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)

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

Mux and Demux

• Analog multiplexing technique that combines analog signals

Frequency-Division Multiplexing (FDM)

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

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.

60 ~ 108 KHz.

312 ~ 552 KHz

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

• Digital multiplexing combining multiple low rate channels into a single high rate channel

Time-Division Multiplexing (TDM)

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

Synchronous TDM

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?

QuestionsInput 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

What if the data rates are not the same among inputs?• Multi-level Multiplexing

• Multiple-slot Multiplexing

Multiple Multiplexing

When the data rate is a multiple of others.

Empty Slot and Stuffing

• Empty slot

• StuffingWhen 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