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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