1. Data Communications and Networking
Chapter 6
Multiplexing
References:
Book Chapter 6
Data and Computer Communications, 6th edition, by William Stallings

2. 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

3. 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

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

5. 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

6. 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.

7. Frequency Division Multiplexing Diagram

8. FDM System

9. 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.

10. 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

11. 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

12. Time Division Multiplexing

13. FDM vs TDM

14. 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.

15. TDM System
N Channels:

16. 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

17. Synchronous TDM vs. Statistical TDM