Multiplexing and Spreading
(Bandwidth Utilization)

1. Multiplexing
• Multiplexing is the set of techniques that allow the simultaneous transmission of multiple signals across a single data link
• Improves link efficiency by “sharing”
• Categories
– Frequency-Division Multiplexing (FDM)
– Wavelength-Division Multiplexing (WDM)
– Time-Division Multiplexing (TDM)
– Code-Division Multiplexing (CDM)

Mux and Demux
• Diving a link into multiple (n) channels using mux and demux

Frequency-Division Multiplexing (FDM)
• Analog multiplexing technique that combines analog signals

FDM Muxing

FDM Demuxing

Example: Voice channel
• We want to combine three voice channels into a link
• Each voice channel occupies 4 kHz
• Link has a bandwidth of 12 kHz, from 20 to 32 kHz

Modulate each of the three voice channels to a different bandwidth

Hierarchical Muxing
60 ~ 108 KHz.
312 ~ 552 KHz
The first multiplexing step combines 12 voice inputs into a basic group, which is formed by having the nth input modulate a carrier at frequency fc= 60 + 4n KHz, where n = 1,2, …, 12.
The next step in the FDM hierarchy involves the combination of five basic groups into a supergroup. This is accomplished by using the nth group to modulate a carrier of frequency fc= 372 + 48nKHz, where n= 1, 2, …, 5.

Guard band
• Example: muxing of five (100kHz) channels

Wavelength-Division Multiplexing
(WDM)
• Is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e
colours) of laser light.

Prisms for Mulxing and Demuxing

Time-Division Multiplexing (TDM)
• Digital multiplexing combining multiple low rate channels into a single high rate channel

Synchronous TDM
• Link has a speed-up of n to combine n channels

TDM Example 1
• Synchronous TDM system
– Combine four 1 Mbps streams
– Unit of data is 1 bit
• Questions
– Input bit duration?
– Output bit duration?
– Output bit rate?
– Output frame rate?

Questions
Input bit duration?
Output bit duration?
Output bit rate?
Output frame rate?
1 bit / 1Mbps = 1 μs
1/4 μs
4 Mbps
1 M frames / s

Multiple Multiplexing
What if the data rates are not the same among inputs?
• Multi-level Multiplexing
When the data rate is a multiple of others.
• Multiple-slot Multiplexing

Empty Slot and Stuffing
• Empty slot
• Stuffing
When the data rate are not multiple integers of each others.

Frame Synchronization
• Synchronization between the multiplexer and demultiplexer is a major issue.
• If not synchronized, a bit belonging to one channel may be received by a wrong channel.
• Additional information is need: frame bits

TDM Example 2
• Synchronous TDM system
– Combine four 250 characters/s streams
– Unit of data is 1 character (8 bits)
– 1 synchronization bit is added to each frame
• Questions
– Input character duration?
– Output frame rate?
– Output frame duration?
– Output bit rate?
1 / 250 = 4 ms
250 frames / s
1 / 250 = 4 ms
33 x 250 bits / s
* Each frame has 33 (= 4 x 8 + 1) bits

Multiplexing Hierarchy
• Telephone system (US)

T-1 Line Multiplexing

European Telephone System

Statistical TDM
• Inefficiency from reserving time slots
– Some inputs have high rate, some have low rate
– Some inputs’ rate changes across time
• Dynamic slot allocation for high efficiency

• Comparison between synchronous TDM and statistical TDM
Addressing for each line

2. Spread Spectrum
• Combine signals from different sources to fit into a larger bandwidth, but prevent eavesdropping and jamming
• For this, spread spectrum techniques add redundancy
• Two well-known methods
– Frequency Hopping Spread Spectrum (FHSS)
– Direct Sequence Spread Spectrum Synchronous
(DSSS)

Idea of Spread Spectrum
• Use more bandwidth

FHSS System
• Frequency hopping spread spectrum (at sender)

Frequency Selection
• Time is divided into cycles of the same length, each cycle has multiple periods (called “hop period”)
– Different frequency is used at a different period
– Each cycle has the same pattern of frequencies
– The patterns are known to both sender and receiver
** pattern during a cycle

FHSS Cycles
• Frequency usage i.e., time

FDM vs. FHSS
• Recall the reasons why we spread signals
– Security issues – eavesdropping, and jamming

DSSS System
• Direct Sequence Spread Spectrum
Synchronous (at sender)

Spread Signal of DSSS
• At each period, codes change instead of freq.
• To decode, one should know the seq. of codes

Homework
• Exercise in Chap. 6
– 16
– 18 (Assume that the system can support 6 active sources simultaneously.)
– 23