1. Multiplexing and Demultiplexing
Chapter 11
Multiplexing and Demultiplexing
© Bobby Hoggard, Department of Computer Science, East Carolina University
These slides may not be used or duplicated without permission

2. Multiplexing / Demultiplexing
Multiplexing: combining information from several sources for transmission over a shared medium
Demultiplexing: separating combined information from a shared medium back into separate information streams
10010
10010
Source A
Dest A
0011
0011
Source B
Multiplexer
Demultiplexer
Dest B
shared medium
11101
11101
Source C
Dest C

3. Types of Multiplexing
Time Division (TDM) Each sender gets a block of time to send their data Only one sender can transmit at a time
Frequency Division (FDM) Each sender transmits data all at the same time, but on different frequencies
Wavelength Division (WDM) The same as frequency division multiplexing, but FDM is typically used for radio waves whereas WDM is typically used for light
Code Division (CDM) Uses mathematical formulas to combine the data

4. Asynchronous Time Division Multiplexing
Transmits data from each sender, one at a time
There are gaps between transmissions if there is no data to send
Source A
Dest A C B A C B A
Source B
Multiplexer
Demultiplexer
Dest B
Source C
Dest C

5. Synchronous Time Division Multiplexing
Transmits data from each sender, one at a time
There are no gaps or delays between transmissions
Source A
Dest A A C B A C B A C B A C B A C B A C B A
Source B
Multiplexer
Demultiplexer
Dest B
Source C
Dest C

6. Telephone System Multiplexing
The telephone system uses a version of synchronous time division multiplexing
An extra "synch" bit is included after each round to ensure sender/receiver stays synchronized
The synch bit alternates between 0 and 1
synch bits
Source A
Dest A 1 1 C B A C B A 1 C B A
Source B
Multiplexer
Demultiplexer
Dest B
Source C
Dest C

7. Unfilled Slots
Synchronous TDM works well if all sources produce data at a uniform rate
If a source has no data to produce then time slots are actually filled with idle bits
These "idle frames" unnecessarily take up time where another source could be using the network to send data
Dest A A3 A2 A1 B2 B1
Dest B A1 C1 B2 A3 D2 A2 D1 B1 A1
Multiplexer
Demultiplexer C1
Dest C D2 D1
Dest D

8. Statistical TDM Skips any source that does not have data ready Dest AB2 B1
Dest B C1 B2 A3 D2 A2 D1 B1 A1
Multiplexer
Demultiplexer C1
Dest C D2 D1
Dest D

9. Statistical TDM Advantage
Takes less time to send the same amount of data, and therefore increases the overall data rate of the network
Synchronous TDM
Takes 11 time units to send all of the data: C1 B2 A3 D2 A2 D1 B1 A1
Statistical TDM
Takes 8 time units to send the same data: C1 B2 A3 D2 A2 D1 B1 A1

10. Statistical TDM Disadvantage
Extra bits must be added to identify the receivers to which the data is being sent
Receiver X's data is expected in every 4th time block
Synchronous TDM D C B A D C B A D C B A 1 2 3 2 2 1 1 1
Statistical TDM
Since we don't know when Sender X's data will arrive,
extra bits must be added to identify each receiver
1 C
2 B
3 A
2 D
2 A
1 D
1 B
1 A

11. Frequency Division Multiplexing
Each sender transmits data all at the same time
Each sender uses a different channel (i.e. a different carrier frequency)
The different signals are combined together into a single composite wave
Source A
Dest A
Multiplexer
Demultiplexer
Source B
Dest B

12. FDM Demultiplexer is a Set of Filters
Each filter extracts a range of frequencies around the carrier frequency
Dest A
Carrier:
440Hz
Carrier:
850Hz
Carrier:
1275Hz
Filter allows
410Hz – 470Hz
Filter allows
820Hz – 880Hz
Dest B
Filter allows
1245Hz – 1305Hz
Dest C

13. FDM Channels Are Independent
FDM gives the illusion of each sender/receiver pair having a private communication line on a separate physical medium
Any modulation technique can be used on each channel, and each channel can use a different one
Source A
Dest A
Channel A
Source B
Multiplexer
Demultiplexer
Dest B
Channel B
Channel C
Source C
Dest C

14. Example 200KHz channel assignments with 20KHz guard bands
Interference can occur if the frequencies of two channels are too close together
Carrier frequencies on a network are chosen with a gap (guard band) between them to reduce the possibility of interference
Likewise, the FCC regulates radio stations to make sure there's enough spacing between frequencies to reduce interference between stations
Example 200KHz channel assignments with 20KHz guard bands
Channel
Frequencies Used 1
100KHz – 300KHz 2
320KHz – 520KHz 3
540KHz – 740KHz 4
760KHz – 960KHz 5
980KHz – 1180KHz 6
1200KHz – 1400KHz
20 KHz guard band
1400
1200
1000
800
600
400
200 6
KHz 5 4 3 2 1

15. Why Allocate a Range of Frequencies?
Frequency Modulation FM modulates between multiple frequencies Note: Amplitude and Phase modulation technically only need one frequency
Increase Data Rate Use the extra frequencies in the range to transmit individual bits of data
Increase Immunity to Interference Use the extra frequencies in the range to transmit copies of the same data

16. Increased Data Rate
Use the extra frequencies in the range to transmit individual bits of data
Example:
Divide the frequency range into 8 smaller frequency blocks
Transmit one bit of data in each of the smaller frequency blocks
Result: parallel transmission of 1 byte of data
Channel 1:
100KHz to
300KHz
105KHz – 120KHz
280KHz – 295KHz
130KHz – 145KHz
155KHz – 170KHz
180KHz – 195KHz
205KHz – 220KHz
230KHz – 245KHz
255KHz – 270KHz 1
8 bits of data transmitted simultaneously One bit on each sub-channel
It's using frequency division multiplexing
for subsections of a larger
frequency division multiplexed system

17. Increased Immunity to Interference
Use the extra frequencies in the range to transmit copies of the same data
Example:
Divide the frequency range into 3 smaller frequency blocks
Transmit the same data in each of the frequency blocks
Result: a high probability that one of the 3 will make it to the receiver with no interference
Channel 1:
100KHz to
300KHz
100KHz – 150KHz
250KHz – 300KHz
175KHz – 225KHz
error
Interference
203KHz – 267KHz

18. Hierarchial FDM
Because FDM requires each source to have a different frequency range, if sources have the same frequencies, they can be shifted into a different ranges so they can be properly multiplexed
0 – 4KHz
0 – 4KHz
2 telephone
channels
0 – 8KHz
0 – 4KHz
4KHz – 8KHz
0 – 8KHz
0 – 16KHz
0 – 4KHz
4KHz – 8KHz
0 – 4KHz
8KHz – 16KHz
2 telephone
channels
0 – 8KHz
0 – 4KHz

19. 1 jumbo group of 3600 phone channels
Hierarchial FDM
FDM hierarchy used in the telephone system
10 super groups of 60 phone
channels per group
240 KHz each
5 groups of 12 phone
channels per group
48 KHz each
6 master groups of 600 phone
channels per group
2.40 MHz each 12
phone
channels
4 KHz
each
1 jumbo group of phone channels
14.4 MHz total bandwidth

20. Wavelength Division Multiplexing
The application of frequency division multiplexing to optical fiber
The inputs/outputs of this multiplexing are wavelengths of light
A prism can demultiplex by taking white light and spread it out into separate colors
A prism can also multiplex by taking separate colors of light and focus them into a single beam of white light
Source A
Dest A
optical fiber
beam of light
Source B
Dest B
Source C
Dest C

21. Inverse Multiplexing
Using several low speed connections in parallel to achieve a single high speed connection
Ten, 10 Gbps connections
100 Gbps
connection
100 Gbps
connection
Demultiplexer
Multiplexer
10 Gbps connection

22. Code Division Multiplexing
Does not rely on the physical properties of time or frequency, but instead uses mathematical formulas to combine and separate data
Each sender is assigned a chip sequence, which is a binary value used to encode their data
The encoded values are added together and transmitted as a single data value
The receiver uses the chip sequence to help split the data and retrieve the portions that it wants
The chip sequence acts like mathematical filter, to filter out the unwanted data and keep the desired data
Used in the cellular telephone system and in some satellite communications
The specific variation used by the cell phone system is called CDMA (Code Division Multi-Access)

23. CDM Send Example Step #1: Convert the values to vectors of bits
2 Senders / 2 Receivers
Sender
Chip Sequence
Data To Send A
1 0 B
1 1
Step #1: Convert the values to vectors of bits
Use -1 to represent binary 0 and use 1 to represent binary 1
1 0
Chip Sequence A
( 1, -1 )
Data A
( 1, -1, 1, -1 )
1 1
Chip Sequence B
( 1, 1 )
Data B
( -1, 1, 1, -1 )

24. CDM Send Example Step #2: Multiply the chip sequence by the data
2 Senders / 2 Receivers
Sender
Chip Sequence
Data To Send A
1 0 B
1 1
Step #2: Multiply the chip sequence by the data
Chip Sequence A
( 1, -1 )
( 1, -1 )
( -1, 1 )
( 1, -1 )
( -1, 1 )
Data A
( 1, -1, 1, -1 )
Chip Sequence B
( 1, 1 )
( -1, -1 )
( 1, 1 )
( 1, 1 )
( -1, -1 )
Data B
( -1, 1, 1, -1 )

25. Data to be transmitted--- i.e. a sequence of signal strengths
CDM Send Example
2 Senders / 2 Receivers
Sender
Chip Sequence
Data To Send A
1 0 B
1 1
Step #3: Add the values together
( 1, -1 )
( -1, 1 )
( 1, -1 )
( -1, 1 ) +
( -1, -1 )
( 1, 1 )
( 1, 1 )
( -1, -1 )
Data to be transmitted---
i.e. a sequence of signal strengths

26. CDM Receive Example
2 Senders / 2 Receivers
Sender
Chip Sequence
Data To Send A
1 0 B
1 1
Step #1: Treat the received signal strengths as vectors
Receiver A
Receiver B
( 0, -2 )
( 0, 2 )
( 2, 0 )
( -2, 0 )
( 0, -2 )
( 0, 2 )
( 2, 0 )
( -2, 0 )

27. CDM Receive Example ( ) ( * ) *
2 Senders / 2 Receivers
Sender
Chip Sequence
Data To Send A
1 0 B
1 1
Step #2: Multiply the vectors by the chip sequence of the desired sender
Receiver A
Receiver B ( ) (
( 1, -1 ) *
( 0, -2 )
( 0, 2 )
( 2, 0 )
( -2, 0 )
( 1, 1 ) * )
( 0, -2 )
( 0, 2 )
( 2, 0 )
( -2, 0 )
( 0, 2 )
( 0, -2 )
( 2, 0 )
( -2, 0 )
( 0, -2 )
( 0, 2 )
( 2, 0 )
( -2, 0 )

28. CDM Receive Example Step #3: Add each group values Receiver A
2 Senders / 2 Receivers
Sender
Chip Sequence
Data To Send A
1 0 B
1 1
Step #3: Add each group values
Receiver A
Receiver B
( 0, 2 )
( 0, -2 )
( 2, 0 )
( -2, 0 )
( 0, -2 )
( 0, 2 )
( 2, 0 )
( -2, 0 )
0 + 2
2 + 0
0 + 2
2 + 0 2 -2 2 -2 -2 2 2 -2

29. CDM Receive Example 1 1 1 1 Step #4: Convert to binary Receiver A
2 Senders / 2 Receivers
Sender
Chip Sequence
Data To Send A
1 0 B
1 1
Step #4: Convert to binary
Negative values represent binary 0 and positive values represent binary 1
Receiver A
Receiver B 2 -2 2 -2 -2 2 2 -2 1 1 1 1