Telecommunication system is an important and integral part of modern society. In addition to public switched telephone network (PSTN), it plays vital role in radio and television networks, internet and Asynchronous transfer mode (ATM) networks. The switching system provides various services to the subscribers. The switching system is a collection of switching elements arranged and controlled in such a way as to setup a communication path between any two distant points. This chapter demonstrates the switching systems of manual exchanges to the electronic switching systems.
The process of transferring message from one place to another (or line to line) is called switching related to outside the switching plant or systems. There are three types of switching namely a circuit switching, message switching and packet switching. In telecommunication switching, the circuit switching and message switchings are used. The switching technique used in computer communication network or data transfer is packet switching. The circuit switching and message switching are explained in this chapter. The packet switching is dealt in the latter chapter.
Telecommunication is the communication of voice or data over long distances using public switched telephone network (PSTN). PSTN consists of transmission component, switching components and facilities for maintaining equipment, billing system and other internal components. PSTN also referred to as plain old telephone system (POTS). The switching technique used in PSTN is circuit switching in general.
To setup connection between subscribers, the PSTN consists of the transmission systems, switching system and signalling systems. The transmission system is dealt in the chapter 3. This chapter explains various switching systems and the signalling system is explained exclusively in the chapter 7. The PSTN consists of the following hierarchy. Local networks which connect subscribers and local exchanges. Function networks, which interconnect a group of local exchanges serving an area and a trunk exchange. Third one is the trunk network which provides long distance connections nationally and internationally.
A simplest telephone communication system between two subscriber is shown in Fig. 4.1. The telephone set which contains transmitter, receiver and base unit circuits were discussed in chapter 3. The central battery and its use in telecommunication also discussed in chapter 3. In the circuit of Fig. 4.1 shown, there is a quiescent current flowing even in the asbence of sound. This quiescent current is necessary for faithful sound reproduction. The inductor L offers no dc resistance but is nominally opposes the voice frequencies.
The audio signal transmitted from calling subscriber not only heared at called subscriber receiver, but also at senders receiver. The audio signal heared at the generating end is called.
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