1. Data Communications & Networking
Lecture-14
Course Instructor : Sehrish Rafiq
Department Of Computer Science
University Of Peshawar

2. Lecture Overview
 WDM
 Similarityto FDM
 Applications of WDM
 TDM
 Data rate in TDM
 Interleaving

3. WDM
(Wave Division Multiplexing)
 WDM is designed to use the high data rate
capability of fiber optic cable.
 Usinga fiber-optic cable for one single line wastes
the available bandwidth.
 Multiplexing allows us to connect several lines in
to one.

4. Similarity to FDM
 WDM is conceptually the same as FDM,except
that the multiplexing and demultiplexing involve
optical signals transmitted through fiber-optic
channels.
 We are combining different signals of different
frequencies.
 However the difference is that the frequencies are
very high.

5. WDM

6. Prisms in WDM

7. Applications of WDM
 Oneapplication of WDM is the SONET network in
which multiple optical fiber lines are multiplexed
and demultiplexed.
 Note:-A new Method called DWDM(dense WDM)
can Multiplex a very large number of channels by
spacing channels closer to one another. It
achieves even greater efficiency.

8. TDM
(Time Division Multiplexing)
 TDMis a digital process that allows several
connections to share the high bandwidth of a link.
 Instead
of sharing a portion of bandwidth as in
FDM,time is shared.
 Each connection occupies a portion of time in the
link.

9. TDM

10. Time slots and frames
 The dataflow of each connection is divided in to units and
the link combines one unit of each connection to make a
frame.
 The size of the unit can be 1 bit or several bits.
 For n input connections, a frame is organized in to a
minimum of n time slots.
 Each slot carrying one unit from each connection.
 Time slots are combined in to frames.
 A frame consists of one complete cycle of time slots with
one slot dedicated to each sending device.

11. Data rate in TDM
 In TDM, the data rate of the link that carries data
from n connections must be n times the data rate
of a connection to guarantee the flow of data.
 The duration of a unit in a connection is n times
the duration of a time slot in a frame.
 Ifwe consider that the bit rate and bit duration
are inverses of each other then the above
requirement makes sense.

12. TDM

13. Example 1
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?
Solution
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 µs.
4. The duration of a frame 1 ms.

14. Interleaving
 TDM can be visualized as two fast rotating switches, one on
the multiplexing side and the other on the demultiplexing
side.
 The switches are synchronized and rotate at the same
speed.
 On the multiplexing side, as the switch opens in front of a
connection, that connection has the opportunity to send a
unit on to the path .
 This process is called interleaving.
 On the demultiplexing side, as the switch opens in front of
a connection that connection has the opportunity to receive
a unit from the path.

15. Interleaving

16. Example 2
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.
Solution
Each frame carries 1 byte from each channel.
So the size of each frame is 4 bytes or 32 bits.
Each channel is sending 100 bytes per second and a frame
carries 1 byte from each channel, the frame rate must be 100
frames per second and each frame contains 32 bits so the bit
rate is 100x 32 or 3200bps.
This is actually four times the bit rate of each channel which
is 100x8=800bps.

17. Example 2

18. Example 3
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?
Solution
The link carries 50,000 frames per second since each frame
contains 2 bits per channel.
The frame duration is therefore 1/50000s .
The frame rate is 50000 frames per second.
Each frame carries 8 bits.
The bitrate is 50000x8=400000bits or 400kps.
Each bit duration is 1/400000s.
The frame duration is 8 times the bit duration because each
frame is carrying 8 bits.

19. Example 3

20. Thanks!!!