Multiplexing

Report
Data and Computer
Communications
Chapter 8 – Multiplexing
Multiplexing
 multiple
links on 1 physical line
 common on long-haul, high capacity, links
 have FDM, TDM, STDM alternatives
Frequency Division
Multiplexing
FDM
System
Overview
FDM Voiceband Example
Problem
#
Given the following information, find the
maximum bandwidth for each signal
source.
 A) FDM multiplexing
 B) Total available BW is 7900 Hz
 C) Three signal sources
 D) A 200-Hz guard band between each
pair of source.
Wavelength Division
Multiplexing

FDM with multiple beams of light at different freq
 carried over optical fiber links



architecture similar to other FDM systems




commercial systems with 160 channels of 10 Gbps
lab demo of 256 channels 39.8 Gbps
multiplexer consolidates laser sources for
transmission over single fiber
Optical amplifiers amplify all wavelengths
Demux separates channels at the destination
also have Dense Wavelength Division
Multiplexing (DWDM)
Synchronous Time Division
Multiplexing
TDM
System
Overview
#
Four channels are TD multiplexed. Each
channel sends 100 bytes/s and 1byte per
channel is multiplexed. Find the size and
duration of the frame, as well as the bit
rate of the link.
 # 2 end offices, 1 node in between with 1MHz trunks, average phone makes four
long distance calls / 8-hour workday, each
call of 6-minutes duration, long distance
calls are 10% of the total.
 Find the maximum number of phones at
each end office. (50000)
TDM Link Control

no headers and trailers
 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
corresponding source must be quenched
leaving empty slots
error control

errors detected & handled on individual channel
Data Link Control on TDM
Framing

no flag or SYNC chars bracketing TDM frames
 must still provide synchronizing mechanism
between source and destination clocks
 Added-digit framing




one control bit added to each TDM frame
identifiable bit pattern used as control channel
e.g. alternating 01010101…unlikely on a data channel
compare incoming bit patterns on each channel with
known sync pattern
Pulse Stuffing

Pulse Stuffing



stuff extra dummy bits or pulses into each incoming
signal until it matches local clock
have outgoing data rate (excluding framing bits)
higher than sum of incoming rates
stuffed pulses inserted at fixed locations in frame and
removed at de-multiplexer
TDM Example
#
Four analog signals are to be TDM over
a channel. Each baseband analog signal
is band-limited to 500 Hz. Find the total bit
rate of the channel considering 4-bit PCM
samples.
 # A TDM system accommodates four 300bps digital inputs and one analog input of
500-Hz BW (encoded into 4-bit PCM). If
20 frames are sent per second, find the
size of the frame and the analog and
digital bits per frame.
Statistical TDM
 in
Synch TDM many slots are wasted
 Statistical TDM allocates time slots
dynamically based on demand
 multiplexer scans input lines and collects
data until frame is full
 output data rate is lower than aggregate
input line rates
 may have problems during peak periods

must buffer inputs
Statistical TDM Frame Format
Frame Optimization
-
Relative addressing
-
Omit address, use ‘bit-map’ register
-
Length field: encode length
Output Capacity = 5000
bps
Output Capacity = 7000
bps
Input
Output
Backlog
Output
Backlog
6
5
1
6
0
9
5
5
7
2
3
5
3
5
0
7
5
5
7
0
2
5
2
2
0
2
4
0
2
0
2
2
0
2
0
Summary
 looked
at multiplexing multiple channels
on a single link
 FDM
 TDM
 Statistical TDM

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