1. 6.1 Chapter 6 Bandwidth Utilization: Multiplexing and Spreading Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Bandwidth Bandwidth is an important resource in data communication. Being an important resource it should be used wisely. Wise utilization of the bandwidth is application dependent. Bandwidth can be utilized in two modes: Multiplexing Spreading 6.2

3. 6.3 Bandwidth utilization multiplexingspreading GOAL Efficiency GOAL Privacy and Anti-Jamming

4. 6.4 Bandwidth utilization is the wise use of available bandwidth to achieve specific goals. Efficiency can be achieved by multiplexing; privacy and anti-jamming can be achieved by spreading. Note

5. 6.5 6-1 MULTIPLEXING Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link can be shared. Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. As data and telecommunications use increases, so does traffic. Frequency-Division Multiplexing Wavelength-Division Multiplexing Synchronous Time-Division Multiplexing Statistical Time-Division Multiplexing Topics discussed in this section:

6. Link Vs. Channel 6.6 The word channel refers to the portion of a link that carries a transmission between a given pair of lines NOTE

7. 6.7 Figure 6.1 Dividing a link into channels

8. 6.8 Figure 6.2 Categories of multiplexing

9. 6.9 Figure 6.3 Frequency-division multiplexing

10. Frequency division multiplexing Frequency division multiplexing is an analog technique that can be applied when the bandwidth of a link is greater than the combined bandwidths of the signals to be transmitted. 6.10

11. Guard Bands Channels in FDM are separated by strips of unused bandwidth called “ guard bands” to prevent signals from overlapping. 6.11

12. Carrier frequency We know that bandwidth of the link comprises a range of frequencies. Bandwidth=highest f – lowest f These frequencies are actually carrier frequencies. Carrier frequencies are fixed frequencies that are altered (modified) to “carry” the original data frequencies. In other words we ride our data on the carrier frequency. 6.12

13. Why we need Carriers? This concept can be understood with an analogy. Suppose you are sitting along with your friends and gossiping. The people sitting around you can hear you. Your voice cannot be heard at the same time on a farther distance. To make the voice heard by others at a further distance we use carrier frequency. 6.13

14. Why we need Carriers? (2) Carrier frequency is just like a fast horse which takes your voice quickly as possible to a far distant location. 6.14

15. Where Carrier Frequencies are used? Telephone/ mobile communication Radio transmission TV broadcast Cable TV transmission Wireless transmissions LANs and WANs 6.15

16. 6.16 FDM is an analog multiplexing technique that combines analog signals. Note

17. 6.17 Figure 6.4 FDM process

18. FDM Process (multiplexing) Figure 6.4 is a conceptual illustration of the multiplexing process. Each source generates a signal of a similar frequency range. Inside the multiplexer, these similar signals modulates different carrier frequencies (f1, f2, f3). The resulting modulated signals are then combined into a composite signal, which is then sent over the transmission media. 6.18

19. FDM Process (Demultiplexing) Figure 6.5 shows the conceptual illustration of the demultiplexing process. There are a series of filters to decompose the multiplexed signal into its constituent component signals. The individual signals are then passed to a demodulator that separates them from their carriers and passes them to the output lines. 6.19

20. 6.20 Figure 6.5 FDM demultiplexing example

21. Is digital transmission possible through FDM? Yes we can surely have by converting a digital signal into an analog signal before FDM is used to multiplex them. 6.21

22. Applications of FDM A very common application of FDM is AM and FM radio broadcasting. Another common application is in TV broadcasting. First generation of cell phones also used FDM. Radio uses the air as the transmission medium. 6.22

23. AM (Amplitude Modulation) A special band from 530 to 1700kHz is assigned to AM radio. All station share this band. Each station is allocated 10kHz of bandwidth. Each station uses a different carrier frequency. The signal that goes to the air is a combination of signals. 6.23

24. AM (2) A receiver receives all these signals, but filters (by tuning) only the one which is desired. If there was no multiplexing only one AM station could broadcast on air. 6.24

25. FM (frequency modulation) Same as AM FM has a wider band i.e. 88 to 108MHz. Each station requires 200kHz 6.25

26. TV broadcast and Cell Communication Each TV channel has its own bandwidth of 6 MHz. In first generation cell phone each user is assigned 30kHz channel, one for sending and one for receiving. 6.26

27. 6.27 Assume that a voice channel occupies a bandwidth of 4 kHz. We need to combine three voice channels into a link with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the configuration, using the frequency domain. Assume there are no guard bands. Solution We shift (modulate) each of the three voice channels to a different bandwidth, as shown in Figure 6.6. We use the 20- to 24-kHz bandwidth for the first channel, the 24- to 28-kHz bandwidth for the second channel, and the 28- to 32-kHz bandwidth for the third one. Then we combine them as shown in Figure 6.6. Example 6.1

28. 6.28 Figure 6.6 Example 6.1

29. 6.29 Five channels, each with a 100-kHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 kHz between the channels to prevent interference? Solution For five channels, we need at least four guard bands. This means that the required bandwidth is at least 5 × 100 + 4 × 10 = 540 kHz, as shown in Figure 6.7. Example 6.2

30. 6.30 Figure 6.7 Example 6.2

31. Wave length division multiplexing Same as FDM except that multiplexing and demultiplexing involves optical signals transmitted through fiber-optic channels. The idea is the same: we are combining different signals of different frequencies, the difference is that the frequencies are very high. 6.31

32. Why we call it WDM? The reason for calling it WDM is that “we are dealing with light signals”. And colors of different shades are distinguished by the factor of wavelength. That’s why we call it WDM. 6.32

33. WDM (continued) Figure 6.10 gives a conceptual view of a WDM multiplexer and demultiplexer. Very narrow bands of light from different sources are combined to make a wider band of light. At the receiver, the signals are separated by the demultiplexer. 6.33

34. 6.34 Figure 6.10 Wavelength-division multiplexing

35. 6.35 WDM is an analog multiplexing technique to combine optical signals. Note

36. WDM Process WDM technology is very complex as we have to combine light of different shades from different sources into one. But the idea is very simple….. The combining and splitting of light sources are easily handled by a prism. This is illustrated in figure 6.11 6.36

37. 6.37 Figure 6.11 Prisms in wavelength-division multiplexing and demultiplexing

38. Application of WDM WDM is designed to be used with the fiber optic. The purpose is to support high & high data rates. One application of WDM is the SONET network in which multiple optical fiber lines are multiplexed and demultiplexed. A new method dense WDM (DWDM), can multiplex a very large number of channels by spacing channels very close to one another 6.38

39. Time Division Multiplexing Synchronous TDM Statistical TDM 6.39

40. 6.40 Figure 6.12 TDM

41. 6.41 TDM is a digital multiplexing technique for combining several low-rate channels into one high-rate one. Note

42. 6.42 Figure 6.18 Empty slots

43. 6.43 Figure 6.26 TDM slot comparison

44. 6.44 6-1 SPREAD SPECTRUM In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but our goals are to prevent eavesdropping and jamming. To achieve these goals, spread spectrum techniques add redundancy. Frequency Hopping Spread Spectrum (FHSS) Direct Sequence Spread Spectrum Synchronous (DSSS) Topics discussed in this section:

45. Spread Spectrum It is designed to be used in wireless applications. In wireless communication all stations use air (or vacuum) as a medium. threats encountered are: Interception by an eavesdropper. Signal jamming from a malicious intruder. 6.45

46. Basic technique The basic technique followed in spread spectrum is the spreading of the signal. If the required bandwidth for each station is B, spread spectrum expands it to Bss, such that Bss >> B. 6.46

47. 6.47 Figure 6.27 Spread spectrum

48. Frequency Hopping Spread Spectrum (FHSS) Each source Uses ‘M’ different carrier frequencies to send data. At one moment, the signal modulates one carrier frequency. At the next moment, the signal modulates another carrier frequency. Although the modulation is done using one carrier frequency at a time, M frequencies are used in the long run. The bandwidth is B FHSS >>B 6.48

49. 6.49 Figure 6.31 Bandwidth sharing

50. FHSS Process A pseudorandom code generator, also called pseudorandom noise(PN), creates a k-bit pattern for every hopping period T h. Frequencies are assigned to the k-bit pattern in the frequency table. The frequency synthesizer creates a carrier signal for that frequency, and the source signal is modulated on the carrier frequency. 6.50

51. 6.51 Figure 6.28 Frequency hopping spread spectrum (FHSS)

52. FHSS Process Example Suppose we have decided to have 8 hops, i.e. eight hopping frequencies. Extremely low for real applications. In this case M=8 and k=3 The pseudorandom code generator will create eight different 3-bit patterns. These are mapped to eight different frequencies in the frequency table. 6.52

53. 6.53 Figure 6.29 Frequency selection in FHSS

54. 6.54 Figure 6.30 FHSS cycles

55. FHSS Conclusion If there are many k-bit patterns and the hopping period is short, a sender and receiver can have privacy. If an intruder tries to intercept the transmitted signal, she can only access a small piece of data because she does not know the spreading sequence to quickly adapt herself to the next hop. This scheme has also and anti-jamming effect. 6.55

56. Direct Sequence Spread Spectrum The DSSS also expands the bandwidth of the original signal but the process is different. Each data bit is replaced by ‘n’ bit code. The code of ‘n’ bits is called chips. 6.56

57. 6.57 Figure 6.32 DSSS

58. 6.58 Figure 6.33 DSSS example