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

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.

Frequency Division Multiplexing Diagram

FDM System

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

Time Division Multiplexing

FDM vs TDM

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

Synchronous TDM vs. Statistical TDM