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Multiplexing and Spreading

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1 Multiplexing and Spreading
: Data Communication and Computer Networks Asst. Prof. Chaiporn Jaikaeo, Ph.D. Computer Engineering Department Kasetsart University, Bangkok, Thailand Adapted from lecture slides by Behrouz A. Forouzan © The McGraw-Hill Companies, Inc. All rights reserved

2 Sharing Medium A link is divided into channels 1 link, n channels
n signals (input) M U X D E M U X n signals (output) Multiplexer Demultiplexer

3 Frequency Division Multiplexing (FDM)
Medium BW > Channel BW Each signal is modulated to a different carrier frequency E.g., broadcast radio Channel allocated even if no data An analog multiplexing technique to combine signals

4 Conceptual View of FDM Channel 3 Channel 2 Channel 1 Time f3 f2 f1
Frequency Time

5 FDM: Multiplexing Process

6 FDM: Demultiplexing Process

7 Guard Bands Strips of unused bandwidth to prevent signals from overlapping 3 kHz f f 3 kHz FDM f 3 kHz FDM (no guard band) f 3 kHz f 3 kHz FDM (with guard band)

8 FDM: Example 1 Five voice channels, each with 3-kHz bandwidth, are to be multiplexed together. If there is a need for a guard band of 1.5 kHz, what is the minimum bandwidth of the link? f 3 kHz 1.5 kHz 21 kHz

9 FDM: Example 2 Four digital-data channels, 1 Mbps each, use a satellite channel of 1 MHz. Design an appropriate configuration using FDM

10 Analog Hierarchy Used by AT&T … … 12 voice channels F D M F D M F D M
4 kHz 48 kHz 12 voice channels 12 voice channels 4 kHz 240 kHz 60 voice channels group F D M 2.52 MHz 600 voice channels 5 groups supergroup F D M 10 supergropus master group MHz 3600 voice channels F D M 6 master groups Jumbo group F D M

11 Example The Advanced Mobile Phone System (AMPS) uses two bands. The first band of 824 to 849 MHz is used for sending, and 869 to 894 MHz is used for receiving. Each user has a bandwidth of 30 kHz in each direction. How many people can use their cellular phones simultaneously? Solution Each band is 25 MHz. If we divide 25 MHz by 30 kHz, we get In reality, the band is divided into 832 channels. Of these, 42 channels are used for control, which means only 790 channels are available for cellular phone users.

12 Wavelength Division Multiplexing (WDM)
An analog multiplexing technique to combine optical signals WDM is a special case of FDM WDM 1 2 3 1+2+3 1 1 Multiplexer Demultiplexer 1+2+3 Fiber-optic cable 2 2 3 3

13 Time Division Multiplexing (TDM)
A digital multiplexing technique to combine data Medium Data Rate > Signal Data Rate Multiple digital signals interleaved in time Time slots are preassigned to sources and fixed are allocated even if no data do not have to be evenly distributed among sources one unit T D M A B C Frame Time slot

14 Conceptual View of TDM M U X 1 2 3 1 Data flow 2 D E M U X 3 Time
Frequency Channel 3 Channel 2 Channel 1

15 TDM Frames A frame consists of one complete cycle of time slots

16 TDM: Example Four 1-Kbps connections are multiplexed together. A unit is 1 bit. Find: the duration of 1 bit before multiplexing, the transmission rate of the link, the duration of a time slot, and the duration of a frame?

17 Empty Slot

18 Synchronization Multiplexer and demultiplexer must be synchronized
Framing bits are used to provide synchronization Synchronization pattern … … B C A 1 B C A B C A 1 B C A framing bit

19 Synchronization: Example
Identify all the framing bits from this bit sequence output by a multiplexer 2 channels 1 unit = 2 bits 1 1 1 1 1 1 1 1 1

20 Multi-Level Multiplexing

21 TDM of Different Data Rates
Data rate from one source may be faster than the others More than one time slot can be assigned to certain sources 4 kbps 1 frame B C A A MUX 8 kbps 16 kbps B 4 kbps C 4 kbps 8 kbps 16 kbps MUX A B C Note: assume 1 unit = 1 bit

22 TDM of Different Data Rates
B 1 frame A B 5 kbps MUX A A 8 kbps 3 kbps B B Note: assume 1 unit = 1 bit

23 Bit Padding Different data rates may not be integer multiples of each other Solution – Bit padding 1 ms 5 kbps 3 kbps A B 6 kbps A B Bit Padding MUX 9 kbps Extra bit A B 3000 cycles/s A Note: assume 1 unit = 1 bit B

24 Digital Signal (DS) Hierarchy
64 kbps 1.544 Mbps 24 DS-0 24 channels DS-0 T D M DS-0 6.312 Mbps 4 DS-1 DS-1 Mbps 7 DS-2 DS-2 T D M Mbps 6 DS-3 DS-3 T D M T D M DS-4 1 kbps = 1,000 bps 1 Mbps = 1,000 kbps

25 DS Services and T Lines DS-0, DS-1, etc, are services
T lines are used to implement these services Service Line Rate (Mbps) Voice Channels DS-1 T-1 1.544 24 DS-2 T-2 6.312 96 DS-3 T-3 44.736 672 DS-4 T-4 4032

26 T Lines and Analog Signals

27 T-1 Frame Structure

28 E Lines European's version of T lines Also used in Thailand E Line
Rate (Mbps) Voice Channels E-1 2.048 30 E-2 8.448 120 E-3 34.368 480 E-4 1920

29 Inverse Multiplexing Inverse MUX Inverse DEMUX low-speed lines
high-speed line low-speed lines

30 Spread Spectrum Spread signal to use larger bandwidth
To prevent eavesdropping To reduce effect from interference

31 Frequency-Hopping SS "FHSS" – Frequency-Hopping Spread Spectrum
Used in Bluetooth technology

32 FHSS Cycles

33 Direct-Sequence SS "DSSS" – Direct-Sequence Spread Spectrum
Used in Wireless LANs

34 DSSS and Interference Amplitude Narrow Band Signal
Frequency Amplitude Narrow Band Signal Narrow Band Interference Spread Spectrum Signal

35 DSSS Example

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