MultiplexingCEN 220/CIS 192 Advanced Data Communications and NetworkingData and Computer Communications, W. Stallings 9/E, Chapter 8

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Multiplexing“It was impossible to get a conversation going, everybody was talking too much.”

—Yogi Berra

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Multiplexingmultiple links on 1 physical linecommon on long-haul, high capacity, linkshave FDM, TDM, STDM alternatives

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Frequency Division Multiplexing (FDM)

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FDMSystem

Overview

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FDM Voiceband Example

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Analog Carrier Systemslong-distance links use an FDM hierarchyAT&T (USA) and ITU-T (International) variantsGroup

12 voice channels (4 kHz each) = 48 kHz Range 60 kHz to 108 kHz

Supergroup FDM of 5 group signals supports 60 channels carriers between 420 kHz and 612 kHz

Mastergroup FDM of 10 supergroups supports 600 channels

original signal can be modulated many times

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North American and International FDM Carrier Standards

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Wavelength Division Multiplexing (WDM)

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ITU WDM Channel Spacing (G.692)

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Synchronous Time Division Multiplexing

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TDM SystemOverview

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TDM Link Controlno headers and trailersdata link control protocols not neededflow 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

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Data Link Control on TDM

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Framingno flag or SYNC chars bracketing TDM framesmust still provide synchronizing mechanism between source and destination clocksadded digit framing is most common

one control bit added to each TDM frame identifiable bit pattern used as control channel alternating pattern 101010…unlikely to be sustained on a

data channel receivers compare incoming bits of frame position to the

expected pattern

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Pulse Stuffing

– problem of synchronizing various data sources

– variation among clocks could cause loss of synchronization

– issue of data rates from different sources not related by a simple rational number

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TDM Example

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Digital Carrier Systems

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North American and International TDM Carrier Standards

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DS-1 Transmission Format

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SONET/SDHSynchronous Optical Network (ANSI)

Synchronous Digital Hierarchy (ITU-T)

high speed capability of optical fiber

defines hierarchy of signal rates– Synchronous Transport Signal level 1 (STS-1) or

– Optical Carrier level 1 (OC-1) is 51.84 Mbps

– carries one DS-3 or multiple (DS1 DS1C DS2) plus ITU-T rates (e.g., 2.048 Mbps)

– multiple STS-1 combine into STS-N signal

– ITU-T lowest rate is 155.52 Mbps (STM-1)

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SONET/SDH Signal Hierarchy

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SONET Frame Format

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Statistical TDM

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Statistical TDM Frame Format

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Single-Server Queues with Constant Service Times and Poisson (Random) Arrivals

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Cable Modems

– dedicate two cable TV channels to data transfer

– each channel shared by number of subscribers using statistical TDM

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Cable Spectrum DivisionTo support both cable television programming and data channels, the cable spectrum is divided in to three ranges:

user-to-network data (upstream): 5 - 40 MHz television delivery (downstream): 50 - 550 MHz network to user data (downstream): 550 - 750 MHz

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Cable Modem Scheme

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Asymmetrical Digital Subscriber Line (ADSL)link between subscriber and networkuses currently installed twisted pair cableis Asymmetric - bigger downstream than upuses Frequency Division Multiplexing

reserve lowest 25 kHz for voice (POTS) uses echo cancellation or FDM to give two bands

has a range of up to 5.5 km

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ADSL Channel Configuration

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Discrete Multitone (DMT)

multiple carrier signals at different frequenciesdivide into 4 kHz subchannelstest and use subchannels with better SNR256 downstream subchannels at 4 kHz (60 kbps)

in theory 15.36 Mbps, in practice 1.5-9 Mbps

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DMT Transmitter

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Broadband – Customer SideDSL link is between provider and customer

a splitter allows simultaneous telephone and data service

data services use a DSL modem sometimes referred to as G.DMT modem

DSL data signal can be divided into a video stream and a data stream

the data stream connects the modem to a router which enables a customer to support a wireless local area network

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Broadband – Provider Sidea splitter separates telephone from Internet

voice traffic is connected to public switched telephone network (PSTN)

data traffic connects to a DSL multiplexer (DSLAM) which multiplexes multiple customer DSL connections to a single high-speed ATM line.

ATM line connects ATM switches to a router which provides entry to the Internet

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xDSLhigh 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 separate bands for separate services

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Comparison of xDSL Alternatives

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SummaryMultiplexing multiple channels on single link

FDM analog carrier systems wavelength division multiplexing

TDM TDM link control pulse stuffing

statistical TDM

broadband

ADSL and xDSL