Data Communications and Networking Chapter 6 Multiplexing References

Data Communications and Networking Chapter 6 Multiplexing References: Book Chapter 6 Data and Computer Communications, 6 th 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 Wi. Fi: 1 hour 2

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. 3

Multiplexing How to share the capacity of a data link? § FDM: Frequency Division Multiplexing § TDM: Time Division Multiplexing • Synchronous TDM • Statistical TDM 4

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 5

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. 6

Frequency Division Multiplexing Diagram 7

FDM System 8

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. 9

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 1550 nm wavelength range • 50 GHz per channel 10

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 11

Time Division Multiplexing 12

FDM vs TDM 13

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. 14

TDM System N Channels: 15

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 16

Synchronous TDM vs. Statistical TDM 17