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Multiplexing and Spreading 01204325: Data Communication and Computer Networks Asst. Prof. Chaiporn Jaikaeo, Ph.D.

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Presentation on theme: "Multiplexing and Spreading 01204325: Data Communication and Computer Networks Asst. Prof. Chaiporn Jaikaeo, Ph.D."— Presentation transcript:

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 2 Sharing Medium MUXMUX DEMUXDEMUX A link is divided into channels A link is divided into channels n signals (input) n signals (output) 1 link, n channels MultiplexerDemultiplexer

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

4 4 Conceptual View of FDM Channel 3 Channel 2 Channel 1 f1f1 f2f2 f3f3 Frequency Time

5 5 FDM: Multiplexing Process

6 6 FDM: Demultiplexing Process

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

8 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 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 10 Analog Hierarchy FDMFDM FDMFDM FDMFDM FDMFDM 5 groups … 10 supergropus 6 master groups group supergroup master group Jumbo group 4 kHz … 12 voice channels Used by AT&T Used by AT&T 48 kHz 12 voice channels 240 kHz 60 voice channels 2.52 MHz 600 voice channels MHz 3600 voice channels

11 11 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. Example

12 12 MultiplexerDemultiplexer Wavelength Division Multiplexing (WDM) WDM is a special case of FDM WDM is a special case of FDM Fiber-optic cable WDM An analog multiplexing technique to combine optical signals

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

14 14 Conceptual View of TDM Time Frequency Channel 3 Channel 2 Channel 1 Channel 3 Channel 2 Channel 1 Channel 3 Channel 2 Channel MUXMUX DEMUXDEMUX Data flow

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

16 16 TDM: Example Four 1-Kbps connections are multiplexed together. A unit is 1 bit. Find: (1) the duration of 1 bit before multiplexing, (2) the transmission rate of the link, (3) the duration of a time slot, and (4) the duration of a frame? Four 1-Kbps connections are multiplexed together. A unit is 1 bit. Find: (1) the duration of 1 bit before multiplexing, (2) the transmission rate of the link, (3) the duration of a time slot, and (4) the duration of a frame?

17 17 Empty Slot

18 18 Synchronization BCA1 … … Synchronization pattern Multiplexer and demultiplexer must be synchronized Multiplexer and demultiplexer must be synchronized Framing bits are used to provide synchronization Framing bits are used to provide synchronization BCA0BCA1BCA0 framing bit

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

20 20 Multi-Level Multiplexing

21 21 TDM of Different Data Rates Data rate from one source may be faster than the others Data rate from one source may be faster than the others More than one time slot can be assigned to certain sources More than one time slot can be assigned to certain sources 4 kbps 8 kbps16 kbps MUX A B C 1 frame BCAB Note: assume 1 unit = 1 bit 4 kbps 8 kbps 4 kbps 16 kbps MUX A B C

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

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

24 24 Digital Signal (DS) Hierarchy TDMTDM TDMTDM TDMTDM DS-1 DS-2 DS-3 DS-4 DS-0 TDMTDM … 24 channels Mbps 24 DS Mbps 4 DS Mbps 7 DS Mbps 6 DS-3 64 kbps 1 kbps = 1,000 bps 1 Mbps = 1,000 kbps

25 25 DS Services and T Lines ServiceLine Rate (Mbps) Voice Channels DS-1T DS-2 DS-2T DS-3 DS-3T DS-4T DS-0, DS-1, etc, are services DS-0, DS-1, etc, are services T lines are used to implement these services T lines are used to implement these services

26 26 T Lines and Analog Signals

27 27 T-1 Frame Structure

28 28 E Lines European's version of T lines European's version of T lines Also used in Thailand Also used in Thailand E Line Rate (Mbps) Voice Channels E E E E

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

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

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

32 32 FHSS Cycles

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

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

35 35 DSSS Example


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