1、 - CCITT RECNN*E-450 92 M 4462591 0576555 950 M INTERNATIONAL TELECOMMUNICATION UNION CCITT THE INTERNATIONAL TELEGRAPH AND TELEPHONE CONSULTATIVE COMMITTEE TELEPHONE NETWORK AND ISDN QUALITY OF SERVICE, NETWORK MANAGEMENT AND TRAFFIC ENGINEERING E.850 (1 0/92) CONNECTION RETAIN ABILITY OBJECTIVE FO
2、R THE INTERNATIONAL TELEPHONE SERVICE Recommendation E.850 CCITT RECMN*E-850 92 m 4862593 0576556 97 m FOREWORD The CCITT (the International Telegraph and Telephone Consultative Committee) is a permanent organ of the International Telecommunication Union (ITU). CCITT is responsible for studying tech
3、nical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The Plenary Assembly of CCITT which meets every four years, establishes the topics for study and approves Recommendations prepared by its Study Groups. The
4、approval of Recommendations by the members of CCITT between Plenary Assemblies is covered by the procedure laid down in CCITT Resolution No. 2 (Melbourne, 1988). Recommendation E.850 was revised by Study Group II and was approved under the Resolution No. 2 procedure on the 30th of October 1992. CCII
5、T NOTE In this Recommendation, the expression “Administration” is used for conciseness to indicate both a telecommunication administration and a recognized private operating agency. O IT 1993 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means,
6、electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. Recommendation E.8501) CONNECTION RETAINABILITY OBJECTIVE FOR THE INTERNATIONAL TELEPHONE SERVICE) (revised 1992) Introduction This Recommendation is one of a set of Recommendations, comprisin
7、g Recommendations E.810, E.830, E.845, E.850 and E.855 concenied with the accessibility, reL2inability and integrity of telephone services. The CCIT, cansidering (a) that “premature release” is defined in Recommendation E.800 as the event that an established connection will be released for a reason
8、other than intentionally by any of the parties involved in the calk (b) that premature release is a measure of Connection retainability; (c) that a prematurely released connection is considered high in annoyance as perceived by telephone users; (d) that the probability of a premature release is a fu
9、nction of network component failure intensity and call holding time; (e) that the objective should take account of the expectations and tolerances of users to the premature relese impairment as well as the capabilities of current technology; (f) that the objective might not be met at the present tim
10、e but should be viewed as a long-term goal; (g) that the objective should take into account the concerns of network planners and system designers, provide useful guidance to each, and it can be used by Administrations in a consistent way to measure connection retainability performance; (h) that conn
11、ection retainability is defined in Recommendation E.800, recommends 1 Definitions A prematurely released telephone connection is known as a cutoff call when the connection is completely 1) when a single interruption occurs lasting for longer than ten seconds which causes the transmission quality of
12、the connection to be unsuitable for voice communications; 2) when a succession of interruptions occur lasting less than ten seconds where the product of the average duration of each interruption and the frequency of occurrence (i.e. average number of interruptiondseconds) exceeds 0.005. broken, or )
13、 Formerly G. 18 1, in Rrd Rook, Fascicle III.1. ) Some of the terms in this Recommendation, for example, the noun “measure”, are used in the sense of their definition given in Recommendation E.80. Recommendation E.850 (10/92) 1 CCITT REC!N*E.850 72 4862573 0576558 bbT W 2 A measure to quantify telep
14、hone connection retainability performance The measure to be used shall be the complement of connection retainability, namely the probability of a prematurely released telephone connection when normalized to a call holding time of one minute (Pr). The estimator of the premature release probability is
15、 the premature release call ratio (Pre) which is defined as: where N is the number of telephone calls successfully established in some period of time, Tis the mean call holding tirne in minutes and RN is the number of telephone calls successfully completed out of such N calls (see Annex A ,and Annex
16、 B). 3 Overall objective for premature release probability The provisional objective for the normalized premature release probability (PJ shall be such that the performance is better than the values given below: for typical international connections: 2 x 10-4 Pr I4 x 10-4 for 90th percentile interna
17、tional connections: 4 x 10-4 5 F5 8 x 10-4 for worst case intemational connections: 8 x 10-4 I p; 1.6 x 10-3 Note I - It is intended to establish a single value for Pr, P; or p; in the future. Note 2 - The typical 90th percentile and worst case connections mentioned above shall be assumed to be thos
18、e hypothetical reference connections (HRXs) given in Recommendation E.830. Note 3 - See Annex B. 4 Allocation of the overall objective It is desirable, for planning purposes, to allocate the overall objective for a typical connection to the national systems and the international chain of the HRX. Th
19、e overall objective is given by: where Prill and Pril, are the premature release probabilities for originating and terminating national systems respectively and Pri is the premature release probability of the international chain. The allocation of the overall objective to national systems and intern
20、ational chain shall be as follows: 2 Recommendation E.850 (10/92) then Nore I - 01 is provisiondly recommended as being equal to 2. Thus, for example, if: Pr = 3 x 10-4 P, = 0.6 x 10-4 Note 2 - Further allocation of the overall objective to the circuits and exchanges used in a connection might also
21、be desirable. Note 3 - Objectives for the permissible probability of premature release of an established telephone connection in Integrated Digital Networks (IDNs) and mixed (analogue/digital) networks, due to transit digital or local and combined locdtraisit exchange malfunctions, are specified in
22、the Recommendations Q.504 or Q.514. ANNEX A (to Recommendation E.850) Relationship between the premature release probability and its estimator The following relationship exists between the premature release probability normalized to a 1-minute holding time (Pr) ,and its estimator P,: = Pry if such l
23、imit exists lim lim N+w = N j.00 On the other h,and, for the purpose of nehvork design, the probability of a premature release with a mean call holding time of T ininutes, P(Z, 0, cai be expressed using the formula where aid Zj is the average number of failures per minute of ai i component in the hy
24、pothetical connection between two users as shown in Figure A-lE.850. The connection holding time and the time between failures for the individual components are assumed to be exponentially distributed. Recommendation E.850 (10192) 3 CCITT RECMN*E.B5O 92 W 4862591 0576560 218 = II 2, 31 li I I I I To
25、203750-93 FIGURE A-1E.850 Hypothetical connection to estimate the retainability of an established telephone connection In practice, Z T-1 and therefore Pr can be approximated as follows: Also, the following relationship exists: lim N-+w 11 - y = P(Z, T) ANNEX B (to Recommendation E.850) A method to
26、estimate the premature release probability for an international telephone connection In this annex, a method is described which can be used to estimate the premature release probability for an international telephone connection. The method is based on placing end-to-end test calls, whose mean holdin
27、g time is T in minutes, and observing those which are prematurely released due either to transmission or switching failures, or transmission interruptions lasting longer than ten seconds. From the results of Annex A, it follows that the simple estimator of Pr is: If it can be reasonably assumed that
28、 the occurrence or non-occurrence of a premature release for each of the test calls constitutes independent events, then the binomial sampling theory can be used to derive confidence intervals for Pr, and to determine minimum sample sizes (N). In particular, it would be required that N be chosen suc
29、h that: Pr I(RN/N) - PrT I I ePrT/lOO 2 d100 where e is the estimation error in percent, and a is the confidence level in percent. Writing P = Pr x T, it follows from the central limit theorem that, for large N, / NP(l - P)E 2 Zu 100 where Zu is the root of the equation: za (2/n)Esexp (- 1/2 y2) dy
30、= u/100 O 4 Recommendation E.850 (10/92) CCITT RECMN*E=B50 92 48b259L 0576563 154 Neglecting terms of order P2, the inequality (B-1) becomes: In this last formula, P is generally not known. As an example, however, if we have to verify that P is in conformity with the overall objectives of typical co
31、nnections (see 0 31, such that P is in the order of 3 x 10-4, then a choice of u = 90% and e = 40% would lead to N 2 56 720. Similar calculations based on varying assumptions are reproduced in Figure B- lB.850. Based on these results, it is proposed that for an average holding time of T = 1 min, N =
32、 60 OOO. For other dues of T (in ininutes), N = 60 OOO/T. % 400 300 200 100 90 80 -2 70 - Li $60 50 40 30 20 10 =!= O 20 40 60 80 100 T2374-93 Number of premature released calls, C (= N - RN) FIGURE B-lE.850 Relative precision in estimating Pr from large samples when CN= 0.1 Bibliography TORTORELLA (M.): Cutoff calls and telephone equipment reliability, The Bell System Technicul Journal, Vol 60, No. 8, pp. 1861-1890, 1981. Recommendation ES0 (10/92) 5
