Multiplexing and spreading techniques are used to combine multiple signals over a shared medium such as a communications channel. Frequency division multiplexing (FDM) combines signals by assigning each signal a unique, non-overlapping frequency band. Time division multiplexing (TDM) divides the available time on a channel into time slots that are allocated to individual signals in a cyclical, non-overlapping manner. Spread spectrum techniques like frequency-hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS) spread signals across a wider bandwidth to improve interference resistance and security.

1. Multiplexing and Spreading
Data Communication and
Computer Networks
Presentation By:-
Pawan Singh Raikhola

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

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

5. 5
FDM: Multiplexing Process

6. 6
FDM: Demultiplexing Process

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

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 3 kHz 3 kHz 3 kHz 3 kHz
1.5 kHz 1.5 kHz 1.5 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
F
D
M F
D
M
F
D
M
F
D
M
5
groups
…
10
supergropus
6
master
groups
group
supergroup
master
group
Jumbo
group
4 kHz
4 kHz
4 kHz
…
12
voice
channels
 Used by AT&T
48 kHz
12 voice channels
240 kHz
60 voice channels
2.52 MHz
600 voice channels
16.984 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 833.33. 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
Multiplexer Demultiplexer
Wavelength Division Multiplexing (WDM)
 WDM is a special case of FDM
1
2
3
1
2
3
1+2+3
Fiber-optic cable
WDM WDM
1
2
3
1
2
3
1+2+3
An analog multiplexing technique to combine optical signals

13. 13
 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
Time Division Multiplexing (TDM)
T
D
M
A
B
C
A
B
C
A
B
C
A
B
C
Frame
Time slot
A digital multiplexing technique to combine data
one unit

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

15. 15
TDM Frames
 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?

17. 17
Empty Slot

18. 18
Synchronization
B
C A 1
… 1 0 1 0 …
Synchronization pattern
 Multiplexer and demultiplexer must be
synchronized
 Framing bits are used to provide synchronization
B
C A 0 B
C A 1 B
C A 0
framing bit

19. 19
Synchronization: Example
 Identify all the framing bits from this bit
sequence output by a multiplexer
 2 channels
 1 unit = 2 bits
0 0 0 1 1 0 1 1 1 0 0 0 0 1 0 0 1 0 1 1

20. 20
Multi-Level Multiplexing

21. 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
4 kbps
8 kbps 16 kbps
MUX
A
B
C
1 frame
B
C A
B
Note: assume 1 unit = 1 bit
4 kbps
4 kbps
8 kbps
4 kbps
4 kbps
16 kbps
MUX
A
B
C

22. 22
TDM of Different Data Rates
5 kbps
3 kbps
8 kbps
A
B
A
B A A A A
B B
B B B
A A A A A
1 frame
A
B A A A A
B B
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
 Solution – Bit padding
9 kbps
Bit
Padding
MUX
A A
B A A
B
B A
A A
B
3000 cycles/s
Extra bit
1 ms
Note: assume 1 unit = 1 bit
A A
B A A
B
B A
A A
A A
A
B
B
B

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

25. 25
DS Services and T Lines
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-4 274.176 4032
 DS-0, DS-1, etc, are 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
 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-4 139.264 1920

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

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

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

32. 32
FHSS Cycles

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

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

35. 35
DSSS Example