Front end processors fep if information is to be

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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 mu...
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This document was uploaded on 04/07/2014.

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