Tài liệu Chapter 8 :Multiplexing pdf

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FDM System

FDM of Three Voiceband Signals

Analog Carrier Systems
❚
AT&T (USA)
❚
Hierarchy of FDM schemes
❚
Group
❙
12 voice channels (4kHz each) = 48kHz
❙
Range 60kHz to 108kHz
❚
Supergroup
❙
60 channel
❙
FDM of 5 group signals on carriers between 420kHz and
612 kHz
❚
Mastergroup
❙
10 supergroups

Synchronous Time Division
Multiplexing
❚
Data rate of medium exceeds data rate of digital
signal to be transmitted
❚
Multiple digital signals interleaved in time
❚
May be at bit level of blocks
❚
Time slots preassigned to sources and fixed
❚
Time slots allocated even if no data
❚
Time slots do not have to be evenly distributed
amongst sources

Time Division Multiplexing

TDM System

TDM Link Control
❚
No headers and tailers
❚
Data link control protocols not needed
❚
Flow control
❙
Data rate of multiplexed line is fixed
❙
If one channel receiver can not receive data, the others
must carry on
❙
The corresponding source must be quenched
❙
This leaves empty slots
❚
Error control
❙
Errors are detected and handled by individual channel
systems

Data Link Control on TDM

Framing
❚
No flag or SYNC characters bracketing TDM
frames
❚
Must provide synchronizing mechanism
❚
Added digit framing
❙
One control bit added to each TDM frame
❘
Looks like another channel - “control channel”
❙
Identifiable bit pattern used on control channel
❙
e.g. alternating 01010101…unlikely on a data channel
❙
Can compare incoming bit patterns on each channel
with sync pattern

Pulse Stuffing
❚
Problem - Synchronizing data sources
❚
Clocks in different sources drifting
❚
Data rates from different sources not related by
simple rational number
❚
Solution - Pulse Stuffing
❙
Outgoing data rate (excluding framing bits) higher than
sum of incoming rates
❙
Stuff extra dummy bits or pulses into each incoming
signal until it matches local clock
❙
Stuffed pulses inserted at fixed locations in frame and
removed at demultiplexer