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1991-12-30
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Recommendation E.540
OVERALL GRADE OF SERVICE OF THE INTERNATIONAL PART OF AN
INTERNATIONAL CONNECTION
1 International Routing Plan envisages
that international traffic relations may be served by any of the
following routing arrangements:
a) direct circuits;
b) transit operation involving one or more transit centres for
all connections,
c) direct high-usage circuits with overflow via one or more
transit centres.
In principle there would be merit in dimensioning international facilities
to provide e grade of service for all relations,
however served. Practical considerations make it advisable to
depart from one universal value.
2 Direct circuit groups are dimensioned,
according to Recommendation E.520 on the basis of p = 1% loss
probability during the mean busy hour.
An exception is permitted for small groups of very long
international circuits for which p = 3% loss probability is
accepted for six or fewer circuits. As the traffic increases the
grade of service improves progressively until p = 1% loss value is
reached for 20 circuits.
3 For the relations served exclusively by transit operation the
grade of service will deteriorate with the number of transit
centres in the connection. Measurements made on
congestion in such circumstances suggest that the overall grade of
service for up to six links in tandem is less than twice the
congestion of any of the six links in the chain. Hence, for a
series of routes, each dimensioned for p = 1%, the overall grade of
service should seldom exceed 2%. An East-West type of connection
would have the advantage of different busy hours on the various
links. Corresponding advantage would not apply to North-South
circuits.
Fascicle II.3 - Rec. E.540 PAGE1
In the case of relations served by high-usage circuits the overflow traffic will route over
at least two links and, hence, will be subject to the same
deterioration of service as in the case for transit traffic. However, a substantial part of the traffic will be
connected over the high-usage circuits and the overall grade of
service will approximate that of the relations served solely by
direct circuits.
It is desirable that at least one high-usage circuit should
always be provided between a CT3 and its homing CT1, even though
the circuit may not be wholly justified on economic considerations
alone. However, such a circuit should not be provided unless there
is a measurable amount of traffic which exists, or can be foreseen
in the busy hour. The provision of such circuits would
improve the transmission as well as the grade of service; these
considerations should encourage an increase both in traffic and in
the revenue-earning capacity of the circuits provided.
The overall grade of service for the international part of a
connection is a contributory factor to the overall grade of service
from the calling party in one country to the called party in
another.
PAGE1 Fascicle II.3 - Rec. E.540