequency (3 to 30 MHz) radio transmission Binary 1 Binary 0

52. Topic 4 –Data Encoding and Modulation52 Digital Data, Analog Signals –Multiple FSK (MFSK) is a signal that is more bandwidth efficient, but also susceptible to errors More than two frequencies are used in this case s i (t) = A cos2Πf i t, 1 <= I <= M Where f i = f c +(2i-1-M)f d f d = the difference frequency f c = the carrier frequency L = number of bits per signal element M = 2 L = number of different signal elements

53. Topic 4 –Data Encoding and Modulation53 Analog Data, Digital Signals –The digital data can be encoded as a digital signal using a code other than NRZ-L Thus an extra step is required –The digital data can be converted into an analog signal, using one of the modulation techniques discussed in the previous section The analog data, as they have been digitised, can be treated as digital data, even though transmission requirements (e.g., use of microwave) dictate that an analog signal be used

54. Topic 4 –Data Encoding and Modulation54 Pulse Code Modulation (PCM) –The digital signal consists of blocks of n bits, where each n-bit number is the amplitude of a PCM pulse –On reception, the process is reversed to reproduce the analog signal –However, by quantising the PAM pulse, the original signal is now only approximated and cannot be recovered exactly This effect is known as quantising error or quantising noise The signal-to-noise ratio for quantising noise can be expressed as SNR dB = 20log2 n + 1.76dB = 6.02n +1.76dB

55. Topic 4 –Data Encoding and Modulation55 Pulse Code Modulation (PCM) –Typically, the PCM scheme is refined using a technique known a s nonlinear encoding That is, in effect, the quantisation levels are not equally spaced –The problem with equal spacing is that the mean absolute error for each sample is the same, regardless of signal level As a result, lower amplitude values are relatively more distorted –By using a greater number of quantising steps for signals of large amplitude, a marked reduction in overall signal distortion is achieved

56. Topic 4 –Data Encoding and Modulation56 Pulse Code Modulation (PCM) –The same effect (with non-equally spaced quantisation) can be achieved using uniform quantising but companding (compressing-expanding) the input analog signal –Companding is a process that compresses the intensity range of a signal by imparting more gain to weak signals than to strong signals on input At the output, the reverse operation is performed Nonlinear encoding can significantly improve the PCM SNR ratio –For voice signals, improvements of 24 – 30 dB have been achieved

57. Topic 4 –Data Encoding and Modulation57 Analog Data, Digital Signals

58. Topic 4 –Data Encoding and Modulation58 Delta Modulation (DM) –One of the most popular alternatives to PCM is delta modulation (DM) –With DM, an analog function is approximated by a staircase function that moves up or down by one quantisation level (δ) a each sampling interval (T s ) –The important characteristic of the staircase function is that its behaviour is binary: At each sampling time, the function moves up or down a constant amount δ Thus, the output of the delta modulation process can be represented as a single binary digit for each sample

59. Topic 4 –Data Encoding and Modulation59 Delta Modulation (DM) –In essence a bit stream is produced by approximating the derivative of an analog signal rather than its amplitude A 1 is generated if the staircase function is to go up during the next interval while a 0 is generated otherwise –There are two important parameters in a DM scheme: The size of the step assigned to each binary digit, δ and The sampling rate δ must be chosen to produce a balance between two types of errors or noise

60. Topic 4 –Data Encoding and Modulation60 Delta Modulation (DM)

61. Topic 4 –Data Encoding and Modulation61 Delta Modulation (DM) –When the analog waveform is changed very slowly, there will be quantising noise »This noise is increased as δ is increased –When the analog waveform is changing more rapidly than the staircase can follow, there is slope overload noise »This noise is decreased as δ is decreased –It is clear that accuracy of the scheme can be increased by increased by increasing the sampling rate »However, this increases the data rate of the output signal

62. Topic 4 –Data Encoding and Modulation62 Analog Data, Analog Signals –Modulation permits frequency division multiplexing (FDM) The principal techniques for modulation using analog data are: –Amplitude modulation (AM) –Frequency modulation (FM) –Phase modulation (PM) AM is the simplest form of modulation and is mathematically expressed as: s(t) = [1 +n a x(t)] cos2Πf c t

63. Topic 4 –Data Encoding and Modulation63 Analog Data, Analog Signals FM and PM are special cases of angle modulation expressed as: s(t) = A c cos[2Πf c t + φ(t)] –For PM, the phase is proportional to the modulating signal: φ(t) = n p m(t) where n p is the phase modulating index –For FM the derivative of the phase is proportional to the modulating signal φ’(t) = n f m(t) where n f is the frequency modulating index

64. Topic 4 –Data Encoding and Modulation64 Analog Data, Analog Signals Modulation has been defined as the process of combining an input signal m(t) and a carrier at frequency f c to produce a signal whose bandwidth is usually centred on f c There are two principal reasons for analog modulation of analog signals –A higher frequency may be required for effective transmission For unguided transmission, it is virtually impossible to transmit baseband signals as the required antennas will be many kilometres in diameter