Data Communications- Multiplexing

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Multiplexing• Multiplexing allows several transmission

sources to share a larger transmission capacity.

• 2 common forms of multiplexing—Frequency division multiplexing (FDM)—Time division multiplexing (TDM)

• Synchronous TDM• Statistical / Asynchronous TDM

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Frequency Division Multiplexing• FDM• Possible if useful bandwidth of medium

exceeds(melebihi) required bandwidth of channel

[ bandwidth/frequency range : medium>signal ]

Each signal is modulated to a different carrier frequency

• Carrier frequencies separated so signals do not overlap (guard bands)—e.g. broadcast radio

• Channel allocated even if no data

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Frequency Division MultiplexingDiagram

FTM

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Synchronous Time Division Multiplexing (TDM)• Possible if data rate of medium exceeds

data rate of digital signal to be transmitted

[ data rate : medium>signal ]• Multiple digital signals interleaved in time• Interleaving can be at bit level of blocks• Time slots pre-assigned to sources and

fixed• Time slots allocated even if no data to

send

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Time Division Multiplexing

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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—The corresponding source must be quenched—This leaves empty slots

• Error control—Errors are detected and handled by individual

channel systems

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Synchronous TDM

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TDM, Multiplexing

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TDM, Demultiplexing

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Statistical TDM• In Synchronous TDM many slots are wasted• Statistical TDM allocates time slots

dynamically based on demand• Multiplexer scans input lines (buffer) and

collects data until frame full, and then send the frame

• Data rate on line lower than aggregate(sum of) rates of input lines

• Thus, statistical TDM can use a lower data rate to support as many devices as asynchronous multiplexer

Synchronous TDM compared with Statistical TDM

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Asynchronous TDM

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Frames and Addresses

a. Only three lines sending data

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Frames and Addresses

c. All five lines sending data

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Statistical TDM Frame Formats

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Performance• Output data rate less than aggregate (the

sum of the data rates of the inputs) input rates

• May cause problems during peak periods—Buffer inputs—Keep buffer size to minimum to reduce delay

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Cable Modem Outline• Cable TV provider provides dedicated 2 channels

to support data transfer to/from cable modem—One for transmission in each direction

• Each channel shared by number of subscribers—Scheme needed to allocate capacity (fraction of

down/upstream capacity)—Statistical TDM

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Cable Modem Operation• Downstream

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

downstream capacity• May get 500kbps to 1.5Mbps

—Also used to allocate upstream time slots to subscribers

• Upstream—User requests timeslots on shared upstream channel

• Dedicated slots for this

—Headend scheduler sends back assignment of future time slots to subscriber

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Cable Modem Scheme

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Asymmetrical Digital Subscriber Line• ADSL• Link between subscriber and network

—Local loop

• Uses currently installed twisted pair cable—Can carry broader spectrum—1 MHz or more

• Most widely publicized of a family of new modem technologies designed to provide high-speed digital data transmission over ordinary telephone wire

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ADSL Design• Asymmetric

—Greater capacity downstream than upstream

• ADSL uses Frequency division multiplexing (FDM)—Reserve 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

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ADSL Channel Configuration

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xDSL• High data rate DSL• Single line DSL• Very high data rate DSL• Various types of xDSL

—Asynchronous Digital Subscriber Line (ADSL)—High Bit-rate Digital Subscriber Line (HDSL) —Symmetric Digital Subscriber Line (SDSL) —Very High Bit-rate Digital Subscriber Line

(VDSL)

Comparison of XDSL Alternatives

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