08-Multiplexing.ppt

Data and Computer
Communications
Eighth Edition
by William Stallings
Lecture slides by Lawrie Brown
Chapter 8 – Multiplexing

Multiplexing
It was impossible to get a conversation
going, everybody was talking too much.
 Yogi Berra

Multiplexing
 multiple links on 1 physical line
 common on long-haul, high capacity, links
 have FDM, TDM, STDM alternatives

Frequency Division
Multiplexing

Analog Carrier Systems
 long-distance links use an FDM hierarchy
 AT&T (USA) and ITU-T (International) variants
 Group
 12 voice channels (4kHz each) = 48kHz
 in range 60kHz to 108kHz
 Supergroup
 FDM of 5 group signals supports 60 channels
 on carriers between 420kHz and 612 kHz
 Mastergroup
 FDM of 10 supergroups supports 600 channels
 so original signal can be modulated many times

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 (1550nm) 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 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

Framing
 no flag or SYNC chars bracketing TDM frames
 must still provide synchronizing mechanism
between src and dest clocks
 added digit framing
 one control bit added to each TDM frame
 identifiable bit pattern used on control channel
 eg. alternating 01010101…unlikely on a data channel
 compare incoming bit patterns on each channel with
known sync pattern

Pulse Stuffing
 have problem of synchronizing data sources
 with clocks in different sources drifting
 also issue of data rates from different sources
not related by simple rational number
 Pulse Stuffing a common solution
 have 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

Digital Carrier Systems
 long-distance links use an TDM hierarchy
 AT&T (USA) and ITU-T (International) variants
 US system based on DS-1 format
 can carry mixed voice and data signals
 24 channels used for total data rate 1.544Mbps
 each voice channel contains one word of
digitized data (PCM, 8000 samples per sec)
 same format for 56kbps digital data
 can interleave DS-1 channels for higher rates
 DS-2 is four DS-1 at 6.312Mbps

SONET/SDH
 Synchronous Optical Network (ANSI)
 Synchronous Digital Hierarchy (ITU-T)
 have hierarchy of signal rates
 Synchronous Transport Signal level 1 (STS-1)
or Optical Carrier level 1 (OC-1) is 51.84Mbps
 carries one DS-3 or multiple (DS1 DS1C DS2)
plus ITU-T rates (eg. 2.048Mbps)
 multiple STS-1 combine into STS-N signal
 ITU-T lowest rate is 155.52Mbps (STM-1)

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 full
 line data rate lower than aggregate input
line rates
 may have problems during peak periods
 must buffer inputs

Cable Modems
 dedicate two cable TV channels to data transfer
 each channel shared by number of subscribers,
using statistical TDM
 Downstream
 cable scheduler delivers data in small packets
 active subscribers share downstream capacity
 also allocates upstream time slots to subscribers
 Upstream
 user requests timeslots on shared upstream channel
 Headend scheduler notifies subscriber of slots to use

Asymmetrical Digital
Subscriber Line (ADSL)
 link between subscriber and network
 uses currently installed twisted pair cable
 is Asymmetric - bigger downstream than up
 uses Frequency division multiplexing
 reserve lowest 25kHz for voice (POTS)
 uses echo cancellation or FDM to give two bands
 has a range of up to 5.5km

Discrete Multitone (DMT)
 multiple carrier signals at different frequencies
 divide into 4kHz subchannels
 test and use subchannels with better SNR
 256 downstream subchannels at 4kHz (60kbps)
 in theory 15.36Mbps, in practice 1.5-9Mbps

xDSL
 High data rate DSL (HDSL)
 2B1Q coding on dual twisted pairs
 up to 2Mbps over 3.7km
 Single line DSL
 2B1Q coding on single twisted pair
(residential) with echo cancelling
 up to 2Mbps over 3.7km
 Very high data rate DSL
 DMT/QAM for very high data rates
 over separate bands for separate services

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

08-Multiplexing.ppt

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