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 walkytalky 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 Simplex Mode Fire 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 Step-by-step Switching Crossbar Space division Time division switching Digital Analog Space switch Time switch Combination switch 1.3 BASIC TELEPHONE EQUIPMENTS 1.3.1 MICROPHONE: Construction and Principle Cover Alu Diaphragm Silk Pouch Gold-plated Cups Metal Housing Carbon Powder Insulator Rings 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 accordingly 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). Instantaneous resistance of the circuit = r i = r 0 r sin ωt where, r 0 = quiescent resistance of MIC when there is no speech signal r = maximum variation in resistance offered by the carbon granules, r < r 0 Here the negative sign indicates, resistance decreases when the carbon granules compressed. Ignoring the external impedances, Current I = V/ (r 0 r sin ωt) = I o (1 m sin ωt) 1 Therefore modulation index m = r/r 0 < 1 and I 0 = V/r 0 = quiescent current Therefore, I I 0 (1 + m sin ωt) = > 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 poles of magnet and diaphragm, Φ i = Φ 0 + Φ sin ωt using alike notations. 1.4 BASIC TELEPHONE COMMUNICATION 1.4.1 Simplex I 1 I 2 MIC L 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. As the voice signal impinges the diaphragm, the concentration of the carbon granules in the box changes. This leads to change in resistance and accordingly 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. In the loudspeaker there is an electromagnet and a diaphragm is placed in front of the electromagnet. As the current varies, the flux linked with the electromagnet changes and the diaphragm vibrates accordingly and we get the required sound output. According to the construction discussed before, the MIC works as a variable resistance in the total circuit. Therefore, according to the figure 1.7, the current I 1 will be changed with respect to the pronounced voice. As the current generated from the DC source, i.e., (I 1 + I 2 ) is constant. So, I 2 will also be changed according to pronounced expression. The I 2 will cause the vibration variation of the loudspeaker; consequently, loudspeaker can vibrate according to the pronounced expression in front of the MIC. In this communication, the inductor is behaving as a short circuit for dc supply and allows the current to flow in both the MIC and Loudspeaker circuits. The inductor blocks the voice signal being an ac signal and hence it goes to the loudspeaker circuit. 1.4.2 Duplex M 1 E 1 L E 2 V M 2 Fig. 1.8 Duplex telephone circuit
INTRODUCTION 7 This system works in the same way as the simplex does. The difference here is that both subscribers 1 and 2 can communicate to each other, which was not possible in simplex system. In half duplex, at the time of 1st subscriber MIC M 1 and speaker E 1 are on, whereas, MIC M 2 and speaker E 2 are on when subscriber 2 is speaking. A circuit called duplexer does the switching. But when we allow communication in both directions, this system has a drawback. This circuit suffers from the problem of side tone i.e., speech can be heard on both sides simultaneously, which is undesirable. So, the side tone needs to be coupled. The modified circuit diagram of the half duplex communication is given below. R E 2 A P Q Z b = Zin R M 1 Equivalent Impedance to the RHS of Section AB To the Next Section Z in B Fig. 1.9 Side tone coupling circuit In this figure only one section of the entire circuit is shown. The other portion is the exact replica of this one. There are two inductors P and Q of same value. Let the impedance viewed from this section into the other be Z in. An impedance of Z b =Z in is connected in the circuit as shown in the figure. When a subscriber speaks into M 1, equal currents flow through inductors P and Q (since P = Q and Z in = Z b ) but in opposite direction. So, no voltage is induced in the secondary coil and hence E 2 has no output. When a subscriber speaks into M 2 equal currents flow through inductors P and Q, but now in same direction. So, a net voltage is induced in the secondary of the coil and the speech is heard at E 2. The same is valid for the other section as well. 1.5 SWITCHING SYSTEM The function of a switching system is to establish a electrical connection between the inlets (inputs) and outlets (outputs) of an exchange. This is done with the help of switching matrix or the switching network. A switching network is a part of a switching system. The figure below shows a switching network with N input lines and M output lines. When N = M, the network is called a symmetric network. The inlets and the outlets can be connected to local subscribers or to trunks. Trunks are the links that run between two or more switching systems. 1.5.1 Folded and Non-folded Network The Fig. 1.10 is showing 1 port (1 type of inlets and 1 type of outlets) and 2 port networks. As, the networks do not have any control over the output through any feedback, the network is called as open loop. Here, the ith instant output is dependent on the ith instant input only. When, the ith instant output is dependent on the ith instant input and also the (i 1)th instant output, the network becomes close loop, i.e., folded.
8 LINE COMMUNICATION SYSTEM N Inlets M Outlets Fig. 1.10 One-port and two-port open loop network If all the inlets and outlets of the network are connected to the subscriber lines, the network is then called a folded network. In a folded network with N subscribers, there can be at most N/2 number of simultaneous calls. If the system design permits N/2 simultaneous calls, the network is said to be nonblocking network. But the probability that all the subscribers will make a call or speak to each other simultaneous is very low, unless everyone goes crazy! So, it is economical to design a network which will meet the estimated maximum average simultaneous traffic, known as busy hour traffic. In this case, it may occasionally happen (e.g., in New Year s Eve or Christmas Eve) that a subscriber requesting a connection is denied a connection due to availability of free path in the network. In such an event, the subscriber is said to be blocked and the network is termed as a blocking network. N Subscriber Lines Switching Network Fig. 1.11 Folded network If in a switching network, all the inlets and outlets are used for inter-system or inter-exchange transmission, then the switching exchange is termed as a non-folded network or a transit network. A transit network doesn t support local subscribers. A switching system provides three different forms of signalling: 1. Subscriber Loop Signalling 2. Inter-exchange Signalling 3. Intra-exchange Signalling
INTRODUCTION 9 N Incoming Trunks Switching Network N Outgoing Trunks Fig. 1.12 Non-folded network REFERENCES 1. www.hit.bme.hu/~pfliegel/telecomm/04acoust.htm 2. Viswanathan, T. (2004), Telecommunication Switching Systems and Networks, PHI Learning Pvt. Ltd., 2004. EXERCISE 1. What do you mean by simplex, half-duplex and full-duplex communication system? Define with proper example. 2. What is the necessity of introducing switching office or exchange? What are the problems with point-to-point communication system? What is maximum capacity of an exchange? 3. What is control function? 4. What are the disadvantages with manual switching exchanges? How automatic exchange can overcome those problems? 5. What do you mean by folded and non-folded network? Describe with proper figure. 6. In basic telephone communication system, what type of analog modulation scheme is employed? Define its modulation index both in earphone and microphone side. 7. What is side-tone? Is it a problem? If yes, how can we overcome the problem?