The Ericsson Group 1968 - History of Ericsson - History of Ericsson

Fig. 2
Relation between costs, number and type
of circuits
Fig. 3
Calculation scheme for determination of
number and type of circuits
The Solution
As is known, the number of circuits on a high usage route is determined
principally by the traffic offered between the exchanges and the cost ratio
between high usage and tandem circuits.
The cost of a circuit is dependent on the type of circuit to be installed.
Finally the type of circuit depends on the number of circuits.
The mutual relations between costs, number and type are illustrated in fig. 2.
On these grounds the calculation must start with an approximate assumption,
e.g. that the cost ratio between high usage and tandem routes has a single
value for all circuits in the network.
On the basis of the number of circuits and given cost data for different types
of circuits, transmission and signalling schemes, and distances, the types of
circuits to be used in different parts of the network can be determined.
As soon as the type of circuit has been decided for all cable runs through
which a route passes, new and more exact values of the cost ratio between
high usage and tandem routes can be calculated from given cost data, and the
procedure can be repeated until sufficient accuracy is attained.
This iterative procedure is illustrated in fig. 3.
The planning procedure has now been described in broad outline. There
remains, however, to describe the method of determining the types of circuits
on routes and cable runs when the number of circuits on each route and cable
run is known.
Separation Process
The determination of the extent to which a route shall consist of physical or
multiplex circuits is done on the basis of the calculated number of circuits
between exchanges and tandem stages.
In the following separation process the following three stages may be
distinguished for each choice of junction matrix:
1. Specification of unconditional physical routes.
2. Specification of remaining routes.
3. Specification on cable runs and calculation of total costs.
Specification of Unconditional Physical Routes
The first step is, on the basis of a cable run matrix describing permissible
cable routes in the network (fig. 4 a), to determine the routes which must
unconditionally consist of physical circuits throughout their length. This is
done by comparing the cost of a physical circuit with a calculated lower limit
for the cost of a multiplex circuit.
The result of this calculation is final and is not affected by changes in the
cable run matrix which must be made later in conjunction with the introduction
of a multiplex system.
61