William Stallings Data and Computer Communications 7 th Edition

1. William StallingsData and Computer Communications7th Edition Chapter 8 Multiplexing

2. Multiplexing

3. Multiplexing Categories

4. Frequency Division Multiplexing • Frequency Division Multiplexing • Analog signaling is used to transmit the signals. • Broadcast radio and television, cable television, and the AMPS cellular phone systems use frequency division multiplexing. • This technique is the oldest multiplexing technique. • Since it involves analog signaling, it is more susceptible to noise.

5. Frequency Division MultiplexingDiagram

6. Frequency Division Multiplexing • Frequency Division Multiplexing (FDM) • Primarily used for Analog data. • Bandwidth of medium equals or exceeds the sum of all channel bandwidths • Each signal is modulated to a different carrier frequency • Carrier frequencies separated so signals do not overlap (guard bands) • Can carry multiple telephone conversations over a single transmission channel. • Frequencies in each call are changed so they can be placed side-by-side in a wide-band channel and be transmitted as a group. • At the other end, the frequencies in each call are changed back to the original frequencies.

7. Frequency Division Multiplexing • Frequency Division Multiplexing (FDM) • More efficient in terms of bandwidth than digital systems. • Downside is that noise is amplified along with the voice. • Has been replaced with Time Division Multiplexing (TDM). • e.g. broadcast radio • Channel allocated even if no data

8. Frequency Division Multiplexing • FDM is an analog multiplexing technique that combines signals.

9. Frequency Division Multiplexing • Frequency Division Multiplexing (FDM) • Assignment of non-overlapping frequency ranges to each “user” or signal on a medium. • A multiplexor accepts multiple analog inputs and assigns frequencies to each device. Modulation is used to move input signals into the assigned frequency ranges. • The multiplexor is attached to a high-speed communications line. A corresponding demultiplexor on the other end of the line separates the multiplexed signals. • FDM is only used with analog signals.

10. FDM System Figure 8.4

11. FDM of Three Voiceband Signals

12. FDM Process

13. FDM Demultiplexing Process

14. FDM Question #1 • Question • 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 without the use of guard bands. • Solution • Shift (modulate) each of the three voice channels to a different bandwidth, as shown on the next slide.

15. Frequency Division Multiplexing of Three Voice Calls

16. Frequency Division Multiplexing

17. Frequency Division Multiplexing • Why stop there? • Group • 12 Voice channels • Frequency range: 60 – 108 kHz • Super Group • 5 Groups • 60 Voice channels • Frequency range: 312 – 552 kHz • Master Group • 10 Super Groups • 600 Voice channels • Frequency range: 564 – 3084 kHz

18. Frequency Division Multiplexing • Why stop there? (cont.) • Jumbo Group • 6 Master Groups • 3600 Voice channels • Frequency range: 564 – 17,548 • Jumbo Group Multiplex • 3 Jumbo Groups • 10,800 Voice channels • Frequency range: 3124 – 60,556 kHz

19. Frequency Division Multiplexing

20. FDM Question #2 • Question • 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 x 100) + (4 x 10) = 540 KHz

21. FDM Question #2

22. Time Division Multiplexing • Time Division Multiplexing • Digital Technology. • Can carry multiple telephone conversations over a single transmission channel. • Analog speech signals are sampled and converted to pulses. • The samples are then coded by Pulse Code Modulation (PCM) • All the samples are transmitted in series over the same channel, one at a time. • At the other end, the signals are demodulated and each sample from each channel is routed to the proper channel.

23. Time Division Multiplexing • Time Division Multiplexing • Sharing of the signal is accomplished by dividing available transmission time on a medium among users. • Digital signaling is used exclusively. • Time division multiplexing comes in two basic forms: • Synchronous time division multiplexing, and • Statistical, or asynchronous time division multiplexing.

24. Time Division Multiplexing

25. Time Division Multiplexing

26. TDM System n

27. Wavelength Division Multiplexing • Multiple beams of light at different frequency • Carried by optical fiber • A form of FDM • Each color of light (wavelength) carries separate data channel • 1997 Bell Labs • 100 beams • Each at 10 Gbps • Giving 1 terabit per second (1 Tbps) • Commercial systems of 160 channels of 10 Gbps now available • Lab systems (Alcatel) 256 channels at 39.8 Gbps each • 10.1 Tbps • Over 100km

28. WDM Operation • Same general architecture as other FDM • Number of sources generating laser beams at different frequencies • Multiplexer consolidates sources for transmission over single fiber • Optical amplifiers amplify all wavelengths • Typically tens of km apart • Demux separates channels at the destination • Mostly 1550nm wavelength range • Was 200MHz per channel • Now 50GHz

29. Dense Wavelength Division Multiplexing • DWDM • No official or standard definition • Implies more channels more closely spaced that WDM • 200GHz or less