Minimization Of Congestion In The Nairobi City Local Telephone System
Mwangi, Elijah M
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Electrical communication which is the fastest, most reliable and flexible among all types of communications also suffers from severe limitations and set-backs with no simple or unique solutions. In Radio Communications the problem is to fit the various input signals in an already overcrowded frequency band, to combat fading and to reduce the effects of noise. In the Telephone system the basic aim is to ensure that almost each and every calling subscriber is admitted into the eve~ busy system and is served, subject to the size of the switching plant and connecting trunks available. To design such a system it is necessary to have a clear understanding of the properties of calls flowing through. Telephone service may be demanded at any hour of the day or night for business, social or any other requirements of the subscriber. The exact time at which any particular call is made depends upon a very wide& range of factors which are unique to that subscriber alone. In the City of Nairobi there is a continuous flow of heavy and moderate traffic during office hours but during the night the number of calls decreases to a very low level. This is due to the fact that a large proportion of the City's population have access to telephone facilities during the office hours only. Calls are originated by subscribers without any knowledge of the demands of other subscribers. It is to be anticipated, therefore, that there will be wide variation in the number of calls originated rni nut.e by minute. The factors which influence the incidence of calls are so very diverse that it can be stated with reasonable accuracy that telephone traffic originates in a 'pure chance' manner. This statement is not strictly true since the probable number of calls originated "at anyone time is to some extent dependent upon the number of conversations already in progress. If, for example, 50 subscribers out of a common group of 100 are engaged in conversation the likelihood of demands for further calls is much less than would be the case if none of the subscribers was already conversing, Under practical conditions the number of subscribers is usually so large in relation to the number of simultaneous conversations that this factor is not of material consequence. In addition to the normal flow of calls abnormal external events may stimulate a flow of calls to an extent that the service given by the exchanges deteriorates to a very low level. The arrival of calls to a telephone exchange, or the input process as it is usually called, can therefore be regarded as a chance phenomenon. As such it is governed by the probability laws of the inter-arrival times and the number of calls during a certain period of time. Therefore the input process forms a stochastic process. Similarly conversation time or the time during which a device is continuously engaged is also a chance phenomenon. This arises due to the fact that the calling habits of subscribers vary between those who make a few calls but converse for a long period of time, and those subscribers who converse for a short time but make a larger number of calls. Thus, the duration of a call, or the holding time is regulated by a probability law and also leads to a stochastic process. The stochastic processes encountered are of the same nature as those of fields remote from telephony such as highway and air traffic control, operational research and various aspects of queuing theory. The aim of this research project is to attempt to improve the grade of service of the Kenya P. & T. Corporation local telephone system within the Nairobi city by minimizing the congestion to a level of 1%. It is necessary to provide alternate paths for the telephone traffic to its destination so as to minimize congestion. These alternate paths are used in common with traffic from other sources. On the desination of a call from one exchange to another, the attempt is first made to seize a direct line to the destination if it exists, and if unsuccessful the call overflows to the alternate route and then to the destination via a tandem exchange. The number of direct lines designed to carry the heavy traffic and the number of alternate lines designed to carry the overflow traffic are so balanced as to keep the total cost of the network including switching costs at a minimum. The introduction of local call timing is another effectively of reducing congestion. Local call timing is an arrangement whereby after conversing for a certain period of time, the subscriber is either disconnected or is charged more if he wishes to retain the service. Some subscribers are known to converse for an unnecessarily long interval of time. This bars other subscribers access t.o the telephone system and increases an 'unexpressed , demand for telephone services. It is an aim of this project to examine t.he holding time of calls originating from various exchanges so as to come up with a suitable time interval t.o be used as a local holding time limit .