EEL4930University of FloridaFall 2003 1 EEL4930 Computer Networks Dr. George Stallings – Chapter 8...

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EEL4930EEL4930 University of FloridaUniversity of Florida Fall 2003Fall 2003

EEL4930EEL4930Computer NetworksComputer Networks

Dr. GeorgeDr. George

Stallings – Chapter 8Stallings – Chapter 8

MultiplexingMultiplexing

NOTE: Many figures and other materials in this presentation are borrowed from required and reference textbooks cited on the class web page.

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Multiplexing (1)Multiplexing (1)

Common applications in long-haul comm.Common applications in long-haul comm. e.g. trunks on long-haul networks are high-capacity e.g. trunks on long-haul networks are high-capacity

fiber, coaxial, or microwave linksfiber, coaxial, or microwave links

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Multiplexing (2)Multiplexing (2)

Two traditional approachesTwo traditional approaches Time-Division Multiplexing (TDM)Time-Division Multiplexing (TDM) Frequency-Division Multiplexing (FDM)Frequency-Division Multiplexing (FDM)

L1

L2

L3

R1

R2

R3Switch 1 Switch 2

L1

L2

L3

R1

R2

R3Switch 1 Switch 2

L1

L2

L3

R1

R2

R3Switch 1 Switch 2

L1

L2

L3

R1

R2

R3Switch 1 Switch 2

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FDMFDM

I can give you I can give you somesome of the of the bandwidth bandwidth allall of the time. of the time.

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FDMFDM

MotivationMotivation Useful bandwidth of transmission medium exceeds Useful bandwidth of transmission medium exceeds

required bandwidth of channelrequired bandwidth of channel

CharacteristicsCharacteristics Each signal is modulated to a different carrier Each signal is modulated to a different carrier

frequency (subcarrier) – each forms a channelfrequency (subcarrier) – each forms a channel Carrier frequencies separated so signals do not Carrier frequencies separated so signals do not

overlap (guard bands)overlap (guard bands) Channel allocated even if no dataChannel allocated even if no data

e.g. broadcast radio, broadcast and cable TVe.g. broadcast radio, broadcast and cable TV

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Example of FDM ChannelsExample of FDM Channels

(a) The original bandwidths. (b) The bandwidths raised in frequency.(c) The multiplexed channel.

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FDM SystemFDM SystemComposite signal may be shifted to another carrier frequency as additional

modulation step

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Analog Carrier SystemsAnalog Carrier Systems

Long-distance carrier system in U.S. and world to Long-distance carrier system in U.S. and world to transmit transmit voicebandvoiceband signals over high-capacity trans. links signals over high-capacity trans. linksAT&T (USA) designed hierarchy of FDM schemesAT&T (USA) designed hierarchy of FDM schemesGroup of 12 channelsGroup of 12 channels

12 voice channels (4 kHz each) combined by FDM to produce 12 voice channels (4 kHz each) combined by FDM to produce group signal with 48kHz bandwidthgroup signal with 48kHz bandwidth

Spectrum of 60 – 108 kHzSpectrum of 60 – 108 kHz

SupergroupSupergroup 60 channels60 channels Each group treated as single signal each with 48 kHz bandwidthEach group treated as single signal each with 48 kHz bandwidth Supergroup formed by FDM of 5 group signals Supergroup formed by FDM of 5 group signals Spectrum of 312 - 552 kHzSpectrum of 312 - 552 kHz

MastergroupMastergroup 10 supergroups, 600 channels10 supergroups, 600 channels

More beyond (e.g. Jumbogroup of 3600 channels)More beyond (e.g. Jumbogroup of 3600 channels)

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Wavelength-Division Multiplexing (WDM)Wavelength-Division Multiplexing (WDM)

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WDM HighlightsWDM HighlightsBasic ideaBasic idea

Multiple beams of light at different frequenciesMultiple beams of light at different frequencies Carried by optical fiberCarried by optical fiber A form of FDMA form of FDM Each color of light (wavelength) carries separate data channelEach color of light (wavelength) carries separate data channel

Landmark for WDM @ Bell Labs in 1997Landmark for WDM @ Bell Labs in 1997 100 beams100 beams Each at 10 GbpsEach at 10 Gbps 1 Terabit per second (Tbps) on a single fiber!1 Terabit per second (Tbps) on a single fiber!

Commercial systems of 160 channels each @ 10 Gbps Commercial systems of 160 channels each @ 10 Gbps now availablenow availableLab systems going fartherLab systems going farther

e.g. 256 channels each @ 39.8 Gbpse.g. 256 channels each @ 39.8 Gbps 10+ Tbps in total10+ Tbps in total Operating over a span of 100+ kmOperating over a span of 100+ km

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WDM OperationWDM OperationSame general architecture as other FDM systemsSame general architecture as other FDM systems

No. of sources generating laser beams @ different wavelengthsNo. of sources generating laser beams @ different wavelengths Multiplexer consolidates sources for xmit over single fiberMultiplexer consolidates sources for xmit over single fiber Optical amplifiers amplify all wavelengthsOptical amplifiers amplify all wavelengths

Typically tens of kilometers apartTypically tens of kilometers apart Demux separates channels at destination for multiple receiversDemux separates channels at destination for multiple receivers

e.g. 1550nm wavelen. (~192-196THz) range is common, e.g. 1550nm wavelen. (~192-196THz) range is common, 50GHz spacing per channel (i.e. ~.4nm per channel)50GHz spacing per channel (i.e. ~.4nm per channel)

ITU WDM channel spacing (G.692) standard accommodates ITU WDM channel spacing (G.692) standard accommodates eighty 50GHzeighty 50GHz channels! channels!

Dense WDM (DWDM)Dense WDM (DWDM) No official or standard definitionNo official or standard definition Implies more channels, more closely spaced, than WDMImplies more channels, more closely spaced, than WDM

Coarse WDM (CWDM)Coarse WDM (CWDM) Wider spacing, less channelsWider spacing, less channels

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Synchronous TDMSynchronous TDM

I can give you I can give you allall of the bandwidth of the bandwidth somesome of the time. of the time.

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Synchronous TDMSynchronous TDM

Time-Division Multiplexing (TDM)Time-Division Multiplexing (TDM) Generally digital signals carrying digital dataGenerally digital signals carrying digital data

MotivationMotivation Data rate of medium exceeds data rate of digital signal to be Data rate of medium exceeds data rate of digital signal to be

transmittedtransmitted

CharacteristicsCharacteristics Multiple digital signals interleaved in timeMultiple digital signals interleaved in time May be at bit level or blocks/charactersMay be at bit level or blocks/characters Time slots pre-assigned to sources and fixed (reason it is called Time slots pre-assigned to sources and fixed (reason it is called

“synchronous” TDM)“synchronous” TDM) Time slots allocated even if no dataTime slots allocated even if no data Time slots do not have to be evenly distributed amongst sourcesTime slots do not have to be evenly distributed amongst sources Slot length equals transmitter buffer length (bit or character)Slot length equals transmitter buffer length (bit or character)

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Synchronous TDM SystemSynchronous TDM System

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TDM Link ControlTDM Link Control

No headers and trailers in frameNo headers and trailers in frameTDM data-link protocols not neededTDM data-link protocols not neededBut what about flow control?But what about flow control?

Data rate of multiplexed line is fixedData rate of multiplexed line is fixed If one channel receiver cannot receive data, others must carry If one channel receiver cannot receive data, others must carry

onon Corresponding source must cease its flow of dataCorresponding source must cease its flow of data This step leaves empty slotsThis step leaves empty slots

But what about error control?But what about error control? Errors detected and handled by individual channel systemsErrors detected and handled by individual channel systems

Thus, flow and error control provided as needed on per-Thus, flow and error control provided as needed on per-channel basis using protocol such as HDLCchannel basis using protocol such as HDLCMUX/DeMUX transparent to stationsMUX/DeMUX transparent to stations

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Data Link Control on TDMData Link Control on TDM

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FramingFraming

No flag or SYNC characters to bracket TDM No flag or SYNC characters to bracket TDM framesframesBut must provide synchronizing mechanismBut must provide synchronizing mechanismCommon mechanism is Common mechanism is added-digit framingadded-digit framing One control bit added to each TDM frameOne control bit added to each TDM frame

Looks like another channel - “control channel”Looks like another channel - “control channel”We’ll see it later in DS-1 frame formatWe’ll see it later in DS-1 frame format

Identifiable bit pattern, from frame to frame, used on Identifiable bit pattern, from frame to frame, used on control channelcontrol channel

e.g. alternating 10101010… unlikely on data channele.g. alternating 10101010… unlikely on data channel Receiver can compare incoming bit patterns on each Receiver can compare incoming bit patterns on each

channel with sync patternchannel with sync patternIf pattern breaks down, receiver enters frame search modeIf pattern breaks down, receiver enters frame search mode

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Pulse StuffingPulse Stuffing

ProblemProblem Synchronizing data sourcesSynchronizing data sources Clocks in different sources driftingClocks in different sources drifting Data rates from different sources not related by Data rates from different sources not related by

simple rational numbersimple rational number

Solution: Pulse StuffingSolution: Pulse Stuffing Outgoing data rate (excluding framing bits) higher Outgoing data rate (excluding framing bits) higher

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

signal until it matches local clocksignal until it matches local clock Stuffed pulses inserted at fixed locations in frame and Stuffed pulses inserted at fixed locations in frame and

removed at demultiplexerremoved at demultiplexer

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TDM of Analog & Digital SourcesTDM of Analog & Digital Sources

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Digital Carrier Systems (1)Digital Carrier Systems (1)

Hierarchy of Synchronous TDMHierarchy of Synchronous TDM USA/Canada/Japan use one systemUSA/Canada/Japan use one system ITU-T use a similar (but different) systemITU-T use a similar (but different) system

US (AT&T) system based on DS-1 formatUS (AT&T) system based on DS-1 format Multiplexes 24 channelsMultiplexes 24 channels Each frame has 8 bits per channel, plus one Each frame has 8 bits per channel, plus one

framing bitframing bit 24 x 8 + 1 = 193 bits per frame24 x 8 + 1 = 193 bits per frame 125us per frame 125us per frame 8000 frames/second 8000 frames/second

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Digital Carrier Systems (2)Digital Carrier Systems (2)

For voiceFor voice Analog voice signal digitized with PCM at 8000 samples/secAnalog voice signal digitized with PCM at 8000 samples/sec Thus, each channel slot and thus each frame must repeat sameThus, each channel slot and thus each frame must repeat same Data rate = 8000 frames/sec x 193 bits/frame = 1.544 MbpsData rate = 8000 frames/sec x 193 bits/frame = 1.544 Mbps Five out of six frames have 8-bit PCM samples per channelFive out of six frames have 8-bit PCM samples per channel Sixth frame, each channel is 7-bit PCM word plus signaling bitSixth frame, each channel is 7-bit PCM word plus signaling bit Signaling bits form stream for each voice channel containing Signaling bits form stream for each voice channel containing

control and routing info (e.g. call connection or termination)control and routing info (e.g. call connection or termination)

Same DS-1 format also for digital dataSame DS-1 format also for digital data 23 channels of data23 channels of data

7 bits per frame plus indicator bit for data or systems control7 bits per frame plus indicator bit for data or systems controlThus, 7 x 8000 = 56 kbps data rate per channelThus, 7 x 8000 = 56 kbps data rate per channel

24th channel reserved for special sync byte24th channel reserved for special sync byteAllows faster and more reliable reframing after a framing errorAllows faster and more reliable reframing after a framing error

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Mixed DataMixed Data

DS-1 can carry mixed voice and data DS-1 can carry mixed voice and data signalssignals 24 channels used24 channels used No sync byteNo sync byte

Higher-level multiplexing Higher-level multiplexing Interleave DS-1 channelsInterleave DS-1 channels e.g. DS-2 is four DS-1s giving 6.312Mbpse.g. DS-2 is four DS-1s giving 6.312Mbps

1.544 Mbps x 4 = 6.176 Mbps1.544 Mbps x 4 = 6.176 MbpsRemaining is for framing and control bitsRemaining is for framing and control bits

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T1/DS-1 Carrier Standard (1.544 Mbps)T1/DS-1 Carrier Standard (1.544 Mbps)

NoteNote T-1 (a.k.a.T1) is digital carrier facility of 24 channels used to T-1 (a.k.a.T1) is digital carrier facility of 24 channels used to

transmit DS-1 (a.k.a. DS1) formatted digital signals transmit DS-1 (a.k.a. DS1) formatted digital signals Similarly for othersSimilarly for others

T-2/DS-2 @ 6.312 Mbps w/ 96 channelsT-2/DS-2 @ 6.312 Mbps w/ 96 channelsT-3/DS-3 @ 44.736 Mbps w/ 672 channelsT-3/DS-3 @ 44.736 Mbps w/ 672 channelsT-4/DS-4 @ 274.176 Mbps w/ 4032 channels)T-4/DS-4 @ 274.176 Mbps w/ 4032 channels)

Hyphens are optional Hyphens are optional

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Multiplexing T1 streams onto higher carriers

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SONET/SDHSONET/SDH

Synchronous Optical Network (SONET)Synchronous Optical Network (SONET) Optical transmission interfaceOptical transmission interface Developed by BellCore, standardized by ANSIDeveloped by BellCore, standardized by ANSI

Synchronous Digital Hierarchy (SDH)Synchronous Digital Hierarchy (SDH) ITU-T standard compatible with SONETITU-T standard compatible with SONET

Signal HierarchySignal Hierarchy OC-x rate is optical equivalent of STS-x electrical signalOC-x rate is optical equivalent of STS-x electrical signal Synch Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1)Synch Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1)

51.84 Mbps data rate, 50.112 Mbps payload rate51.84 Mbps data rate, 50.112 Mbps payload rate

Could carry DS-3 or group of lower rate signals (e.g. DS1 or DS2) plus ITU-T TDM Could carry DS-3 or group of lower rate signals (e.g. DS1 or DS2) plus ITU-T TDM carrier rates (e.g. Level-1 at 2.048 Mbps)carrier rates (e.g. Level-1 at 2.048 Mbps)

Multiple STS-1 are combined into STS-N signal; some common onesMultiple STS-1 are combined into STS-N signal; some common onesOC-3 has 155.52 Mbps data rate, 150.336 payload rateOC-3 has 155.52 Mbps data rate, 150.336 payload rate

OC-12 has 622.08 Mbps data rate, 601.344 payload rateOC-12 has 622.08 Mbps data rate, 601.344 payload rate

OC-48 has 2488.32 Mbps data rate, 2405.376 payload rateOC-48 has 2488.32 Mbps data rate, 2405.376 payload rate

OC-192 has 9953.28 Mbps data rate, 9621.504 payload rateOC-192 has 9953.28 Mbps data rate, 9621.504 payload rate ITU-T lowest rate is STM-1 = STS-3 = OC-3ITU-T lowest rate is STM-1 = STS-3 = OC-3

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SONET systemSONET system

Consists of switches, multiplexers, and repeaters, all connected by Consists of switches, multiplexers, and repeaters, all connected by fiberfiberExample path from source to destination shown belowExample path from source to destination shown below

Fiber from one device to another is a Fiber from one device to another is a sectionsection A run between two MUXs (perhaps with repeaters in middle) is a A run between two MUXs (perhaps with repeaters in middle) is a lineline Connection between source and destination (perhaps with MUXs and Connection between source and destination (perhaps with MUXs and

repeaters in middle) is a repeaters in middle) is a pathpath Topology can be a mesh, but often a dual ring Topology can be a mesh, but often a dual ring

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SONET physical layerSONET physical layer

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SONET frame formatSONET frame format

Building block is STS-1 = OC-1 frameBuilding block is STS-1 = OC-1 frame 810 octets transmitted once every 125us810 octets transmitted once every 125us

810 x 8000 frames/s = 6.48 MB/s = 51.84 Mb/s810 x 8000 frames/s = 6.48 MB/s = 51.84 Mb/s Viewed as matrix of 9 rows of 90 octets eachViewed as matrix of 9 rows of 90 octets each

First 3 columns (27 octets) are overheadFirst 3 columns (27 octets) are overhead 9 of 27 for section-related overhead9 of 27 for section-related overhead

Generated/checked at start/end of each sectionGenerated/checked at start/end of each section 18 of 27 for line-related overhead18 of 27 for line-related overhead

Generated/checked at start/end of each lineGenerated/checked at start/end of each line

Remainder is payloadRemainder is payload 9 x 87 = 783 octets9 x 87 = 783 octets Except one column of path overheadExcept one column of path overhead

Injected somewhere in payloadInjected somewhere in payloadLine-related overhead contains pointer to its locationLine-related overhead contains pointer to its locationIndicates start of Synchronous Payload Envelope (i.e. user data)Indicates start of Synchronous Payload Envelope (i.e. user data)More on next two slidesMore on next two slides

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SONET Frame FormatSONET Frame Format

• Section overhead: for single point-to-point fiber run• Line overhead: for multiplexing multiple data streams (tributaries) onto single line and

demultiplexing at other end• Path overhead: for end-to-end issues

Transmitted row by row (LeftRight, TopBottom)

Lowest rate in ITU-T SDH is STM-1 @ 155.52 Mb/s; thus STM-1 OC-3, STM-4 OC12, etc.

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SONET STS-1 Overhead OctetsSONET STS-1 Overhead Octets

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Two back-to-back SONET framesTwo back-to-back SONET frames

SPE can begin anywhere within frameSPE can begin anywhere within frame As we said, pointer to first byte contained in first row of line overheadAs we said, pointer to first byte contained in first row of line overhead First column of SPE is path overhead (i.e. header for end-to-end path First column of SPE is path overhead (i.e. header for end-to-end path

sublayer protocol)sublayer protocol) SPE may begin anywhere within frame and even span two frames for SPE may begin anywhere within frame and even span two frames for

flexibilityflexibilitye.g. If payload arrives at source while dummy SONET frame being e.g. If payload arrives at source while dummy SONET frame being constructed, can be inserted into current frame instead of waiting for nextconstructed, can be inserted into current frame instead of waiting for next

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Multiplexing in SONETMultiplexing in SONET

To prevent long runs of 0s or 1s

Multiplexing done on a byte-by-byte basis (e.g. when three STS-1 tributaries merged into one STS-3 stream, MUX first outputs one octet from tributary #1, then one from #2, then one from #3, then one from #1, etc.)

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SONET and SDH multiplex ratesSONET and SDH multiplex rates

Other interesting ones of late: OC-192 (9.95328 Gb/s gross), OC-768 (39.81312 Gb/s)

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SONET odds and endsSONET odds and ends

Higher rates need not necessarily be multiplexedHigher rates need not necessarily be multiplexedWhen carrier not multiplexed, it carries data from single When carrier not multiplexed, it carries data from single source, and its designation is appended with “c” for source, and its designation is appended with “c” for “concatenated”“concatenated”

e.g. OC-3c, OC-12ce.g. OC-3c, OC-12c e.g. ATM on OC-3c from single computer at 155.52 Mbpse.g. ATM on OC-3c from single computer at 155.52 Mbps

Amount of actual user data in concatenated stream is Amount of actual user data in concatenated stream is slightly higher than in multiplexed streamslightly higher than in multiplexed stream

ExampleExampleWith OC-3c stream, only one path overhead column per SPEWith OC-3c stream, only one path overhead column per SPEWith OC-3 stream, three path overhead columns, one from each of With OC-3 stream, three path overhead columns, one from each of three independent OC-1 streams three independent OC-1 streams

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Statistical TDMStatistical TDMI can give you I can give you allall of the bandwidth of the bandwidth somesome of the time. How often/much of the time. How often/much depends on how busy you get. depends on how busy you get.

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Statistical TDM (1)Statistical TDM (1)

MovitationMovitation In Synchronous TDM, static allocation implies that many slots may be In Synchronous TDM, static allocation implies that many slots may be

wastedwasted Not all attached devices may be transmitting all the timeNot all attached devices may be transmitting all the time

CharacteristicsCharacteristics Statistical TDM allocates time slots Statistical TDM allocates time slots dynamicallydynamically based on demand based on demand Multiplexer scans input lines and collects data until frame fullMultiplexer scans input lines and collects data until frame full Does not send empty slots if there is data to send from any sourceDoes not send empty slots if there is data to send from any source Data rate on line is lower than aggregate of the rates of input linesData rate on line is lower than aggregate of the rates of input lines We have We have nn I/O lines into our statistical multiplexer, but only I/O lines into our statistical multiplexer, but only kk time slots time slots

available where available where kk < < nn Of course, data arrives and must be distributed to I/O lines Of course, data arrives and must be distributed to I/O lines

unpredictably hereunpredictably hereAddress info. required to ensure proper deliveryAddress info. required to ensure proper delivery

Implies more overhead per slot than with synch. TDMImplies more overhead per slot than with synch. TDM

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Statistical TDM (2)Statistical TDM (2)

““On-demand” TDMOn-demand” TDM Function of MUX to scan input buffers, collect data Function of MUX to scan input buffers, collect data

until a frame is filled, then send frame on linkuntil a frame is filled, then send frame on link Uses synchronous protocol such as HDLCUses synchronous protocol such as HDLC

…

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Statistical TDM Frame FormatsStatistical TDM Frame Formats

Two options with Statistical TDM frame

First case (b), only one source of data included per frame; length of data is variable; works well under light load.

Second case (c), multiple sources per frame; more overhead but more efficient for heavier loads.

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PerformancePerformance

Data rate on line is lower than aggregate of the Data rate on line is lower than aggregate of the rates of inputs rates of inputs (@ peak; o/w on average)(@ peak; o/w on average) Buffer data contending for the linkBuffer data contending for the link Buffer (queue) overflow a symptom of congestionBuffer (queue) overflow a symptom of congestion

Pros and cons of lower data rate on linePros and cons of lower data rate on line Good in getting more for lessGood in getting more for less Bad during peak periodsBad during peak periods

Tradeoff between buffer size and line data rateTradeoff between buffer size and line data rate Reduction in one requires increase in other!Reduction in one requires increase in other! Why would we want to limit buffer size given how Why would we want to limit buffer size given how

cheap memory technology has become?cheap memory technology has become?More buffering implies longer delayMore buffering implies longer delayThus, real tradeoff is system response time versus speed of Thus, real tradeoff is system response time versus speed of multiplexed linemultiplexed line

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Buffer Buffer Size Size and and

DelayDelayExample shown here w/ random arrivals

Data in 1000-bit frames

M is effective capacity of muxed line

As offered load (from sum of mux inputs) approaches muxed line

capacity, avg. # of frames buffered rises quickly.

This increase in turn causes quick rise in delay experienced.

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Example: ADSLExample: ADSL

Asymmetrical Digital Subscriber Line (ADSL)Asymmetrical Digital Subscriber Line (ADSL)ProblemProblem In high-speed, wide-area, public digital networks, key In high-speed, wide-area, public digital networks, key

challenge is link between subscriber and networkchallenge is link between subscriber and networkDigital subscriber lineDigital subscriber line

High expense to run new lines to homes and High expense to run new lines to homes and businessesbusinesses

SolutionSolution Exploit installed base of twisted-pair wire already Exploit installed base of twisted-pair wire already

present from telephone servicepresent from telephone service Can carry broader spectrum than just 4 kHz voiceCan carry broader spectrum than just 4 kHz voice 1 MHz bandwidth or more1 MHz bandwidth or more

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ADSL DesignADSL Design

AsymmetricAsymmetric Greater capacity downstream than upstreamGreater capacity downstream than upstream

Novel use of FDMNovel use of FDM Exploits 1 MHz capacity of twisted-pair wireExploits 1 MHz capacity of twisted-pair wire Reserve lowest 25 kHz for voiceReserve lowest 25 kHz for voice

Plain old telephone service (POTS)Plain old telephone service (POTS)Additional bandwidth included here to prevent crosstalk Additional bandwidth included here to prevent crosstalk between voice and data channelsbetween voice and data channels

Use echo cancellation or FDM to allocate two bands Use echo cancellation or FDM to allocate two bands (upstream and downstream) or channels(upstream and downstream) or channels

Then use FDM Then use FDM withinwithin these two bands these two bandsSerial bit stream split into multiple parallel bit streamsSerial bit stream split into multiple parallel bit streamsEach portion carried on separate freq. band (subchannel)Each portion carried on separate freq. band (subchannel)

Range up to 5.5 km depending on quality of cableRange up to 5.5 km depending on quality of cable

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ADSL Channel ConfigurationADSL Channel Configuration

Two alternativesTwo alternatives FDM for POTS and two data FDM for POTS and two data

bands (a)bands (a) Echo cancellation permits Echo cancellation permits

upstream to overlap lower upstream to overlap lower portion of downstream (b)portion of downstream (b)

Echo cancellationEcho cancellation: signal : signal processing technique for processing technique for simultaneous transmission in simultaneous transmission in both directions on single lineboth directions on single line

Transmitter subtracts echo of Transmitter subtracts echo of its own transmission from its own transmission from incoming signal to recover incoming signal to recover signal sent by other sidesignal sent by other side

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DMT (1)DMT (1)Discrete Multitone (DMT) ModulationDiscrete Multitone (DMT) ModulationMultiple carrier signals at different frequenciesMultiple carrier signals at different frequencies

Transmission band divided into 4 kHz subchannelsTransmission band divided into 4 kHz subchannels Sending bits on each subchannelSending bits on each subchannel

InitializationInitialization DMT modem sends test signal on each subchannel to determine SNRDMT modem sends test signal on each subchannel to determine SNR Assigns more bits to subchannels with better SNR, less to poorer onesAssigns more bits to subchannels with better SNR, less to poorer ones Each subchannel carries data at rate from 0 to 60 kbpsEach subchannel carries data at rate from 0 to 60 kbps Typical example below (increasing attenuation at higher frequencies)Typical example below (increasing attenuation at higher frequencies)

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DMT (2)DMT (2)

DMT TransmissionDMT Transmission After init., bit stream divided into substreams (one per subchannel)After init., bit stream divided into substreams (one per subchannel) Each substream converted to analog signal using quadrature amplitude Each substream converted to analog signal using quadrature amplitude

modulation (QAM); each QAM signal occupies distinct 4 kHz freq. bandmodulation (QAM); each QAM signal occupies distinct 4 kHz freq. band Sum of substream data rates equals total data rateSum of substream data rates equals total data rate

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ADSL/DMT Data RateADSL/DMT Data Rate

Common ADSL/DMT designs employ 256 Common ADSL/DMT designs employ 256 subchannels on downstream channelsubchannels on downstream channel In theory, at 60 kbps per subchannel, can achieve In theory, at 60 kbps per subchannel, can achieve

total data rate of 60 kbps x 256 = 15.36 Mbpstotal data rate of 60 kbps x 256 = 15.36 Mbps However, in practice, impairments limit However, in practice, impairments limit

implementations with range from 1.5 to 9 Mbpsimplementations with range from 1.5 to 9 MbpsDepends on line distance and qualityDepends on line distance and quality

e.g. BellSouth “FastAccess” DSLe.g. BellSouth “FastAccess” DSL Residential serviceResidential service

Up to 1.5 Mbps downstreamUp to 1.5 Mbps downstream

Up to 256 kbps upstreamUp to 256 kbps upstream

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Example: Cable ModemExample: Cable ModemBased on cable TV, FDM of analog signalsBased on cable TV, FDM of analog signals

Two FDM channels dedicated, one for upstream Two FDM channels dedicated, one for upstream and one for downstreamand one for downstream

Each channel shared by number of subscribersEach channel shared by number of subscribers Statistical TDM used to allocate capacityStatistical TDM used to allocate capacity

Topology looks like a treeTopology looks like a tree Head-end controller at provider is the rootHead-end controller at provider is the root Cable modems at customers are the leavesCable modems at customers are the leaves

Role of cable modemRole of cable modem IP packets modulated by provider at head-end IP packets modulated by provider at head-end

controller then sent downstreamcontroller then sent downstream Cable modem demodulates signals back to IP Cable modem demodulates signals back to IP

packetspackets Downstream data received by all cable modems; Downstream data received by all cable modems;

total bandwidth shared; modem filters for local usetotal bandwidth shared; modem filters for local use Upstream data sent in bursts typically via Upstream data sent in bursts typically via

statistical TDM, via reserved and contention slotsstatistical TDM, via reserved and contention slotsThus, performance depends on several factorsThus, performance depends on several factors

Bandwidth of channelsBandwidth of channels Provider constraintsProvider constraints

e.g. local provider may connect to Internet via T-1e.g. local provider may connect to Internet via T-1 Load from fellow local subscribersLoad from fellow local subscribers

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Cable Modem OperationCable Modem Operation

DownstreamDownstream Cable scheduler delivers data in small packetsCable scheduler delivers data in small packets If more than one subscriber active, each gets fraction If more than one subscriber active, each gets fraction

of downstream capacityof downstream capacitye.g. may get 500 kbps to 1.5 Mbpse.g. may get 500 kbps to 1.5 Mbps

Also used to allocate upstream time slots to Also used to allocate upstream time slots to subscriberssubscribers

UpstreamUpstream User requests timeslots on shared upstream channelUser requests timeslots on shared upstream channel

Dedicated slots for thisDedicated slots for this Headend scheduler sends back assignment of future Headend scheduler sends back assignment of future

time slots to subscribertime slots to subscriber

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Cable Modem SchemeCable Modem Scheme

EEL4930University of FloridaFall 2003 1 EEL4930 Computer Networks Dr. George Stallings – Chapter 8...

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