| dc.description.abstract | 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 . | en |
