Multiplexing

1
Multiplexing

• NETE 0510
• Presented by
• Dr.Apichan Kanjanavapastit

2
What is Multiplexing

• Multiplexing is the set of techniques that allows
  the simultaneous transmission of multiple signals
  across a single data link
• Todays technology includes high-bandwidth media
  such as optical fiber, each of these has a
  carrying capacity far in excess of that needed
  for the average transmission signal
• An efficient system maximizes the utilization of
  all facilities

3
Many to One/One to Many

• In a multiplexed system, n devices share the
  capacity of one link
• The stream of traffic from each device is sent to
  a multiplexer (MUX), which combines them into a
  single stream (many to one)
• At the receiving end, that stream is fed into a
  demultiplexer (DEMUX), which separates the stream
  back into its component transmissions (one to
  many) and directs them to their intended
  receiving devices

4
Categories of Multiplexing

• Signals are multiplexed using 3 basic techniques
  frequency-division multiplexing (FDM),
  time-division multiplexing (TDM), and
  wave-division multiplexing (WDM)
• TDM is further subdivided into synchronous TDM
  (usually just called TDM) and asynchronous TDM,
  also called statistical TDM or concentrator

5
Frequency-Division Multiplexing (FDM)

• FDM is an analog technique
• In FDM, signals generated by each sending device
  modulate different carrier frequencies
• Carrier frequencies are separated by enough
  bandwidth to accommodate the modulated signal

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The FDM Process Multiplexing
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The FDM Process Demultiplexing
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Amplitude Modulation Techniques for Analog Signals

• Amplitude Modulation (AM)
• Double-sideband suppressed carrier (DSB-SC)
  modulation
• Single-sideband suppressed carrier (SSB)
  modulation

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Amplitude Modulation (AM)
AM modulator
0
4 kHz
10 kHz
6 kHz
14 kHz
10 kHz
10
Double-sideband suppressed carrier modulation
(DSB-SC)
DSB-SC modulator
0
4 kHz
10 kHz
6 kHz
14 kHz
10 kHz
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Single-sideband suppressed carrier (SSB)
DSB-SC modulator
Band Pass Filter
10 kHz
6 kHz
10 kHz
6 kHz
14 kHz
0
4 kHz
SSB
10 kHz
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Example1

• Five channels, each with a 100-kHz bandwidth, are
  to be multiplexed together. What is the minimum
  bandwidth of the link if there is a need for a
  guard band of 10 kHz between the channels to
  prevent interference?

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Example2

• Four digital signals, each transmitting at data
  rate of 1 Mbps, use a satellite channel of 1 MHz.
  Design an appropriate configuration, using FDM.

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ITU-T Multiplexing Plan for Analog Telephone
System
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ITU-T Multiplexing Plan for Analog Telephone
System (cont.)
Incoming Ch.
BPF
104-108 kHz
1
108 kHz
BPF
Multiplex signal
104-108 kHz
2
104 kHz
BPF
104-108 kHz
12
64 kHz
9
8
7
6
5
4
3
2
1
12
11
10
80
84
88
92
96
100
104
108
64
60
68
76
72
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12 voice frequency Channel input (0-4 kHz)
Supergroup output 60VF. 240 kHz bandwidth
108
F108
F3396
3048
2844
F104
104
F3148
Group output 12 VF. 48 kHz bandwidth
2836
Mastergroup output 600VF. 2.52 MHz bandwidth
2569
100
F100
F2900
2588
F96
96
2348
F612
552
F2652
2340
92
F92
2100
504
F564
F2356
2044
F88
88
1804
F516
456
84
F84
F2108
1796
408
F468
1556
F80
80
F1860
F420
360
1548
1308
76
F76
312
F1612
1300
5 Group inputs
1060
F72
72
10 Supergroup inputs
F1364
1052
68
F68
812
F1116
Supergroup multiplexer
804
F64
64
564
60
Mastergroup multiplexer
Group multiplexer
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Application Example of FDM Cable TV

• Coaxial cable has a bandwidth up to several
  hundreds megahertz
• The bandwidth of the coaxial cable is normally
  divided into 6 MHz using FDM
• Each band provides a TV channel or data
  transmission

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Wave-Division Multiplexing (WDM)

• WDM is conceptually the same as FDM, except that
  the multiplexing and demultiplexing involve light
  signals transmitted through fiber optic channels
• Combining and splitting of light sources are
  easily handled by a prism

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Recall Attenuation in Optical Fiber
First Window
Second Window
Third Window
ATTENUATION (dB/km)
WAVELENGTH (nm)
1310nm
1550nm
850nm
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Time-Division Multiplexing (TDM)

• TDM is a digital process that can be applied when
  the data rate capacity of the transmission medium
  is greater than the data rate required by the
  sending and receiving ends
• TDM can be implemented in 2 ways TDM and
  asynchronous TDM

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Synchronous TDM

• In synchronous TDM, the term synchronous has a
  different meaning from synchronous transmission
• Hear synchronous means that the multiplexer
  allocates exactly the same time slot to each
  device at all times, whether or not a device has
  anything to transmit
• Time slots are grouped into frames. A frame
  consists of one complete cycle of time slots

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Interleaving in Synchronous TDM

• Synchronous TDM can be compared to a very fast
  rotating switch
• As the switch opens in front of a device, that
  device has the opportunity to send a specified
  amount of data into the path
• The switch moves from device to device at a
  constant rate and in a fixed order. This process
  is called interleaving which can be done by bit,
  by byte, or by any other data unit

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Interleaving in Synchronous TDM (cont.)
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Example3

• The data rate for each input connection is 1
  kbps. If 1 bit at a time is multiplexed (a unit
  is 1 bit), what is the duration of (a) each input
  slot, (b) each output slot, and (c) each frame?

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Example4

• The figure below shows synchronous TDM with a
  data stream for each input and one data stream
  for the output. The unit of data is 1 bit. Find
  (a) the input bit duration, (b) the output bit
  duration, (c) the output bit rate, and (d) the
  output frame rate.

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Example5

• Four channels are multiplexed using TDM. If each
  channel sends 100 bytes /s and we multiplex 1
  byte per channel, show the frame traveling on the
  link, the size of the frame, the duration of a
  frame, the frame rate, and the bit rate for the
  link.

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Example6

• A multiplexer combines four 100-kbps channels
  using a time slot of 2 bits. Show the output with
  four arbitrary inputs. What is the frame rate?
  What is the frame duration? What is the bit rate?
  What is the bit duration?

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Multiple Slots Multiplexing
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Example7

• Two channels, one with a bit rate of 100 kbps and
  another with a bit rate of 200 kbps, are to be
  multiplexed. How this can be achieved? What is
  the frame rate? What is the frame duration? What
  is the bit rate of the link?

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Framing Bits in Synchronous TDM

• Because the time slot order in a synchronous TDM
  system does not vary from frame to frame, very
  little overhead information needs to be included
  in each frame
• However, various factors can cause timing
  inconsistencies. For this reason, one or more
  synchronization bits are usually added to the
  beginning of each frame

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Framing Bits in Synchronous TDM (cont.)

• These bits, called framing bits, that allows the
  demultiplexer to synchronize with the incoming
  stream so that it can separate the time slot
  accurately
• In most cases, this synchronization information
  consists of one bit per frame, alternating
  between 0 and 1

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Example of a Data Rate Calculation in
Synchronous TDM

• There are 4 input sources on a synchronous TDM
  link, where transmissions are interleaved by
  character
• If each source is creating 250 characters per
  second, and each frame is carrying 1 character
  from each source, the transmission path must be
  able to carry 250 frames per second
• Assuming that each frame is 33 bits long 32 bits
  for the 4 characters plus 1 framing bit the data
  rate of the multiplexed line will be 8250 bps
  (250 frames with 33 bits per frame)

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Bit Stuffing in Synchronous TDM

• Recall that the time-slot length is fixed and it
  is possible to connect devices of different data
  rate to a synchronous TDM
• For this technique to work, the different data
  rates must be integer multiples of each other
• When the speeds are not integer multiple of each
  other, they can be made to behave as if they
  were, by a technique called bit stuffing

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Bit Stuffing in Synchronous TDM (cont.)

• The multiplexer adds extra bits to a devices
  source stream to force the speed relationships
  among the various devices into integer multiples
  of each other
• The extra bit are then discarded by the
  demultiplexer

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Asynchronous TDM

• In synchronous TDM, since the time slots are
  preassigned and fixed, whenever a connected
  device is not transmitting, the corresponding
  slot is empty and that much of the path is wasted
• Asynchronous TDM or statistical TDM is designed
  to avoid this type of waste
• In this context, asynchronous means flexible or
  not fixed
• The number of time slots in asynchronous TDM
  frame is based on a statistical analysis of the
  number of input lines that are likely to be
  transmitting at any given time

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Asynchronous TDM (cont.)

• Like synchronous TDM, asynchronous TDM allows a
  number of lower-speed input lines to be
  multiplexed to a single higher-speed line
• Unlike synchronous TDM, the total speed of the
  input lines can be greater than the capacity of
  the path
• In asynchronous TDM, the frame can contain a
  lower number of time slots when compared with
  synchronous TDM

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Example of Asynchronous TDM Frames
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Addressing and Overhead in Asynchronous TDM

• Addressing and Overhead
• In the absence of fixed positional relationships,
  each time slot must carry an address telling the
  demultiplexer how to direct the data.