wo every 18 months. This prediction has held for the past 30 or SO years. Over the same period, the cost of such processing has also decreased by the same ratio or more. Since microprocessors work well with digital signals, costs of digital processing and storage devices have decreased signif- icantly vis-à-vis analogue devices. Digital technologies also facilitate easier information systems integration between business and trade partners. 1.0E+00 1.0E-01 1.0E-02 1.0E-03 1.0E-04 1.0E-05 - -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 Signal to noise ratio (SNR) in dB Figure 5.13 A typical BER curve From Analogue to Digital There are also disadvantages to digital communications. Two of these are noted above, namely the loss of quality due to filtering and quantisation noise. One important complexity associated with digital communications is the need to follow well defined protocols. Transmission of information in bursts of 1's and O's must be standardised SO machines can understand each other. These include the size of signal representation in binary form, quanti- sation levels and their boundaries, error detection and handling, data frame sizes, and SO on. Such detailed standardisation is in contrast with analogue communications where standards were mainly needed for signalling and call set-up purposes. Case Study 5.1: Integrated Services Digital Networks Voice communications in a digital form is common in many forms today. Mobile tele- phony is digital, and SO are voice over IP applications such as Skype. However, this has come about only recently and most of these systems date to the mid- to late 1990s. The first major digital voice communications standard, Integrated Services for Digital Network (ISDN) was only developed in the 1980s. ISDN was designed to carry digital voice over twisted pair copper telephone wires and was expected to eventually replace the analogue telephone standard. ISDN used a waveform digitisation of voice signals. It filtered voice at 4 kHz, and sampled it at the Nyquist rate of 8 kHz