CSN200 Introduction to Telecommunications, Winter 2000 Lecture-28 Multiplexing
Trunks and Multiplexing:

What happens, when there is only one media available between two local offices, as shown below?

Local
Office
Local
Office
℡
℡
℡
℡
℡
℡
℡
℡
℡
℡
In this case the media is the space and the link between the two local offices is the Microwave link.
The users from the left want to communicate the users on the right. They have to share the common
media. This can be done in two ways:
First, the users can take turns; each one periodically uses the microwave link for a short period of time.
This time-sharing is known as Time division Multiplexing (TDM).
Second, the bandwidth of the microwave link can be divided among logical channels, with each user

Combining the signals from four 9600 bps terminals into one signal of 38,400 bps.
Combining 24 digital voice channels (64,000 bps) into one 1,544,000 bps (T-1) circuit.
Combining 672 digital voice channels into one 44,373,000 bps (T-3) circuit.

• A Demultiplexer does the opposite of a multiplexer; it separates a multiplexed signal back into its
separate signals, for distribution to multiple users.

Types of Multiplexers:
1. FDM Frequency Division Multiplexing
2. TDM Time Division Multiplexing

Frequency Division Multiplexing: (Chapter-3)
• FDM is an analog technique where each signal or device is allocated a separate range of
frequencies to use on the channel.
• This is similar to AM radio, for example, where each radio station transmits its signal on a different
carrier frequency (the allocated spectrum is 1100KHz, from 550KHz to 1650KHz, and that can
accommodate 1100KHz/10KHz = 110 channels).
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CSN200 Introduction to Telecommunications, Winter 2000 Lecture-28 Multiplexing
• Other standards also exists - up to 230000 voice channels together (4x230000 = 920000 kHz
Bandwidth)

Time Division Multiplexing: (Chapter-3)
Time Division Multiplexing is used by the digital system where as FDM is used by the analogue
system.
In TDM one character is taken from each terminal in turn and transmitted down the circuit.
The analog signals are digitized in the end office by a device called a codec (coder-decoder). The
codec makes 8000 samples per second because Nyquist theorem says that this is sufficient to
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CSN200 Introduction to Telecommunications, Winter 2000 Lecture-28 Multiplexing
reproduce the original information back (4 kHz voice signal).
The sampling rate should be at least double the frequency of the signal being sampled.
• TDM is a digital technique in which different digital signals take turns using the available channel.
• Each signal uses the entire bandwidth for a short time and then waits for its turn again.
• Data from different signals is interleaved; each signal having its own timeslot, much like boxcars on
a train where each car carries freight for one user. The difference is that the sequence of boxcars
repeats itself endlessly.
• TDM generally more efficient than FDM, because it does not need guard bands. Guard bands could
be used to transmit data.
• TDM systems may waste bandwidth if all time slots are not unused.

North America’s T Carrier Digital Circuits form an interconnected synchronous digital
communications system of leased circuits using Time Division Multiplexing.
T Carrier
Circuit
Voice

V
ƒ kHz
112

TDM

100 104 108

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