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.














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