Computer Networks

Chapter 6 - Multiplexing

Spring 2006 Computer Networks 2

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

Spring 2006 Computer Networks 3

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

Spring 2006 Computer Networks 4

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

Spring 2006 Computer Networks 5

FDM Multiplexing Process

Time domain

Frequency domain

Spring 2006 Computer Networks 6

FDM –Demultiplexing Process

Time domain

Frequency domain

Spring 2006 Computer Networks 7

The Analog Hierarchy

Spring 2006 Computer Networks 8

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

Spring 2006 Computer Networks 9

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 = 29600Solving this equation for n will show that 7 channels can be multiplexed.n= (29600-200)/(4000+200) = 29400/4200=7

Spring 2006 Computer Networks 10

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

Spring 2006 Computer Networks 11

TDM – The Multiplexing Process

The duration of the bit on the high speed channel is 3 times shorter, assuming that three channels are multiplexed.

Spring 2006 Computer Networks 12

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

Spring 2006 Computer Networks 13

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 = 64,000bps20 signals × 64,000bps = 1,280,000bps.

Spring 2006 Computer Networks 14

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

Spring 2006 Computer Networks 15

Digital Hierarchy

Telephone companies group TDM slots in higher rate services that are standardized.

Spring 2006 Computer Networks 16

American vs. European Standards

Line Rate (Mbps)

Voice Channels

E1 2.048 32

E2 8.448 4xE1

E3 34.368 4xE2

E4 139.264 4xE3

Line Rate (Mbps)

Voice Channels

T1 1.544 24

T2 6.312 96

T3 44.375 672

T4 274.176 178 T1

European American

Spring 2006 Computer Networks 17

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.

Spring 2006 Computer Networks 18

Statistical Multiplex -Buffering the Packets

Link rate, RX(t)

Packet buffer

Packets for one output

Data Hdr

Data Hdr

Data Hdr

RR

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