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 circuits	Originating 
LE–ISC	International 
ISC–ISC	Terminating 
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	  –	  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