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 call. 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.
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