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Unformatted text preview: ion is assigned a frequency range within a bandwidth of radio
frequencies. Several radio stations may be transmitting speech signals
simultaneously over the physical channel, which is "ether" in this case. A radio
receiver antenna receives signals transmitted by all the stations. Finally, the tuning
dial in the radio is used to isolate the speech signal of the station tuned. In FDM,
the signals to be transmitted must be analog signals. Thus, digital signals must be
converted to analog form if they are to use FDM.
2. Time-Division Multiplexing (TDM). The bit rate of transmission medium
always exceeds the required rate of the digital signal. This fact is utilized for time- division multiplexing. In TDM, the total time available in the channel is divided
between several users and each user of the channel is allotted a time slice (a small
time interval) during which he/she may transmit a message. That is, multiple data
streams belonging to different users are interleaved into one data stream at the
sending end of the channel. At the receiving end, a demultiplexer is used to
reassemble individual chunks of messages sent by the users into full messages,
Figure 17.13 shows multiplexing of three different signals using TDM. The same
idea may be extended for multiplexing several signals.
Although TDM may be used to multiplex digital or analog signals, its usage is
more appropriate for digital data transmission. It is also very much suitable for
communication between computers because such communications occur in short
fast bursts. Besides this, TDM is generally more efficient as more subchannels can
be derived. It is up to the network designers to allocate time slots to different
channels. It is common to have 32 low-speed terminals connected to one highspeed line.
Whether or not to use multiplexing usually depends upon economics. The cost of
high-speed modems and multiplexers is very high compared to the cost of lowspeed modems. However, if line costs are high due to long distances, then
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This document was uploaded on 04/07/2014.
- Spring '14