All drawings appearing in this Recommendation have been done in Autocad.
          Recommendation E.830
                  MODELS FOR THE ALLOCATION OF INTERNATIONAL TELEPHONE
                 CONNECTION RETAINABILITY, ACCESSIBILITY AND INTEGRITY
                Introduction
                This Recommendation is one of a set  of  closely  related  Recommendations,
          comprising Recommendations E.810, E.830, E.845, E.850 and  E.855  concerned  with
          the accessibility, retainability and integrity of telephone services.
                The CCITT,
          considering
                that there is a need to establish hypothetical reference connection  models
          to  allocate  overall  connection  retainability,  accessibility  and   integrity
          objectives to the component parts of international connections,
          recommends
                three models for retainability (one of which is for a typical, or  average,
          international connection), and one model for accessibility and integrity.
          1      Retainability models
                The  models  are  shown  respectively,  in  Figures  1/E.830,  2/E.830  and
          3/E.830. As indicated by Figure 1/E.830, the typical connection has two  circuits
          in each of the national systems, and one in the international chain. In the  90th
          percentile case, there would be three in the national  systems  and  one  in  the
          international chain.
          2      Number of circuits
                The number of circuits in each of the models is  based  on  Table  1/E.830.
          The entries of this table  are  based  on  the  information  contained  in  Table
          1/G.101.
                The mean and model number of national extension circuits are both equal  to
          2. This applies to both originating and terminating national  systems.  The  mean
          number of international circuits is 2.1 and the model number is 2.
                                                 TABLE 1/E.830
                         Probabilities of the number of circuits in the two national 
                        systems and the international chain (expressed as percentages)
            Number of      Originating    International    Terminating 
             circuits         LE-ISC         ISC-ISC        ISC-LE`
                 1             33.8           95.1           32.9
                 2             38.9            4.5           39.5
                 3             20.2            0.3           20.4
                 4              6.0             -             6.1
                 5              1.0      






























                                                         Fascicle II.3 - Rec. E.830   PAGE1

                                                -             1.0
                    LE    Local exchange
                    ISC   International switching centre
                    Note - The possibilities of 6 and 7 circuits in the originating
                    national  system  are  0.005%  and  0.0005%  respectively.  The
                    probabilities of 4, 5 and 6 international circuits  are  0.03%,
                    0.00007% and 0.00009% respectively.
                                         Figure 1/E.830 - CITT 81260

                                        Figure 2/E.830 - CCITT 81270

                                        Figure 3/E.830 - CCITT 81280

         3      Accessibility and integrity model
               The model to be used for allocation of  the  connection  accessibility  and
         integrity objectives found in Recommendations E.845 and E.855 respectively to the
         national portions and international chains of international connections is  shown
         in Figure 4/E.830.
                                        FIGURE 4/E.830 - T0201020-87


















































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