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1991-12-30
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Recommendation E.428
CONNECTION RETENTION
1 Introduction
Connection retention is one of the parameters influencing the quality of
service (QOS) after the call has been set up. As the assessment of this parameter
is difficult and costly, it is recommended to investigate only after information
from sources such as operator trouble reports, subscriber complaints, interviews
and/or service observations indicate that there is a problem.
2 Cut-off call ratio
The cut-off call ratio is the percentage of the established calls that are
released for a reason other than intentional by any of the parties involved in
the call. The cut-off call ratio can only be measured by placing test calls (see
Recommendation G.181).
3 Investigations required
Before measuring the cut-off call ratio the source information leading to
the investigation should be utilized to the extent possible (see Annex B to
Recommendation E.420) and the outcome should be compared with other relevant
sources. For example, subscriber complaints can be followed up by investigating
operator trouble reports.
These investigations should lead to a suspect part of the network.
4 Exchange of information
In case the suspect part of the network is outside the territory of an
Administration, the Administration which has responsibility for the suspect part
of the network should be contacted and informed of the results of the
investigations. In the information given, the type of cut-offs should be
classified by causes such as "absence of answer signal", "artificial clearback
signal", etc.
5 Further investigation required
Within the suspect part of the network, the Administration should look for
obvious causes of cut-offs such as exchange or facility failures. If obvious
causes cannot be identified the Administration should consider test calls.
6 Test calls
One or both Administrations may decide to set up a test call programme.
International standardized test call types are contained in Recommendation
E.424. As stated in that Recommendation, before applying type 3 test calls or
subscriber-to-subscriber type test calls, it should be verified that there are no
evident faults in the national network. This verification can easily be
undertaken by applying non-standardized test calls, for example from the
international centre to a subscriber number in the national network of the same
country (see Recommendation E.424, 4) of S 1).
The usefulness of standardization in these kinds of test calls is under
study in several Administrations.
For making test calls on connection retention, the responding equipment
used should send an answer signal after 10 seconds followed by a continuous tone
in conformity with Recommendation O.61. The threshold level of the detector in
the directing equipment should also be in conformity with Recommendation O.61.
7 Objectives
The long-term objectives as well as the allocations are given in
Recommendation G.181. In the intermediate time the following requirements should
apply: the cut-off call ratio for subscriber-to-subscriber tests should, measured
over 24-hour periods, be below 0.5% for 5 minute calls. In any time-consistent
hour the call cut-off rate may not exceed 3%.
It should be noted that, in general, cut-off problems have a more severe
impact on customers' perception of service on routes with a low answer seizure
ratio than on routes with a low completion rate.
Fascicle II.3 - Rec. E.428 PAGE1