Multiplexing concepts and techniques.ppt

Multiplexing is a fundamental concept in telecommunications and data transmission, enabling the efficient use of communication channels by allowing multiple signals to share a single transmission medium. This paper explores the various types of multiplexing, their applications, advantages, and limitations, as well as the technological advancements that have driven the evolution of multiplexing techniques.

1. Overview of Multiplexing

Multiplexing refers to the process of combining multiple signals or data streams into one signal over a shared medium. The primary goal of multiplexing is to maximize the utilization of the available bandwidth and to reduce the cost of transmitting data. By allowing multiple signals to share the same communication channel, multiplexing reduces the need for additional physical resources, such as cables or frequency bands.

2. Types of Multiplexing

There are several types of multiplexing, each with its own method of combining signals:

2.1 Time Division Multiplexing (TDM)

TDM is a method where the available bandwidth is divided into time slots, with each slot allocated to a different signal. The signals take turns in using the communication channel, with each signal transmitting its data during its designated time slot. TDM can be categorized into two types:

Synchronous TDM: In this type, time slots are pre-assigned to signals in a fixed pattern. Even if a signal has no data to transmit, its time slot remains reserved, leading to potential inefficiency.

Asynchronous TDM (Statistical TDM): Time slots are dynamically assigned based on the need, allowing for more efficient use of bandwidth. Slots are only allocated when there is data to transmit, making this method more efficient than synchronous TDM.

2.2 Frequency Division Multiplexing (FDM)

FDM divides the available bandwidth into multiple frequency bands, with each band allocated to a different signal. Each signal modulates a different carrier frequency, and all the modulated signals are transmitted simultaneously over the same channel. FDM is widely used in radio and television broadcasting, where different channels operate on different frequencies.

2.3 Wavelength Division Multiplexing (WDM)

WDM is a type of multiplexing used in optical fiber communication. It divides the light spectrum into multiple wavelengths, with each wavelength carrying a different signal. WDM has enabled significant increases in the data-carrying capacity of optical fibers. There are two main types of WDM:

Dense Wavelength Division Multiplexing (DWDM): In DWDM, multiple closely spaced wavelengths are used to carry different signals, allowing for the transmission of a large number of data streams over a single fiber.

Coarse Wavelength Division Multiplexing (CWDM): CWDM uses a wider spacing between wavelengths, making it a less expensive option compared to DWDM, though with lower data capacity.

2.4 Code Division Multiplexing (CDM)

CDM, also known as Code Division Multiple Access CDM.

1. Multiplexing
•Multiplexing is the set of techniques that allows the
simultaneous transmission of multiple signals across a
single data link.
•A Multiplexer (MUX) is a device that combines several
signals into a single signal.
•A Demultiplexer (DEMUX) is a device that performs the
inverse operation.

3. Categories of Multiplexing

4. Frequency-division Multiplexing (FDM)
•FDM is an analog technique that can be applied when the bandwidth
of a link is greater than the combined bandwidths of the signals to be
transmitted.

5. Frequency-division Multiplexing (FDM)
•In FDM signals
generated by each device
modulate different carrier
frequencies. These
modulated signals are
combined into a single
composite signal that can
be transported by the
link.
FDM is an analog multiplexing technique
that combines signals.

6. Modulating (Modulation)
•In analog transmission, the sending device produces a high
frequency signal (a sine wave) that acts as a basis for the
information signal. This base signal is called the carrier
signal.
•Digital information is then modulated on the carrier signal
by modifying one or more of its characteristics (amplitude,
frequency, phase). This kind of modification is called
modulation and the information signal is called a
modulating signal.

7. Modulation (Amplitude Shift keying ASK)
Time-domain description

8. Modulation (Amplitude Shift keying ASK)
Frequency-domain description

9. Frequency-division Multiplexing (FDM)
•In FDM signals generated by each device modulate
different carrier frequencies. These modulated signals are
combined into a single composite signal that can be
transported by the link.
•Carrier frequencies are separated by enough bandwidth to
accommodate the modulated signal.
•These bandwidth ranges arte the channels through which
various signals travel.
•Channels must separated by strips of unused bandwidth
(guard bands) to prevent signal overlapping.

12. Frequency-division Multiplexing (FDM)
•In FDM, signals are modulated onto separate carrier
frequencies using either AM or FM modulation.

13. Example 1Example 1
Assume that a voice channel occupies a bandwidth of 4
KHz. We need to combine three voice channels into a
link with a bandwidth of 12 KHz, from 20 to 32 KHz.
Show the configuration using the frequency domain
without the use of guard bands.
SolutionSolution
Shift (modulate) each of the three voice channels to a
different bandwidth, as shown in Figure 6.6.

14. Example 1

15. Example 2Example 2
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?
SolutionSolution
For five channels, we need at least four guard bands.
This means that the required bandwidth is at least
5 x 100 + 4 x 10 = 540 KHz,
as shown in Figure 6.7.

16. Example 2

17. Wave-division Multiplexing (WDM)
•Wave-division multiplexing is conceptually the same as
FDM, except that multiplexing and demultiplexing involve
light signals transmitted through fiber-optic channels.
•The purpose is to combine multiple light sources into one
single light at the multiplexer and do the reverse at the
demultiplexer.
•Combining and splitting of light sources are easily handled
by a prism.

19. Time-division Multiplexing (TDM)
•Time-division multiplexing (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
devices.

20. TDM
TDM is a digital multiplexing technique to
combine data.

21. Time-division Multiplexing (TDM)
•TDM can be implemented in two ways: synchronous TDM
and asynchronous TDM.
•In synchronous time-division multiplexing, the term
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.
•Frames
Time slots are grouped into frames. A frame consists of a one
complete cycle of time slots, including one or more slots
dedicated to each sending device.

23. TDM frames

24. Example 5Example 5
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?
SolutionSolution
We can answer the questions as follows:
1. The duration of 1 bit is 1/1 Kbps, or 0.001 s (1 ms).
2. The rate of the link is 4 Kbps.
3. The duration of each time slot 1/4 ms or 250 s.
4. The duration of a frame 1 ms.

25. In a TDM, the data rate of the link is n In a TDM, the data rate of the link is n
times faster, and the unit duration is n times faster, and the unit duration is n
times shorter.times shorter.

26. Interleaving

27. Example 6Example 6
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.
SolutionSolution
The multiplexer is shown in Figure 6.15.

28. Example 6

29. Example 7Example 7
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?
SolutionSolution
Figure 6.16 shows the output for four arbitrary inputs.

30. Example 7

31. Time-division Multiplexing (TDM)
Framing Bits
Because the time slot order in a synchronous TDM system
doest no vary from frame to frame, very little overhead
information needs to be included in each frame. However,
one or more synchronization bits are usually added to the
beginning of each frame.
These bits, called framing bits, allows the demultiplexer to
synchronize with the incoming stream so that it can
separate the time slot accurately.

32. Framing bits

33. Example
Suppose that we have four input devices on a synchronous TDM
link, where the transmissions are interleaved by character. If
each device is generating 250 characters per second, and each
frame is carrying 1 character from each device, what is the
minimum data rate of this link?

34. The link must be able to carry 250
frames per second.
If we assume that each character
consists of 8 bits, then each
frame has 4x8 + 1= 33 bits
( 32 bits for the four characters
plus 1 framing bit).
On the other hand, each device is
creating 2000bps, because 250
characters per second x 8 bits
=2000 bits per second, and the
link is carrying 8250 bps,
because 250 frames per second
x33 bits is 8250 bps.

35. Example 8Example 8
We have four sources, each creating 250 characters per
second. If the interleaved unit is a character and 1
synchronizing bit is added to each frame, find (1) the data
rate of each source, (2) the duration of each character in
each source, (3) the frame rate, (4) the duration of each
frame, (5) the number of bits in each frame, and (6) the
data rate of the link.
SolutionSolution
See next slide.

36. Solution (continued)Solution (continued)
We can answer the questions as follows:
1. The data rate of each source is 2000 bps = 2 Kbps.
2. The duration of a character is 1/250 s, or 4 ms.
3. The link needs to send 250 frames per second.
4. The duration of each frame is 1/250 s, or 4 ms.
5. Each frame is 4 x 8 + 1 = 33 bits.
6. The data rate of the link is 250 x 33, or 8250 bps.

37. Example 9Example 9
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?
SolutionSolution
We can allocate one slot to the first channel and two slots
to the second channel. Each frame carries 3 bits. The
frame rate is 100,000 frames per second because it
carries 1 bit from the first channel. The frame duration is
1/100,000 s, or 10 ms. The bit rate is 100,000 frames/s x
3 bits/frame, or 300 Kbps.

38. DS hierarchy

39. Table 6.1 DS and T lines ratesTable 6.1 DS and T lines rates
Service Line
Rate
(Mbps)
Voice
Channels
DS-1DS-1 T-1T-1 1.5441.544 2424
DS-2DS-2 T-2T-2 6.3126.312 9696
DS-3DS-3 T-3T-3 44.73644.736 672672
DS-4DS-4 T-4T-4 274.176274.176 40324032

40. Table 6.2 E line ratesTable 6.2 E line rates
E Line
Rate
(Mbps)
Voice
Channels
E-1E-1 2.0482.048 3030
E-2E-2 8.4488.448 120120
E-3E-3 34.36834.368 480480
E-4E-4 139.264139.264 19201920

42. Asynchronous TDM
•Synchronous TDM does not guarantee that the full
capacity of a link is used. Because the time slots are
preassigned and fixed, whenever a connected device is not
transmitting, the corresponding slot is empty.
•Asynchronous time-division multiplexing, or statistical
time-division multiplexing, is designed to avoid this type
of waste.
•Like synchronous TDM, asynchronous TDM allows a
number of lower-speed input lines to be multiplexed to a
single higher-speed line. However, in asynchronous TDM
the total speed of the input lines can be greater than the
capacity of the link.

43. •In an asynchronous system,
if we have n input lines, the
frame contains no more than
m slots, with m less than n.
•The number of time slots in
an asynchronous TDM
frame (m) is based on
statistical analysis of the
number of input lines that
are likely to be transmitting
at any given time.
•In this case any slot is
available to any of the
attached input lines that has
data to send.

45. Asynchronous TDM
Addressing and Overhead
•In asynchronous TDM each time slot must carry an
address telling the demultiplexer how direct the data. This
address, for local use only, is attached by the multiplexer
and discarded by the demultiplexer once it has been read.
•Asynchronous TDM is efficient only when the size of the
time slots kept relatively large.

46. Inverse Multiplexing
•Inverse multiplexing takes the
data stream from one high-
speed line and breaks it into
portions that can be sent across
several lower-speed lines
simultaneously, with no loss in
the collective data rate.

47. Figure 6.21 Multiplexing and inverse multiplexing