ations systems of the Voyager probes? What factors were needed for a link budget analysis in this system? Which parameter varies? Speculate whether the probes would be launched in the absence of understanding the information theory principles. Why is it that communications can be carried out even when the received signal power is significantly below that of noise? Maximum transmission rate calculation uses the second of Shannon's contribution to the field of information theory, namely channel capacity. Channel Capacity One requirement for telecommunications system design is knowledge of the source information generation rate as discussed above, which can then be used to dimension the network to trans- port the information. Clearly the more information generated by a source, the wider the pipe Information Theory Source Transmitter source encoder Channel Source Receiver decoder Noise Figure 4.6 A simple telecommunications block diagram needs to be to carry the stream. Moreover, the pipe needs to be clear and reasonably free of noise: it is a common experience to discontinue a conversation on the phone when the line is too noisy. Figure 4.6 shows a simplified telecommunications block diagram. Shannon showed that the capacity of such a system is a function of two parameters: the bandwidth of the channel and the relative strength of the signal compared with noise. He formulated an equation, now known as Shannon's theorem that calculates the maximum possible transmission rate, or capacity(C) for a band-limited channel in the presence of noise: where B represents the channel bandwidth, and S/N is the ratio of the power of the desired signal to that of the background noise. Shannon theorem teaches that capacity is directly related to bandwidth: in essence the wider a pipe, the more water can pass through it. It also teaches that as signal power relative to noise power increases, that is the cleaner the pipe becomes, the more information it can carry. In turn this means that a faster transmission of signals becomes possib