In general, the channel will distort the message and add noise to it. In order to avoid the distortion, and to reduce the effect of the noise to any desired degree, an encoder is placed between the source and the channel, and a decoder is placed between the channel and the destination. Now, the source sends the transmitted message, which is encoded as the transmitted signal; this is sent through the channel. It emerges as the received signal, which is decoded to give the received message; this arrives at the destination.
The channel is considered to have a noise source that inputs “information” in addition to that in the transmitted signal. The aim of the encoder and decoder is to make the received message resemble, as closely as required, the transmitted message, in spite of the “information” from the noise source.
See also source coding theorem, channel coding theorem.
"Shannons model." A Dictionary of Computing. . Encyclopedia.com. (August 16, 2018). http://www.encyclopedia.com/computing/dictionaries-thesauruses-pictures-and-press-releases/shannons-model
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