1. William Stallings
Data and Computer
Communications
7th Edition
Chapter 8
Multiplexing
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2. 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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3. Multiplexing
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4. 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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5. Frequency Division Multiplexing
Diagram
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6. 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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7. 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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8. 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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9. Time Division Multiplexing
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10. 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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11. 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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12. 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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13. 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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14. Statistical TDM Frame Formats
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15. 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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16. 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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17. 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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18. 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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19. Required Reading
• Stallings chapter 8
• Web sites on
—ADSL
—SONET
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20. 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?
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