Telecommunications In Korea Essay Research Paper INTRODUCTIONTelecommunications — страница 3

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information are being transmitted. Videotex systems connect subscribers to data banks and display economic, financial and scientific information as text or graphics on a television screen. In addition, services such as video-conferencing, teletex and electronic mail are widespread. All of these services could be integrated into a single global network called Integrated Services Digital Network (ISDN). An ISDEN comprises digital switching, computer control and optical fiber digital transmission. Telecommunications network consist of subscriber terminals, subscriber lines, local exchanges, toll exchanges and interexchange circuits. Subscriber terminals have evolved from the simple telephone set designed to provide basic voice telephony. The current trend is toward increasing

intelligence in the terminal, enabling subscribers to perform a range of functions which enhance the telephone utility. The terminals associated with other services such as facsimile and data have also become more versatile and sophisticated. Already the concept is emerging of a multipurpose terminal for text, data and other non telephone services as well as being used as a telephone. An ordinary telephone line will be able to carry many of these services and devices of this kind and would make it easier and cheaper to provide non telephone services to rural and remote areas. Subscriber terminals are normally connected to local exchanges by a physical pair of conductors. The physical conductors are expensive, representing around 30% of the total capital cost of plant in urban

areas. However, in highly populated areas, a suitable local network distribution system can be planned and installed that can be expanded economically. Local networks use overhead or buried cables in various size and flexibility points, allowing sufficient capacity to accommodate new subscriber. Various means of concentrating traffic are available so that more than one subscriber may share each physical pair of conductors. However, local distribution represents the major problem of providing telephone service in rural and remote areas. In rural areas, subscribers may be at a distance over 30 to 50 kilometers from the exchange and the terrain and environment communication transfer even more difficult. Local network cost may represent as much as 70 to 80% of capital plant cost

which are themselves normally 4 to 5 times of urban cost. An alternative to physical conductors, especially in rural areas, is radio telephony. Ultra High Frequency and Super High Frequency systems have direct line of sight paths between transmitter and receiver at a distance of 50 to 70 kilometers. In practice the environment and terrain reduce this distance, Improving the utilization of frequency spectrum is possible by using the cellular radio concept and other methods of dynamic frequency assignment. Manual systems with operators setting up connections using key plugs and jacks are still used in developing countries. If attended for 24 hours a day, these provide reasonable service with low investment, low consumption of power and simple maintenance. In automatic analog

systems, speech is converted into a electrical 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 Telecommunications It 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