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  1. Multiplexing • Multiplexing is the set of techniques that allows multiple signal transmission across a single medium at the same time (simultaneously). • It can be applied when the bandwidth of a medium connecting two devices is greater than the bandwidth needs of the devices. • As such, the use of the medium is maximised. Physical Layer

  2. Figure 6.1Dividing a link into channels

  3. Multiplexing (Continued…) • Multiplexing techniques can be divided into three categories: • Frequency division multiplexing (FDM) • Wave division multiplexing (WDM) • Time division multiplexing (TDM) Physical Layer

  4. FDM • FDM is used when the bandwidth of the medium is greater than the combined bandwidths of the signals to be transmitted across it. • Signals generated by each device modulate different carrier frequencies. • The modulated signals are combined into a single composite signal and transported across the medium (link). Physical Layer

  5. FDM (Continued…) • To ensure successful recovery of data at the receiver, the following need to be followed. • Carrier frequencies are separated by sufficient bandwidth to accommodate the modulated signal. • Channel must be separated by strips of unused bandwidths (guard bands). • Carrier frequencies must not interfere with the original data signal frequencies. Physical Layer

  6. Figure 6.3FDM

  7. Figure 6.4FDM process

  8. Figure 6.5FDM demultiplexing example

  9. Figure 6.6Example 1

  10. Figure 6.7Example 2

  11. FDM (Continued…) • FDM are used in the following applications: • AM (amplitude modulation) and FM (frequency modulation) radio broadcasting – filtering at the receiving end is done by tuning. • TV broadcasting – filtering is done by choosing the desired channel. • First generation mobile phones. Physical Layer

  12. WDM • WDM is designed to make use of the high data rate capability of the fiber-optic cable. • Its operation is similar to the FDM, except that • The multiplexing and demultiplexing involve optical signals transmitted through the fiber-optic channels and • The frequencies involved is very high compared to those used in FDM. • Application - SONET Physical Layer

  13. Figure 6.10WDM

  14. Figure 6.11Prisms in WDM multiplexing and demultiplexing

  15. TDM • TDM is a digital process that allows several connections to share the high bandwidth of a link. • In TDM, a portion of a time is shared instead of bandwidth. • The data flow in each connection is divided into units. • The link combines one unit of each connection to make a frame. • The size of the unit can be 1 or several bits. Physical Layer

  16. TDM (Continued…) • For n connections, n time slots are at least required in each frame. • The data rate of the combined link = n x data rate of each connection. • Therefore, the duration of a unit in a connection is n times longer than its duration in the frame/combined link. • The operation of a TDM can be visualised as a a pair of rotating switch operating at the same speed but opposite direction. Physical Layer

  17. Figure 6.12TDM

  18. Figure 6.13TDM frames

  19. Figure 6.14Interleaving

  20. TDM (Continued…) • Synchronisation is required to ensure that the demultiplexer is able to sort out the incoming data accordingly. • Framing bits, which are alternately numbered, are included in each frame to provide this synchronisation. Physical Layer

  21. Figure 6.17Framing bits