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