 channel and a bank of filters separates the messages at the destination (Figure 4.20). The frequency band of the system is divided into several narrowband channels, one for each user. Each narrowband channel is reserved for one user all the time. FDM has been used in analog carrier systems in the telephone network. The same prin- ciple is also used in analog cellular systems in which each user occupies one FDM channel for the duration of the call. In such a case, we call the process frequency-division multiple access (FDMA) because the frequency-division method is now used to allow multiple users to access the network at the same time. A more modern method of multiplexing is TDM, which puts different messages, for example, PCM words from different users, in nonoverlapping Transmission Shared Ch. 1 Ch. 1 The principle of FDM transmission media Ch. 2 Ch. 2 Ch. 3 Ch. 3 channel 4 Ch. 4 Ch. 4 channel 3 channel 2 Frequency channel 1 division multiplexing, The principle of TDM Frequency division multiplexing, Frame 1 synchronization Figure 4.20 Multiplexing methods FDM and TDM. time slots. Each user channel uses a wider frequency band but only a small fraction of time, one time slot in each frame as shown Figure 4.20. In addi- tion to the user channels, framing information is needed for the switching circuit at the receiver that separates the user channels (time slots) in the demultiplexer. When the demultiplexer detects the frame synchronization word, it knows that this is the start of a new frame and the next time slot con- tains the information of user channel 1. This method of TDM is used in high-capacity transmission systems such as optical line systems but also in digital cellular networks where we call it time-division multiple access (TDMA). One user occupies one time slot of a frame, and the time-division principle allows multiple users to access the net- work at the same time using the same carrier frequency. 4.6.2 PCM Frame Structure We introduced the principle of TDM in the previous section. As an example o