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Data Communications and Networking. Chapter 6 Multiplexing References: Book Chapter 6 Data and Computer Communications, 6th edition, by William Stallings. Schedule. Signal encoding: 5 hours Multiplexing: 1 hour Error detection & correction: 2 hours Flow control & error control: 3 hours
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Data Communications and Networking Chapter 6 Multiplexing References: Book Chapter 6 Data and Computer Communications, 6th edition, by William Stallings
Schedule Signal encoding: 5 hours Multiplexing: 1 hour Error detection & correction: 2 hours Flow control & error control: 3 hours Circuit switching & packet switching: 2.5 hours Routing: 2.5 hours LAN overview: 2 hours Ethernet: 2 hours WiFi: 1 hour
Outline • Frequency division multiplexing • can be used with analog signals. • Synchronous time division multiplexing • can be used with both digital signals and analog signals. • Statistical time division multiplexing • can be used with both digital signals and analog signals.
Multiplexing • How to share the capacity of a data link? • FDM: Frequency Division Multiplexing • TDM: Time Division Multiplexing • Synchronous TDM • Statistical TDM
Frequency Division Multiplexing • FDM: A number of signals can be carried simultaneously. • Each signal is modulated to a different carrier frequency • Carrier frequencies are sufficiently separated so signals do not overlap (guard bands) • Available bandwidth of medium exceeds the sum of all channels • Examples: broadcast radio, cable TV • Channel allocated even if no data
Terminologies • Channel (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.
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, a total of 10.1 Tbps.
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 • Mostly 1550nm wavelength range • 50GHz per channel
Synchronous Time Division Multiplexing • Data rate of medium exceeds data rate of digital signal to be transmitted • Multiple digital signals interleaved in time • Can be at the bit level or in blocks • 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
Terminologies • Frames: a cycle of time slots, each of which is dedicated to a data source. • Channel (TDM): the sequence of slots dedicated to one source, from frame to frame, is called a channel.
TDM System N Channels:
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 • Data rate on line lower than aggregate rates of input lines
