lectrical signal with varying frequency and amplitude and calls are connected through separate switches in the system. The oldest automatic switching system is the step-by-step system with electromechanical switches. Although extensively used, it becomes obsolete.Cross bar systems operate at higher speeds, are less costly to maintain, and give subscribers greater capabilities. But this system also will be obsolete and it is expected to remain in a limited lifetime only to provide for the expansion of existing systems.The most recent generation of analog ESS have Stored Program Control (SPC). In SPC systems the control functions are performed by a computer, and the switching matrix can use solid state electronic crosspoints. Advantages include extensive remote operation and maintenance facilities, built-in test and signaling units and practically no open contacts, which make them less sensitive to dust and humidity. Normally SPC exchanges are built in compact form and so require air conditioning, especially in hot and humid climates. These systems are not yet obsolete, but most manufacturers are switching their product line to digital ESS.Digitalization of TelecommunicationsIt is the digitalization of telecommunications that has accomplished much of its telephone connections. The TDX is a digital system. In digital ESS, voice or signal is converted from the analog signal to a code form consisting of high speed on/off pulses. Pulses of different conversations are separated from each other by discrete time intervals and switched in turn by the time division switching system so that many calls are multiplexed by the same switch. Digital exchanges are cheaper to install and maintain than analog exchanges particularly in the larger sizes. Among the advantages of digital switching are its compatibility with computers and potential cost and space savings when operated in conjunction with digital transmission systems (Lee 27). The l...
