Description:

The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local telephone operators. These consist of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all interconnected by switching centres which allow most telephones to communicate with each other. Originally a network of fixed-line analogy telephone systems, the PSTN is now almost entirely digital in its core network and includes mobile and other networks, as well as fixed telephones. The technical operation of the PSTN adheres to the standards created by the ITU-T. These standards allow different networks in different countries to interconnect seamlessly. The E.163 and E.164 standards provide a single global address space for telephone numbers. The combination of the interconnected networks and the single numbering plan allow telephones around the world to dial each other Commercialization of the telephone began in 1876, with instruments operated in pairs for private use between two locations. Users who wanted to communicate with persons at multiple locations had as many telephones as necessary for the purpose. Alerting another user of the desire to establish a telephone call was accomplished by whistling loudly into the transmitter until the other party heard the alert. Bells were soon added to stations for signalling so an attendant no longer needed to wait for the whistle.

A simplified example of an early circuit-switched telephone network Later telephones took advantage of the exchange principle already employed in telegraph networks. Each telephone was wired to a telephone exchange established for a town or area. For communications outside this exchange area, trunks were installed between exchanges. Networks were designed in a hierarchical manner until they spanned cities, countries, continents, and oceans. Automation introduced pulse dealing between the telephone and the exchange so that each subscriber could directly dial another subscriber connected to the same exchange, but long-distance calling across multiple exchanges required manual switching by operators. Later, more sophisticated address signalling, including multi-frequency signalling methods, enabled direct-dialed long-distance calls by subscribers, culminating in the Signalling System 7 (SS7) network that controlled calls between most exchanges by the end of the 20th century. The growth of the PSTN meant that teletraffic engineering techniques needed to be deployed to deliver quality of service  guarantees for the users. The work of A. K. Erlangen established the mathematical foundations of methods required to determine the capacity requirements and configuration of equipment and the number of personnel required to deliver a specific level of service In the 1970s, the telecommunications industry began implementing packet-switched network data services using the X.25 protocol transported over much of the end-to-end equipment as was already in use in the PSTN. In the 1980s, the industry began planning for digital services assuming they would follow much the same pattern as voice services and conceived end-to-end circuit-switched services, known as the Broadband Integrated Services Digital Network (B-ISDN). The B-ISDN vision was overtaken by the disruptive technology of the Internet.

Thanks &regards

 John Greesham              

Journal coordinator

International Journal of Innovative Research In Computer and Communication Engineering