odulations schemes such as amplitude shift keying (ASK), frequency shift keying (FSK) and phase shift keying (PSK), which are primarily variants of the analogue modulations. Depending on the discrete amplitude levels, frequencies and phase states, a variety of modulation methods can be derived from these basic methods. Both in analogue and digital modulations, the properties of the signal to be varied are similar, but the transmitted signal has a direct impact on the output of modulation. 3.2.2 Multiple Access Schemes Multiplexing The field of telecommunication has come a long way from just communicating between two individual users separately. Today on a single link many users seem to simultaneously communicate. This has been possible due to the efforts of what is now very commonly known as multiplexing. With some 1000 million telephone connections in use around the world today and the number of Internet users continuing to grow rapidly, the network providers have been faced with the task of trying to deal effectively with increased telephone traffic. It was not feasible to give every user his/her own separate channel. Therefore, in response to the growing market needs, a number of methods and technologies have been developed within the last few decades to cater for the increasing marketing needs in as economic a way as possible. In the telecommunication field this resulted in the introduction of one of the very first multiplexing techniques, i.e. the frequency division multiplexing (FDM) technique. In this technique, each individual telephone channel/signal is modulated with a different carrier fre- quency such that the carrier frequencies are sufficiently separated and SO the bandwidths of the sig- nals do not significantly overlap. These channels are separated by guard bands, which are unused por- tions of the channel spectrum, to prevent interference. These modulated signals are then shifted into different frequency ranges, enabling different telephone connections/signals to be sent over the same channel/tel