The purpose of telecommunication switching system is to provide the means to pass information from one terminal (calling subscriber) to other terminal (called subscriber) somewhere. The telecommunication system is divided into four possible elements. They are end systems or instruments, transmission systems switching systems and signalling. In this chapter, the first two elements are explained in detail. Fig. 3.1 shows the telephone network and services.
Fig. 3.1. Telephone network and services.
The end instruments are responsible for transmitting and receiving the sound, data, facsimiles, pictures, voice, video and other informations. The end instrument may be a telephone, fax, telex, computer or any other means. The instrument widely used for communication is telephone and other inistruments are office related. Thus the general customer premise equipment (CPE) is assumed as telephone. Telephones come in variety of designs, colors, and styles. Many telephones are designed to work with the local telephone company. The telephone system section of this chapter explains the concept in detail.
The primary function of transmission system is to provide circuits having the capability of accepting electrical signals at one point and delivering them at destination point with good quality. The transmissions may take place over guided media (copper cables and fiber optic cables) and unguided media (wireless radio, microwave and infrared). Transmission components
(links) define the cable or wireless infrastructure foe transmitting signals. The wiring from the subscriber premises to the local exchange is called local loop or subscriber loop. The guided and unguided media used between the local exchanges and local exchanges and primary, secondary and taritory centers are called trunks.
Signal multiplexing is a technique of combining multiple channels of information over a single circuit of transmission path. To make cost effective use of the transmission system, multiplexing plays a very important role. There are several ways by which signals can be multiplexed. The most important ones are Space Division Multiplexing (SDM). Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM) and they are discussed in detail.
Digital signals are transmitted over media by representing the binary digit as electrical pulses in which a pulse is a signal element. Bandwidth in digital transmission refers to the number of bits per second that can be transmitted over a link. In digital data communication a fundamental requirements is that the receiver knows the starting time and duration of each bit that receiver. The two schemes which meet these requirements are Asynchronous transmission and synchronous transmission. The circuits, techniques and applications of digital switching system are elaborated in the chapter 4.
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