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
PAGE2 Fascicle II.3 - Rec. E.830