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Chapter 1.Pmd 1 Introduction 1.1. Concept 1.2. Point-to-Point Communication to Switching Exchange 1.3. Basic Telephone Equipments 1.4. Basic Telephone Communication 1.5. Switching System ➢ References ➢ Exercise 1.1 CONCEPT Telecommunication networks carry information from one place to another situated at a certain distance apart. The word ‘tele’ means distant and ‘communication’ is the process of exchanging feelings and ideas. Grossly, telecommunication can be of two types depending upon the transmission channel placed between the sender of information and the receiver of information. If the channel is a free space or air, the communication is named as Radio wave telecommunication and if the said channel is transmission line or cable, the communication is Line Communication. In telephone communication, the person who originates a call is referred to as the calling subscriber and the person for whom the call is originated is known as the called subscriber or called-for subscriber. In some cases like computer communication and to some extent in telephone communication, the communicating bodies or entities are also known as source (one who transmits a signal) and destination (one who receives a signal). The main idea behind modern telecommunication is to allow anybody in any part of the world to be able to communicate with anybody in any part of the world. The first technological development in the field of telecommunication was the transmission of telegraphic signals over wire. In March 1876, Alexander Graham Bell invented telephone and demonstrated it. It was basically a point-to-point connection. His discovery laid the foundation for telephone communication. A communication system can be classified into three categories as: 1. Simplex 2. Half Duplex 3. Full Duplex 2 LINE COMMUNICATION SYSTEM Simplex communication is a unidirectional communication system i.e., communication is possible in one direction only. Typically, the transmitter (the one talking) sends a signal and it’s up to the other receiving device (the listener) to figure out what was sent. This type of communication is most efficient when there is a lot of data flow in one direction, and no traffic is needed in the other direction. Broadcast systems like the T.V. and radio signals, fire alarm systems etc. are good examples of Simplex communication system. In Half Duplex communication system, bidirectional communication is possible, but only in one direction at a time. That means one can either transmit or receive a signal at a particular instant of time in this system of communication. One cannot transmit and receive a signal simultaneously. The walky- talky used in defense and by police is a good example of Half Duplex communication system. In Full Duplex communication system simultaneous two-way communication is achieved. Unlike half duplex communication system, one can both transmit and receive a signal simultaneously. Telephone conversation is an appropriate example of Full Duplex communication system. Smoke Alarm Fire Simplex Mode Station “over” “over” or Half-Duplex and Full-Duplex Fig. 1.1 Modes of directional communication 1.2 POINT-TO-POINT COMMUNICATION TO SWITCHING EXCHANGE Fig. 1.2 Point to point communication INTRODUCTION 3 The above figure shows the point-to-point network connection for six subscribers. Here each subscriber is connected directly with the other five subscribers. For six subscribers, the number of links required is equal to 15. Let us assume that there are n subscribers. In order to connect the first subscriber to all other subscribers, we need n–1 links. With this, the second subscriber is already connected to the first one. So, we need another n–2 links to connect the second subscriber to other subscribers. Similarly for the third subscriber we need n–3 links and so on. If the total number of links required in the entire network be L, then we can find it as under. L = (n – 1) + (n – 2) + (n – 3) + …… + 2 + 1 + 0 = n(n – 1)/2 Networks with point-to-point connections among all the subscribers or nodes are known as fully connected networks. For a fully connected network, the total number of links required increases with increase in number of subscribers. Even for a moderate value of n, we find that L is quite high and is not possible to implement on a large scale practically. For example for n =10 we have L = 45; for n = 50 we have L = 1225; for n = 100 we have L = 4950 and for n = 1000 we have L = 499,500. As the number of subscribers increased beyond a certain limit, the need for a so-called switching system or switching office or switching exchange was realized. In this new system, the subscribers are directly connected to the exchange and not to one another as in the case of point-to-point communication. All the calls are now completed and put through by the switching system which establishes a connection between the communicating parties. Now since each subscriber is directly connected to the switching system, the total number of links required for n subscribers is n. Fig. 1.3 Switching exchange This switching system now needs a proper signalling to establish or disconnect a call. Also it should be able to detect whether the called subscriber is busy and convey the same message to the calling subscriber. All the operations which are performed by the switching exchange while establishing or disconnecting a call, are done by control functions. Earlier the switching systems were manual i.e. operator oriented. It was a slow process. Moreover as the number of subscribers increased, more manpower was required and the system became more complex. As a result of the limitations of manual switching, automatic switching systems came into the picture. Fig. 1.4 An operator based manual telephone switching system 4 LINE COMMUNICATION SYSTEM Switching systems in general can be classified as follows: Switching System Automatic Manual Electromechanical Electronic (Stored Program Control) Strowger or Crossbar Space division Time division Step-by-step switching Switching Digital Analog Space switch Time switch Combination switch 1.3 BASIC TELEPHONE EQUIPMENTS 1.3.1 MICROPHONE: Construction and Principle Alu Diaphragm Cover Silk Pouch Gold-plated Cups Metal Housing Insulator Rings Carbon Powder Fig. 1.5 Carbon microphone INTRODUCTION 5 In telephone sets, mass-produced cheap carbon microphones are used. Such a microphone acts as a resistor varying its resistance according to the change of the sound pressure on the diaphragm. This is due to the change of the contact resistance of carbon powder placed between two gold plated contacts. The bottom electrode is fixed and isolated from the housing while the upper electrode moves together with the diaphragm. Since the contact resistance is a non-linear function of the diaphragm displacement, the distortion of the microphone is rather high. The widespread use of this type of microphones is due to its great output signal. It has an acceptable fidelity with large electrical output. There are carbon granules in a metal box. One of the faces of the metal box is used as diaphragm. As the voice signal impinges the diaphragm, the concentration of the carbon granules in the box changes. This leads to change in resistance and accord- ingly the current through the MIC also changes and we get an audio or speech signal equivalent to the voice signal. If a voltage is applied, current through the circuit varies according to the vibration, i.e. the function of MIC regarding the audio signal can be designated as amplitude modulation (AM). ω Instantaneous resistance of the circuit = ri = r0 – r sin t where, r0 = quiescent resistance of MIC when there is no speech signal r = maximum variation in resistance offered by the carbon granules, r < r0 Here the negative sign indicates, resistance decreases when the carbon granules compressed. Ignoring the external impedances, ω ω –1 Current I = V/ (r0 – r sin t) = Io (1 – m sin t) Therefore modulation index m = r/r0 < 1 and I0 = V/r0 = quiescent current ≈ ω Therefore, I I0 (1 + m sin t) = > AM Currently, they are loosing importance as fully electronic telephone sets enter the market. 1.3.2 HEADPHONE: Construction and Principle Headphones produce a sound field restricted only to the ear cavity. Magnetic headphone is one of the most important types as this is used as telephone receiver. A simplified sketch of such a headphone is shown on following fig. The air gap of the fixed part is chosen great so that the flux lines close through the moving anchor. The anchor is attached to a flexible diaphragm. Magnetic field generated by the coil strengthens or weakens the static field so that the anchor vibrates around its default position. This vibration produces a sound pressure in the closed cavity of the ear. The crucial point of the construction is the size of the air gap. For consumer applications, dynamic headphones are most widely used. In fact, these are small dynamic loudspeakers and because of their size, broadband frequency response can more easily be achieved than with conventional loudspeakers. Diaphragm Anchor Housing Coil Fig. 1.6 Magnetic headphone 6 LINE COMMUNICATION SYSTEM As that off MIC side, in EAR side, AM also is being done. Here the instantaneous flux liking the Φ Φ Φ ω poles of magnet and diaphragm, i = 0 + sin t using alike notations. 1.4 BASIC TELEPHONE COMMUNICATION 1.4.1 Simplex I1 I2 L MIC Loudspeaker V Fig. 1.7 Simplex telephone communication The microphone normally used in telephone is made of carbon. It has an acceptable fidelity with large electrical output. There are carbon granules in a metal box. One of the faces of the metal box is used as diaphragm.
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