Lecture 14: Multiplexing (1)zLast LectureyData Communication InterfaceySource: chapter 6

zThis LectureyMultiplexing (1)ySource: chapter8

zNext LectureyMultiplexing (2)ySource: chapter8

Multiplexing

A common application is in long-haul communications§ E.g telephone systems, radio, …

§ High-capacity trunks, such as fiber, coaxial, or microwave links are shared

§ The link is able to carry n separate channels of data, and thereby minimizes infrastructural costs.

MultiplexingzMultiplexer combines data from the n input lines

and transmits over the high-capacity data linkzDemultiplexer accepts multiplexed data stream,

separates data according to channel, and delivers them to the appropriate output lines.zTypes of multiplexingy Frequency Division Multiplexing (FDM)y Time Division Multiplexing (TDM)

Frequency Division MultiplexingzFDM is an analog techniquezUseful bandwidth of medium exceeds required

bandwidth of individual channelszEach signal is modulated to a different carrier

frequencyzCarrier frequencies are separated so signals do

not overlap (the gaps are “guard bands”)ze.g. broadcast radio, TV, telephonezChannel is allocated even if no data

FDM – view 1

FDM – view 2

FDM – view 3

FDM - Demultiplexing

FDM System

Ex: FDM for 3 voiceband signals

Analog Carrier SystemszAT&T (USA)zHierarchy of FDM schemeszGroupy 12 voice channels (4kHz each) = 48kHzyRange 60kHz to 108kHz

zSupergroupy 60 channely FDM of 5 group signals on carriers between 420kHz

and 612 kHz

zMastergroupy 10 supergroups

Time Division Multiplexing (TDM)zData rate of medium exceeds data rate of

individual signal to be transmitted.zMultiple digital signals interleaved in timezTime slots preassigned to sources and fixedzSlots may be at the bit level of blockszTime slots are allocated even if no datazTime slots do not have to be evenly distributed

amongst sources.

TDM – view 1

TDM – view 2

TDM System

TDM Link ControlzNo headers and tailerszData link control protocols not neededzFlow controly Data rate of multiplexed line is fixedy If one channel receiver can not receive data, the

others must carry ony The corresponding source must be quenchedy This leaves empty slots

zError controly Errors are detected and handled by individual

channel systems

Data Link Control on TDM

FramingzNo flag or SYNC characters bracketing TDM frameszMust provide synchronizing mechanismzAdded digit framing (looks like control channel)y One control bit added to each TDM framey e.g. alternating 01010101…unlikely on a data channely Can compare incoming bit patterns on each channel

with sync pattern

Pulse StuffingzProblem - Synchronizing data sourceszClocks in different sources driftingzData rates from different sources not related by

simple rational numberzSolution - Pulse StuffingyOutgoing data rate (excluding framing bits) higher

than sum of incoming ratesy Stuff extra dummy bits or pulses into each incoming

signal until it matches local clocky Stuffed pulses inserted at fixed locations in frame

and removed at demultiplexer

TDM of Analog and Digital Sources