# 4 Sources and examples of channel degradation

# 4.1 Introduction to Chapter 4

#### Having dealt in Chapter 3 with many of the issues surrounding practical filtering for digital communications systems in an idealized channel context, this chapter is aimed at injecting a bit of reality into the design process by considering the characteristics of a range of typical channels. |

#### 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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