Upload
bethany-parsons
View
220
Download
2
Embed Size (px)
Citation preview
Data and Computer Communications
Chapter 8Multiplexing Techniques
Transmission Efficiency: MultiplexingSeveral data sources share a common
transmission medium, with each source having its own channel
Line sharing saves transmission costsHigher data rates mean more cost-
effective transmissionMost individual data sources require
relatively low data rates
Transmission Efficiency: Data compressionReduces the size of data files to move
more information with fewer bitsUsed for transmission and for storage
e.g. ZIP
Often combined with multiplexing to increase efficiency
Alternate Approaches to Terminal SupportDirect point-to-point links Multidrop lineMultiplexer Integrated MUX function in host
Direct Point-to-Point
Multidrop Line
Multiplexer
Integrated MUX in Host
Why multiplexing
share the use of a common channel
DEMUX
Multiplexer Demultiplexer
shared channel
MUX
Multiplexing
Types of Multiplexer
FDM (Frequency Division Multiplexer)
TDM (Time Division Multiplexer)
Frequency Division MultiplexingFDMUseful bandwidth of medium exceeds
required bandwidth of channelEach signal is modulated to a different
carrier frequencyCarrier frequencies separated so signals
do not overlap (guard bands)e.g. broadcast radioChannel allocated even if there is no data
Frequency Division MultiplexingRequires analog signaling & transmissionBandwidth = sum of inputs + guardbandsModulates signals so that each occupies a
different frequency bandStandard for radio broadcasting, analog
telephone network, and television (broadcast, cable, & satellite)
Frequency Division Multiplex
CH2
CH1
CH3
original bandwidth
CH1
CH2
CH3
bandwidths raised infrequency
MUXCH1 CH2 CH3
bandwidths multiplexedinto one channel
f
Frequency Division MultiplexingDiagram
FDM System
FDM of Three Voiceband Signals
Only the lower sideband is used
Analog Carrier SystemsAT&T (USA)Hierarchy of FDM schemesGroup
12 voice channels (4kHz each) = 48kHz Range 60kHz to 108kHz
Supergroup 60 channel FDM of 5 group signals on carriers between
420kHz and 612 kHz
Mastergroup 10 supergroups
Synchronous Time Division MultiplexingData rate of medium exceeds data rate of
digital signal to be transmittedMultiple digital signals interleaved in timeTime slots pre-assigned to sources and
fixedTime slots allocated even if there is no
dataTime slots do not have to be evenly
distributed amongst sources
Synchronous Time-Division Multiplexing (TDM)Used in digital transmissionRequires data rate of the medium to
exceed data rate of signals to be transmitted
Signals “take turns” over mediumSlices of data are organized into frames
Synchronous TDM and PSTN
Used in the modern digital telephone system US, Canada, Japan: DS-1 (T-1), DS-3 (T-3), ... Europe, elsewhere: E-1, E3, … These are listed in table 8.3 Page 249
Data rate of 1.544MbpsUses PCM to digitize voice transmission at
8K samples/sec, frame length of 193bits (8000x193=1.544 Mbps=T1)
Time Division Multiplex
A2
A1
A3
original signal
D2
D1
D3
digitized signal
MUX
data filled in time slot
time slot1 2 3 4
Time Division Multiplexing
TDM System
TDM Link ControlNo headers and trailersData link control protocols not neededFlow control
Data rate of multiplexed line is fixed If one channel receiver cannot receive data, the
others must carry on The corresponding source must be quenched This leaves empty slots
Error control Errors are detected and handled by individual
channel systems
Data Link Control on TDM
FramingNo flag or SYNC characters bracketing
TDM framesMust provide synchronizing mechanismAdded digit framing
One control bit added to each TDM frameLooks like another channel - “control channel”
Identifiable bit pattern used on control channel e.g. alternating 01010101…unlikely on a data
channel Can compare incoming bit patterns on each
channel with sync pattern
Pulse StuffingProblem - Synchronizing data sourcesClocks in different sources driftingData rates from different sources not
related by simple rational numberSolution - Pulse Stuffing
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
TDM of Analog and Digital Sources
Digital Carrier SystemsHierarchy of TDMUSA/Canada/Japan use one systemITU-T use a similar (but different) systemUS system based on DS-1 formatMultiplexes 24 channelsEach frame has 8 bits per channel plus
one framing bit193 bits per frame
Digital Carrier Systems (2)For voice each channel contains one word of
digitized data (PCM, 8000 samples per sec) Data rate 8000x193 = 1.544Mbps Five out of six frames have 8 bit PCM samples Sixth frame is 7 bit PCM word plus signaling bit Signaling bits form stream for each channel
containing control and routing info
Same format for digital data 23 channels of data
7 bits per frame plus indicator bit for data or systems control
24th channel is sync
Mixed DataDS-1 can carry mixed voice and data
signals24 channels usedNo sync byteCan also interleave DS-1 channels
Ds-2 is four DS-1 giving 6.312Mbps
ISDN User Network InterfaceISDN allows multiplexing of devices over
single ISDN lineTwo interfaces
Basic ISDN Interface Primary ISDN Interface
Basic ISDN Interface (1)Digital data exchanged between subscriber
and NTE - Full DuplexSeparate physical line for each directionPseudoternary coding scheme
1=no voltage, 0=positive or negative 750mV +/-10%
Data rate 192kbpsBasic access is two 64kbps B channels and
one 16kbps D channelThis gives 144kbps multiplexed over 192kbpsRemaining capacity used for framing and sync
Basic ISDN Interface (2)B channel is basic user channelDataPCM voiceSeparate logical 64kbps connections o
different destinationsD channel used for control or data
LAPD frames
Each frame 48 bits longOne frame every 250s
Frame Structure
Primary ISDNPoint to pointTypically supporting PBX1.544Mbps
Based on US DS-1 Used on T1 services 23 B plus one D channel
2.048Mbps Based on European standards 30 B plus one D channel Line coding is AMI usingHDB3
Primary ISDN Frame Formats
SONET: Synchronous Optical NetworkSpecification for high-speed digital
transfer via optical fiberRates from 51.84Mbps to 13.2GbpsUses Synchronous TDM
Sonet/SDHSynchronous Optical Network (ANSI)Synchronous Digital Hierarchy (ITU-T)CompatibleSignal Hierarchy
Synchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1)
51.84Mbps Carry DS-3 or group of lower rate signals (DS1
DS1C DS2) plus ITU-T rates (e.g. 2.048Mbps) Multiple STS-1 combined into STS-N signal ITU-T lowest rate is 155.52Mbps (STM-1)
SONET Frame Format
SONET STS-1 Overhead Octets
Statistical Time Division Multiplexingrequires digital signaling & transmissiondata rate capacity required is well below
the sum of connected capacitysame concepts as synchronous TDMuses memory buffers to avoid loss of datawidely used for remote communications
with multiple terminalssimilar to medium-sharing done by LANs
Statistical TDMIn Synchronous TDM many slots are
wastedStatistical TDM allocates time slots
dynamically based on demandMultiplexer scans input lines and collects
data until frame fullData rate on line lower than aggregate
rates of input lines
Statistical TDM
A
B
C
D
Data to be sent
t1 t2 t3
A1 B1 C1D1 C2
D2A2 B2
1st cycle 2nd cycle
Synchronous TDMwaste bandwidth
A1 B1 C2B2
1st cycle 2nd cycle
Statistical TDMextra bandwidthavailable
Statistical TDM Frame Formats
PerformanceOutput data rate less than aggregate
input ratesMay cause problems during peak periods
Buffer inputs Keep buffer size to minimum to reduce delay
Buffer Size and Delay
Asymmetrical Digital Subscriber LineADSLLink between subscriber and network
Local loop
Uses currently installed twisted pair cable Can carry broader spectrum 1 MHz or more
ADSL DesignAsymmetric
Greater capacity downstream than upstream
Frequency division multiplexing Lowest 25kHz for voice
Plain old telephone service (POTS)
Use echo cancellation or FDM to give two bands
Use FDM within bands
Range 5.5km
ADSL Channel Configuration
Discrete MultitoneDMTMultiple carrier signals at different frequenciesSome bits on each channel4kHz subchannelsSend test signal and use subchannels with
better signal to noise ratio256 downstream subchannels at 4kHz
(60kbps) 15.36MHz Impairments bring this down to 1.5Mbps to 9Mbps
DMT Transmitter
xDSLHigh data rate DSLSingle line DSLVery high data rate DSL