tted at a constant rate. In data communica- tion applications we usually work locally and every now and then a high data rate is needed to load graphical information or files. A peak load is typically of the order of 1,000 times higher than the average transmission capacity we The different requirements just explained have supported development of the circuit-switched networks, such as PSTN and ISDN, for voice com- munications and packet-switched networks, such as LANs and the Internet, for data communications. Asynchronous transfer mode (ATM) technology was developed by ITU-T to be suitable and efficient for transferring all types of information. However, the expansion of the Internet has reduced its impor- tance and Internet technology will be developed further to provide a plat- form for all kinds of communications. 3.2 Simplex, Half-Duplex, and Full-Duplex Communication In telecommunications systems the transmission of information may be uni- directional or bidirectional. The unidirectional systems that transmit in one direction only are called simplex, and the bidirectional systems that are able to transmit in both directions are called duplex systems. We can implement bidirectional information transfer with half- or full-duplex transmission as shown in Figure 3.1. In simplex operation the signal is transmitted in one direction only. An example of this principle is broadcast television, where TV signals are sent Signals Carried over the Network Simplex: Source Destination Signal is transmitted in one direction only. Examples: broadcast radio and TV and paging systems. Half-Duplex: Source / Destination / Signals are transmitted in one direction at a time. destination source Examples: Some data and radio systems. Full-Duplex (or Duplex): Source and Source and Signals are transmitted in both directions at destination destination the same time. Examples: Conventional telephone, cellular or mobile telephone systems and ISDN. Figure 3.1 Simplex, half-duplex, and full-duplex transmission. from a transmitter to TV s