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= n 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= nKHz, where n= 1, 2, …, ~ 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?

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

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 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