FDM Useful bandwidth of medium exceeds required bandwidth of channel Each signal is modulated to a different carrier frequency Carrier frequencies separated so signals do not overlap (guard bands) . broadcast radio Channel allocated even if no data | William Stallings Data and Computer Communications Chapter 8 Multiplexing Multiplexing * Frequency Division Multiplexing FDM Useful bandwidth of medium exceeds required bandwidth of channel Each signal is modulated to a different carrier frequency Carrier frequencies separated so signals do not overlap (guard bands) . broadcast radio Channel allocated even if no data * Frequency Division Multiplexing Diagram * FDM System * FDM of Three Voiceband Signals * Analog Carrier Systems AT&T (USA) Hierarchy of FDM schemes Group 12 voice channels (4kHz each) = 48kHz Range 60kHz to 108kHz Supergroup 60 channel FDM of 5 group signals on carriers between 420kHz and 612 kHz Mastergroup 10 supergroups * Synchronous Time Division Multiplexing Data rate of medium exceeds data rate of digital signal to be transmitted Multiple digital signals interleaved in time May be at bit level of blocks Time slots preassigned to sources and fixed Time slots allocated even if no data Time slots do not have to be | William Stallings Data and Computer Communications Chapter 8 Multiplexing Multiplexing * Frequency Division Multiplexing FDM Useful bandwidth of medium exceeds required bandwidth of channel Each signal is modulated to a different carrier frequency Carrier frequencies separated so signals do not overlap (guard bands) . broadcast radio Channel allocated even if no data * Frequency Division Multiplexing Diagram * FDM System * FDM of Three Voiceband Signals * Analog Carrier Systems AT&T (USA) Hierarchy of FDM schemes Group 12 voice channels (4kHz each) = 48kHz Range 60kHz to 108kHz Supergroup 60 channel FDM of 5 group signals on carriers between 420kHz and 612 kHz Mastergroup 10 supergroups * Synchronous Time Division Multiplexing Data rate of medium exceeds data rate of digital signal to be transmitted Multiple digital signals interleaved in time May be at bit level of blocks Time slots preassigned to sources and fixed Time slots allocated even if no data Time slots do not have to be evenly distributed amongst sources * Time Division Multiplexing * TDM System * TDM Link Control No headers and tailers Data 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 * Data Link Control on TDM * Framing No flag or SYNC characters bracketing TDM frames Must provide synchronizing mechanism Added digit framing One control bit added to each TDM frame Looks like another channel - “control channel” Identifiable bit pattern used on control channel . alternating 01010101 unlikely on a data channel Can compare incoming bit patterns on each channel with sync pattern * Pulse Stuffing Problem - Synchronizing data sources Clocks in different sources drifting Data rates from different sources not related by simple rational number Solution - .
