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Tariff Zones
Owing to the large number of subscribers who may be situated at a large
distance from the terminal exchange it has often been advisable, from the
charging point of view, to distinguish between different groups of subscribers
connected to the same terminal exchange. In this way a charging system can
be arranged in which the tariff can be determined strictly according to the
distance between the A and B subscribers. The subscribers connected to the
terminal exchange can be divided into 20 groups. The subscribers within a
group are given a special tariff zone marking. The tariff zone, like the subscriber
class, is signalled before the subscriber's selection information from
the terminal exchange to the transit centre and can therefore be used at the
transit centre for determination of the tariff. The tariff zone (called T on the
signalling diagram) is signalled by a teleprinter signal from the terminal exchange
to the register in the transit centre in conjunction with the signalling
of the subscriber class information.
Subscribers with Different Alphabet on the Same
Terminal Exchange
The connection of subscribers with different alphabets (e.g. Roman and
Arabic) is made possible by the use of classification. In order that both
classes of subscribers may receive printed service signals, however, the system
has two sets of telegraph signalling generators for service signals. Switching
between them is done on the basis of the received class.
Head-on Collision
Early detection of head-on collision through the use of Type B signalling is
made possible by the fact that the call and call-confirmation signals differ in
form. Although the probability of head-on collision is small, especially within
national networks, measures must be taken when such collision occurs. On a
head-on collision on a circuit between two transit centres a reselection of line
repeaters is made. On a head-on collision between a terminal exchange and a
transit centre a reselection is made in the transit centre, while the call is
passed through in the direction from the terminal exchange to the transit
centre.
Retest Function
A call on a trunk circuit must be acknowledged by a call-confirmation signal.
Should this signal not be returned owing to a line fault, for example, a
Fig. 7
Time supervision circuit for recognition of
calling and clearing signals
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