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William StallingsData and Computer Communications7th Edition

Chapter 8Multiplexing

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Multiplexing• Enable two or more transmission sources

to share a common circuit• Most common forms – FDM and TDM• FDM – associated with analog signal,

simultaneous transmission• TDM – associated with digital signal (could

also be analog, but single frequency) with time slices

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Multiplexing

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Frequency Division Multiplexing• FDM• 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• Broadband

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Frequency Division MultiplexingDiagram

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Wavelength Division Multiplexing

• Multiple beams of light at different frequency

• Carried by optical fiber

• A form of FDM

• Each colour of light (wavelength) carries separate

data channel

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WDM Operation• Same general architecture as other FDM• Number of sources generating laser

beams at different frequencies• Multiplexer consolidates sources for

transmission over single fiber• Optical amplifiers amplify all wavelengths

—Typically tens of km apart

• Demux separates channels at the destination

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Synchronous Time Division Multiplexing• Multiple digital signals interleaved in time• May interleave bits, so not necessarily

synchronous transmission• Time slots pre-assigned to sources and

fixed• Time slots allocated even if no data• Time slots do not have to be evenly

distributed amongst sources• Baseband

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Time Division Multiplexing

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Optical TDM • In early days of Fiber Optics, every telco

had its own proprietary optical TDM• After break up of AT&T, phone companies

had to connect to multiple long distance carriers, all with different optical TDMs

• This created the need for standardized optical TDM – SONET – synchronized optical network

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Design Goals of SONET• Enable different carriers to interoperate –

resulted in need of common signaling standard with respect to wavelength, timing, framing structure, etc.

• Needed to unify US, European, and Japanese signalling systems

• Had to provide a way to multiplex multiple digital signals

• Provide support for operations, administration, and maintenance

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SONET/SDH• Synchronous Optical Network (ANSI standard -

USA)• Synchronous Digital Hierarchy (ITU-T standard -

Europe)• Compatible• Both are fiber optic standards for high speed data

transmission• Signal Hierarchy

—Synchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1)

—51.84Mbps—ITU-T lowest rate is 155.52Mbps (STM-1)

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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 and collects

data until frame full

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Statistical TDM Frame Formats

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Performance• Output data rate less than aggregate

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• Two channels from cable TV provider dedicated

to data transfer—One in each direction

• Each channel shared by number of subscribers—Scheme needed to allocate capacity—Statistical TDM

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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

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ADSL Design• Asymmetric

—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

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Required Reading• Stallings chapter 8• Web sites on

—ADSL —SONET

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Chapter 8 Review Questions• Explain the basics of multiplexing.Why is

multiplexing so cost effective?• How is interference avoided by using FDM?• Explain how TDM works. Why is statistical time

division multiplexing more efficient than TDM• Compare and contrast TDM, STDM, and FDM• (note: for purpose of this class, STDM = statistical,

not synchronous)• What is SONET?• Compare and contrast cable modems and DSL• Define upstream and downstream with respect to

subscriber lines? What is the greatest advantage of ADSL?