Elements Of Communication Switching System

The purpose of a telecommunication switching system is to provide the means to pass information from any terminal device to any other terminal device selected by the originator. Telecommunication system can be divided into four main parts. They are

1. End system or Instruments

2. Transmission system

3. Switching system

4. Signaling.

End Systems or Instruments. The end system or instruments are a transmitter or receiver that are responsible for sending information or decoding or inverting received information or message into an intelligible message. End systems in the telelphone network have evolved from analog telephones to digital handsets and cellular phones. However, endless arrays of other devices are being attached to telephone lines, including computer terminals used for data transmission. Fig. 1.3 shows some of the end instruments.

(d) Computer (e) Video conference

Fig. 1.3. Some of the end instruments.

Transmission System. Signals generated by the end system or the instruments should be transported to the destination by some means. The transmission on links conveys the information and control signals between the terminals and switching centers. A transmission link can be characterized by its bandwidth, link attenuation and the propagation delay. To maintain signal quality, the signal must be regenerated after a certain distance.

In general a communication path between two distinct points can be setup be connecting a number of transmission lines in tandem. The transmission links include two-wire lines, coaxial cables microwave radio, optical fibers and satellites. Functionally, the communication channels between switching system are referred to as trunks. Fig. 1.4 shows the various possible transmission media.

(a) Unshielded twistedpair (UTP) cable

(a) Unshielded twistedpair (UTP) cable

(b) Shielded twisted-pair (STP) cable

Conductor

Conductor

Plastic Wire insulator mash shield

PUC coating

Plastic Wire insulator mash shield

(b) Shielded twisted-pair (STP) cable

Fiber Buffer

Jacket (d) Fiber optic cable

Fiber Buffer

PUC coating

(c) Coaxil cable

Jacket (d) Fiber optic cable

(c) Coaxil cable

(f) Satellite communication

Fig. 1.4. Various transmission media.

(f) Satellite communication

Fig. 1.4. Various transmission media.

Switching System. The switching centers receives the control signals, messages or conversations and forwards to the required destination, after necessary modification (link amplifications) if necessary. A 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. A switching center of a telephone network comprising a switching network and its control and support equipment is called a central office.

In computer communication, the switching technique used is known as packet switching or message switch (store and forward switching). In telephone network the switching method used is called circuit switching. Some practical switching system are step-by-step, cross barred relay system, digital swtiching systems, electronic switching system etc.

Signalling Systems. A signalling system in a data communication networks exchanges signalling information effectively between subscribers. The signalling systems are essential building blocks in providing the ultimate objective of a worldwide automatic telephone services standardized. Signalling provides the interface between different national systems. The introduction of signalling system was the big step in improving the PSTN.

The consultative committe on international telegraphy and telephony (CCITT) based in Geneva, recommended seven formats related to signalling. The first five formats related to Inband signalling and the last two in the category of common channel signalling. In In-band signalling, voice information and signalling information travel on common paths, where as in common channel signalling, they travel on separate paths. Further classification and detained study are carried out in later chapter.

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