Ch 6. Multiplexing and Spreading (Bandwidth Utilization)

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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

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