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
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
n
B   Bi
i 1
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 1550nm wavelength range
• 50GHz 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