3 Outline Frequency division multiplexing can be used with analog signals.Synchronous time division multiplexingcan be used with both digital signals and analog signals.Statistical time division multiplexing
4 Multiplexing How to share the capacity of a data link? FDM: Frequency Division MultiplexingTDM: Time Division MultiplexingSynchronous TDMStatistical TDM
5 Frequency Division Multiplexing FDM: A number of signals can be carried simultaneously.Each signal is modulated to a different carrier frequencyCarrier frequencies are sufficiently separated so signals do not overlap (guard bands)Available bandwidth of medium exceeds the sum of all channelsExamples: broadcast radio, cable TVChannel allocated even if no data
6 TerminologiesChannel (FDM): each modulated signal requires a certain bandwidth centered on its carrier frequency, referred to as a channel.Subcarrier: each of the multiple carriers is referred to as a subcarrier. Its frequency is denoted by fi . fi must be chosen so that the bandwidths of various signals do not significantly overlap.
9 Wavelength Division Multiplexing Multiple beams of light at different frequencyCarried by optical fiberA form of FDMEach color of light (wavelength) carries separate data channel1997 Bell Labs100 beamsEach at 10 GbpsGiving 1 terabit per second (Tbps)Commercial systems of 160 channels of 10 Gbps now availableLab systems (Alcatel) 256 channels at 39.8 Gbps each, a total of 10.1 Tbps.
10 WDM Operation Same general architecture as other FDM Number of sources generating laser beams at different frequenciesMultiplexer consolidates sources for transmission over single fiberOptical amplifiers amplify all wavelengthsTypically tens of km apartDemux separates channels at the destinationMostly 1550nm wavelength range50GHz per channel
11 Synchronous Time Division Multiplexing Data rate of medium exceeds data rate of digital signal to be transmittedMultiple digital signals interleaved in timeCan be at the bit level or in blocksTime slots pre-assigned to sources and fixedTime slots allocated even if no dataTime slots do not have to be evenly distributed amongst sources
16 Statistical TDM In Synchronous TDM many slots are wasted Statistical TDM allocates time slots dynamically based on demandMultiplexer scans input lines and collects data until frame fullData rate on line lower than aggregate rates of input lines