All drawings appearing in this Recommendation have been done in Autocad.
         Recommendation E.810
                           MODEL FOR THE SERVEABILITY PERFORMANCE ON A BASIC CALL
                                 IN THE TELEPHONE NETWORK1)
               Introduction
               This Recommendation2) is one of a set of  closely  related  Recommendations
         concerned with the accessibility and  retainability  of  telephone  services,  as
         listed below.
               The CCITT,
         considering
               (a) that there is a desire to establish overall objectives for the  quality
         of service as perceived by the users;
               (b) that such objectives can then be  used  as  a  basis  for  the  design,
         planning, operation and  maintenance  of  telecommunication  networks  and  their
         component parts;
               (c) that Recommendation  E.800  contains  terms  and  definitions  for  the
         quality of service, the reliability and  availability  performances  and  related
         characteristics of services and networks,
         recommends
               that the telephone call model given in this Recommendation  shall  be  used
         by Administrations to design, plan, operate and maintain  their  networks  taking
         into account the objectives given in Recommendations:
               E.830  Models for the allocation  of  international  telephone  connection
                      retainability, accessibility and integrity;
               E.845  Connection accessibility objective for the international  telephone
                  service;
               E.850  Connection retainability objective for the international  telephone
                  service.
               Note - Refer also to the draft Recommendation  on  interruption  objectives
         which is being studied under Question 39/II.
         1      Model of a basic telephone call and its serveability 
               performance
               The following simplified model illustrates the principal phases of a  basic
         telephone ca  .   It   also   interrelates    these    phases    to
         the  service-related  performance  concepts  and  their   principal
         measures  as  well  as  to  the  main  causes  of  failure  in  the
         establishment and retention of  such  a  call  and  its  subsequent
         billing.
              The model also indicates where, in this series of phases, user
         actions or mistakes may influence the call.
         2      Comments to the model and its applications
         2.1    Mathematical modelling
               In a simple case of statistical  independence,  the  probabilities  may  be
         combined into the following mathematical models:
                              P = (P11 . P12) . P2 . (P31 . P32) . P4
         to express the probability of a correctly billed revenue-making call and,
                                 P = (P11 . P12) . P2 . (P31 . P32)
         to express the probability of a successfully completed call.
         2.2    Contributions to causes of call failure
               It is generally  recognized  that  the  various  parts  of  a  national  or
         international network may be of different importance to the successful completion
         of the various phases of a call. For example, the network accessibility is mainly
         determined by the telephone set, the subscriber line and the local exchange;  the
         connection accessibility by the exchanges, transmission  network  and  signalling
         network used; the billing integrity is dependent on the charging facilities  used
         by the network parts  that  constitute  the  connection  and  the  equipment  for
         processing the billing information, etc. In some Administrations,  the  telephone
         set is not considered as a part of the  network  and  in  that  case  it  is  not
         included in the concept of network performance.

         1) Although this Recommendation deals with the telephone service, in principle  the  model
           and the decomposition  of  serveability  performance  can  also  be  applied  to  other
           telecommunication services. The elaboration of  this  principle  is  left  for  further
           study.
         2) Some of the terms in this Recommendation, for example the noun "measure", are  used  in
           the sense of their definition given in Recommendation E.800.



                                                        Fascicle II.3 - Rec. E.810   PAGE1

         2.3    Time aspects of measures
               Depending on the intended application of the measures indicated  in  Figure
         1/E.810, it may be appropriate to express these measures as instantaneous  values
         related to a given instant of time or as a mean over a given time interval.
               Advice on which variant to use should be given in  each  specific  relevant
         Recommendation.
         2.4    Space aspects of averages
               The measures as indicated in Figure  1/E.810  could  be  applied  to  calls
         between particular destinations as traffic weighted averages  over  a  number  of
         destinations, etc.
               Each relevant Recommendation should clearly  specify  which  alternative(s)
         to use.












                                        FIGURE 1/E.810 - T0200373-88












































         PAGE2   Fascicle II.3 - Rec. E.810