All communications links are ultimately limited by background noise
in the system, and so it is essential to have a working knowledge of the statistical properties of noise as they affect data communications
performance. Also, many channels are subject to interference, usually man-made, which can
be equally detrimental to communications integrity. Thirdly, no communications link is
truly distortion free, whether this be caused by imperfections in the processing hardware
or defects within the channel, and the nature and impact of distortion on a digital
communications system must be understood if good design choices are to be made.
In order to 'root' the discussion of noise, interference and distortion in reality, this
chapter concludes with sections outlining the characteristics of two channel types in
everyday use: the telephone channel and the radio channel.
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