A channel is a division in a transmission medium so that it can be used to send multiple streams of information. For example, a radio station may broadcast at 96.1 MHz while another radio station may broadcast at 94.5 MHz In this case, the medium has been divided by frequency and each channel has received a separate frequency to broadcast on. Alternatively, one could allocate each channel a recurring segment of time over which to broadcast—this is known as time-division multiplexing and is used in optic fiber communication.
The channel is the medium that is used to transmit the signal. The channel is often noisy, in the sense that when the signal arrives at the receiver, it may contain noise or static, or it may be slightly garbled. For example, the channel could be the millions of kilometers of empty space between Jupiter and Earth, with noise arising because the received signal is so weak. Or it could be the surface of a CD, with noise occurring because of fingerprints, dust, or scratches on the surface.
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