11 - ericssonhistory.com

tions Commission (Australia) so that the input from the international trunks
at the Sydney transit exchange should not be more than 5 %.
Unique Features
A comparison of Figures 2 and 4 shows that while there are 44 tariff zones
in the Australian automatic telex system there are exchanges in only 1 1 of
these, although there are some hundreds of subscribers in the remaining tariff
zones. The distribution of these is such that they cannot be economically
grouped together in exchanges in their own tariff zones. The telegraph channel
network, which is not exclusive to telex but is still mainly governed by
private wire and public traffic system requirements, often does not provide
a convenient common grouping point for an exchange within the tariff zone;
in fact the transmission considerations previously mentioned tend to make
this undesirable. The telegraph channel network rather tends to form a
simple star pattern in each State based upon the capital city as the centre of
the star.
L M Ericsson offered an ingenious solution enabling these subscribers to
be connected over voice frequency telegraph channels to capital city ARB 111
exchanges but automatically metered according to their remote tariff zone
origin. This is arranged in the signalling system by the automatic transmission
of a 5-unit start/stop tariff zone code identifying the tariff zone of the
calling subscriber, and this is used in the rate setting equipment, which is part
of the parent ARM exchange, with the first two (A, B) digits of the called
subscriber's number to determine the appropriate metering rate. This type of
subscriber has become known as the "out of tariff zone" subscriber, and the
automatically transmitted tariff zone code as the "T" code. The start/stop
alphabet, of course, allows a number of these tariff zones to operate from
one ARM rate-setting centre. Approximately 15 % of Australian subscribers
are connected in this way.
A second feature of the Australian automatic system is a very flexible
classification facility, enabling calls to be examined at the point of entry to
the transit network, thus enabling certain classes of call to be barred from
the general trunk network, or from particular trunk routes, as well as permitting
calls to be examined at the terminating exchange for compatibility
with the calling subscriber's individual class.
In addition the calling subscriber's class may be used to modify ratesetting,
enabling special rates for particular classes of customer. As for the
tariff zone mark, the class mark is automatically signalled as a 5-unit start/
stop code and has become known as the "K" code.
It will be observed that the total information needed for rate-setting purposes
is then the T code, the K code and the A. B digits of the called subscriber's
number.
In a common control system such as that employed in the automatic telex
network, the charging function and the routing function may be separated
and although T, K, and A, B digits are examined for charging purposes, T, K,
A, B, C and D may be examined for routing.
Special Subscriber Facilities
Group search facilities are provided at every terminal exchange, the only
limitation on number allocation being that each number must be in the same
400-line ARB 111 terminal exchange block.
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