1. Computer Networks Chapter 6 - Multiplexing

2. Spring 2006Computer Networks2 Multiplexing  The term “multiplexing” is used whenever it is necessary to share a single device among several devices  In this chapter multiplexing denotes a set of techniques that provide sharing of a single high bandwidth channel among a number of individual users.  The reasons for using multiplexing  Media with high bandwidth are expensive  Sharing is necessary to justify their use

3. Spring 2006Computer Networks3 Categories of Channel Multiplexing  FDM (Frequency Division Multiplex)  The individual user signals are shifted to different frequency channels  WDM (Wavelength Division Multiplex)  Conceptually the same as FDM, except that the multiplexing involves optical signals transmitted over fiber-optic channels  TDM (Time Division Multiplex)  Each user signal is assigned a particular time slot for transmission

4. Spring 2006Computer Networks4 Other Types of Multiplexing  Statistical multiplexing  The channel is shared in time but without particular time slots assigned. Sharing is on the “as needed” basis.  DMT (Discrete Multitone)  Found in digital subscriber lines. It combines hundreds of different signals that are QAM modulated into a single stream  CDM (Code Division Multiplex)  An advanced technique that allows multiple devices to transmit on the same frequency at the same time

5. Spring 2006Computer Networks5 FDM Multiplexing Process Time domain Frequency domain

6. Spring 2006Computer Networks6 FDM –Demultiplexing Process Time domain Frequency domain

7. Spring 2006Computer Networks7 The Analog Hierarchy

8. Spring 2006Computer Networks8 Using FDM  The oldest multiplexing technique. Susceptible to noise  Used in the old days of telephone network to make efficient use of the long distance trunks  Used in AM and FM radio broadcast to provide the use of many stations on a single channel without each interfering with the other  The first generation (1G) of mobile phones used FDM with analog frequency modulation which required 2x30Khz per user

9. Spring 2006Computer Networks9 Example: FDM multiplexing is used for voice channels (each requiring 4000 Hz) with 200-Hz guard band between each signal source. If the total bandwidth is 29.6 KHz, how many channels can be multiplexed? Answer: If n is the number of channels that can be multiplexed then the total bandwidth required i expressed with the following equation. nx4000 + (n-1)x200 = 29600 Solving this equation for n will show that 7 channels can be multiplexed. n= (29600-200)/(4000+200) = 29400/4200=7

10. Spring 2006Computer Networks10 TDM – Time Division Multiplex  Digital process that is nowadays used for long- distance telephone lines.  A portion of time is allocated to each voice channel  The voice signal is first transformed into bits using PCM  The data flow is divided into units called frames. The frame contains as many bits as the number of channels multiplexed

11. Spring 2006Computer Networks11 TDM – The Multiplexing Process The duration of the bit on the high speed channel is 3 times shorter, assuming that three channels are multiplexed.

12. Spring 2006Computer Networks12 TDM -Interleaving Even when a channel has nothing to send, its time slot cannot be used by another channel Synchronization between sender and receiver is important

13. Spring 2006Computer Networks13 Example 20 voice signals are to be multiplexed and transmitted over twisted pair. What is the bandwidth required, in bps, if synchronous time division multiplexing is used, if we use the standard analogue-to-digital sampling rate, if each voice signal has a bandwidth of 4000Hz, and if each sample is converted into an 8-bit value? Answer: Each voice signal is sampled at 2 times the frequency = 8000 samples per second. Each sample is an 8 bit value so: 8000 samples per sec × 8 bits = 64,000bps 20 signals × 64,000bps = 1,280,000bps.

14. Spring 2006Computer Networks14 TDM - Flexibility  It is possible to assign several slots to a fast transmitting device  The channel needs to have the speed that is an integer times higher than the speeds of the regular channel  If this cannot be accomplished, extra dummy bits are added. This process is called bit padding

15. Spring 2006Computer Networks15 Digital Hierarchy  Telephone companies group TDM slots in higher rate services that are standardized.

16. Spring 2006Computer Networks16 American vs. European Standards LineRate (Mbps) Voice Channels E12.04832 E28.4484xE1 E334.3684xE2 E4139.2644xE3 LineRate (Mbps) Voice Channels T11.54424 T26.31296 T344.375672 T4274.176178 T1 European American

17. Spring 2006Computer Networks17 Statistical Multiplexing  Statistical multiplexing is mainly used with computer networks  Only the data from active sources are transmitted using the full capacity of the channel  Bits are grouped into frames, but they can be with different size.  If there is more traffic than the high speed link can transmit, the frames are queued and are served using the first-come firs-served principle.

18. Spring 2006Computer Networks18 Statistical Multiplex -Buffering the Packets Link rate, R X(t) Packet buffer Packets for one output DataHdr DataHdr DataHdr R R R  Bursty flows share the same link efficiently  When too many packets arrive simultaneously, the buffer could not have enough space to keep all of them  Dropping packets is possible when packet buffer is full