3 multiplexing-wdm


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3 multiplexing-wdm

  1. 1. William StallingsData and ComputerCommunications7th EditionChapter 8Multiplexing 1
  2. 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 2
  3. 3. Multiplexing 3
  4. 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 4
  5. 5. Frequency Division MultiplexingDiagram 5
  6. 6. Wavelength DivisionMultiplexing• Multiple beams of light at different frequency• Carried by optical fiber• A form of FDM• Each colour of light (wavelength) carries separate data channel 6
  7. 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 7
  8. 8. Synchronous Time DivisionMultiplexing• 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 8
  9. 9. Time Division Multiplexing 9
  10. 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 10
  11. 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 11
  12. 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) 12
  13. 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 13
  14. 14. Statistical TDM Frame Formats 14
  15. 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 15
  16. 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 16
  17. 17. Asymmetrical DigitalSubscriber Line• ADSL• Link between subscriber and network —Local loop• Uses currently installed twisted pair cable —Can carry broader spectrum —1 MHz or more 17
  18. 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 18
  19. 19. Required Reading• Stallings chapter 8• Web sites on —ADSL —SONET 19
  20. 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? 20