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 RatesB
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