TDM is a digital multiplexing technique that combines data by allocating time slots to each data signal in a frame. FDM is an analog technique that combines signals by modulating them onto separate carrier frequencies. Asynchronous TDM improves efficiency over synchronous TDM by allowing any time slot to transmit any signal, reducing empty slots.

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

25. Interleaving

26. 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.

27. 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.

28. Framing bits

29. DS hierarchy

30. 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

31. 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

33. 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.

34. •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.

36. 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.

37. 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.

38. Figure 6.21 Multiplexing and inverse multiplexing