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
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.ecolours) of laser light.
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 • Multiple-slot 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)
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