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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
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
Frequency Division Multiplexing Diagram
• Wavelength Division Multiplexing
• Multiple beams of light at different frequency
• Carried by optical fiber
• A form of FDM
• Each color of light (wavelength) carries separate data channel
• 1997 Bell Labs
— 100 beams
— Each at 10 Gbps
— Giving 1 terabit per second (Tbps)
• Commercial systems of 160 channels of 10 Gbps now available
• Lab systems (Alcatel) 256 channels at 39.8 Gbps each
— 10.1 Tbps
— Over 100km
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
• Was 200MHz per channel
• Now 50GHz
• 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
Time Division Multiplexing
• TDM Link Control
• No headers and trailers
• Data page 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
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
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
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)
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
• Statistical TDM Frame Formats
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
— 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
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
— 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
• Required Reading
• Stallings chapter 8
• Web sites on
— ADSL
— SONET
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?