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

 commercial systems with 160 channels of 10 Gbps lab demo of 256 channels 39.8 Gbps
 architecture similar to other FDM systems
 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 1-
MHz 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

2
2
7
2
9
3
Input
6
Output Capacity = 5000 bps
Output
5
5
5
5
5
4
2
0
0
5
2
Backlog
1
5
3
Output Capacity = 7000 bps
Output
6
7
2
7
5
2
2
0
0
0
0
Backlog
0
2
0

Summary
 looked at multiplexing multiple channels on a single link
 FDM
 TDM
 Statistical TDM