1、INTERNATIONAL TELECOMMUNICATION UNION CCITT THE INTERNATIONAL TELEGRAPH AND TELEPHONE CON SU LTATIV E COM M ITTE E TELEPHONE NETWORK AND ISDN QUALITY OF SERVICE, NETWORK MANAGEMENT AND TRAFFIC ENGINEERING E.830 (1 0/92) MODELS FOR THE SPECIFICATION, EVALUATION AND ALLOCATION OF SERVEABILITY AND SERV
2、ICE INTEGRITY Recommendation E.830 FOREWORD The CCITT (the International Telegraph and Telephone Consultative Committee) is a permanent organ of die International Telecommunication Union (ITU). CCITT is responsible for studying technical, operating and tariff questions and issuing Recommendations on
3、 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 approval of Recommendations by the members of CCITT between Plenary
4、Assemblies is covered by the procedure laid down in CCITT Resolution No. 2 (Melbourne, 1988). Recommendation E.830 was revised by Study Group II and was approved under the Resolution No. 2 procedure on the 30th of October 1992. CCIT NOTE In this Recommendation, the expression “Administration” is use
5、d for. conciseness to indicate both a telecommunication administration and a recognized private operating agency. O ITU 1993 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or rnechatiical, including photocopying and microfilm, w
6、ithout permission in writing from the ITU. Recornmendation E.830 MODEIS FOR THE SPECIFICATION, EVALUATION AND ALLOCATION OF SERVEABILITY AND SERVICE INTEGRITY (revised 1992) 1 Introduction This Recoinmetidation is one of a set of closely related Recommendations comprising E.8 10, E.825, E.830, E.X45
7、, E.850 aiid E.855, concenied with the accessibility, retainability and integrity of telephone services. The CCITT, c.msicierir that there is a iieed for having means (models) for the allocation of serveability and service integrity perfonnarice aiid for its evaluation from the characteristics of ne
8、twork components, the use of the inodels described in this Recoinmendatioii. These models depend on the measure under coiisideratioii, i.e. accessibility, retahiability and service integrity. 2 Description of models In gerieral two types of models are required: one for the allocation of the performa
9、nce to the different iictwork portioiis aiid the other for the evaluatioii of the overall performance froin the network compotieiit characteristics. 2.1 All(ic.clticiri niociel The purpose of such a model is to apportion a given overall measure to international network and to two iiatioiial iietwrks
10、. Figure 1E.830 shows the inodel to be used for allocatioii purposes. Note - The Iuteniatioiial Switching Ceiitres (ISCs) are iiicluded in the international portion. 4 LA LA -7 r- National portion International portion National portion TOWO-CJ FIGURE 1/E.830 Model for the allocation The diagram of F
11、igure 1E.830 should be coiisidered for typical, 90th percentile ,and worst case. For the case of retainability and service integrity, these three situations are summarized in Table 1E.830 which gives the iiuinber of exchanges to be iiicluded in aii iiiteniatioiid coiiiiectioii for retaiiiability obj
12、ective iillocititii. Recommendation E.830 (10/92) 1 - CCITT RECMN*E-830 92 48b259L 0576553 O88 M Typical 90th percentile Worst case TABLE UE.830 Originating network International network Destination network International National exchanges switching centers National exchanges (ISCS) Local Others Oth
13、ers Local 1 1 2 1 1 1 2 2 2 1 1 4 5 4 1 2.2 Evciluution models The purpose of an evaluation model is to derive the overall measures from measures related to different network portions, network sections or network components. In case of already established connection (retainability, integrity) the ev
14、aluation model is simple. In case of coiiiicctions to be established (network and connection accessibility) the evaluation model requires more detailed consideration as indicated in the following. Note - The following only refers to the derivation of overall measure from network components character
15、istics. The method for combining measures related to different network portions is for further study. 2.2.1 Models for accessibility performunce evuluution Some call attempts fail due to the customers (busy party, no response, misnumbering), some fail due to the network. In the latter case, the caus
16、es can be: - a lack of network capacity, in its fault free condition (poor dimensioning or heavy traffic load); - failure of one of its components, resulting in a more or less severe congestion, depending on the role of the network component in the network. A network model is required to describe in
17、 a simplified manner the behaviour of a (circuit switched) network under traffic load and in the presence of failures. The probability that a call attempt fails depends on both the traffic at the time of the call attempt and the arnount of resources available for the connection required at the same
18、time. As a consequence, in order to describe the behaviour of a network in presence of failures, it is necessary to evaluate: - - their effect, which requires the description of a logical network model in terms of traffic engineering; their probabilities, which require the description of a physical
19、network model in terms of its components. & 3 Recommendation E.830 (10/92) CCITT RECNN*E.830 92 m 4862593 0576554 TL4 m A logical network model is a means to evaluate the performance of the network in terms of traffic and pcuticularly the effect of failures in the physical network. It is made of nod
20、es interconnected by circuit groups (see Recointnetidiitiuti E.60, Q 3.3, also called trunk group, in a hierarchial order. It shows the traffic load of each circuit group (busy hour). If a circuit group is subdivided into circuit subgroups (see Recommendation E.600, 0 3.6) which are routed on separa
21、te links, then the logical model must distinguish them. Thus, the consequence of failure on a link between these two nodes will result in a known reduction of the amount of circuits available between these nodes. The derivation of the effect of each failure (link or nodes) in terms of blocking proba
22、bility can be made by The physical network model describes the way in which the logical network model in implemented. It is tnade of exchanges and of transmission links, the implementation of which is to be described in detail: The failures probability of a link can be derived from the availabilitie
23、s of its components, taking in account The physicd network model should also provide information on the architecture of the exchange to allow the In the case of connection accessibility, it is necessary to identify the origin and the destination of the The origin aiid the destination may belong to d
24、ifferent categories: a single subscriber, a number of combining the traffic load and the reduced capacity of the network to handle the considered traffic load. multiplexers, digital sections and their length, protection rules and topology. the protection measures in that link. evoluatioti of the pro
25、babilities of their main failure modes. connection ti) be established. suhscrihers, an exchruige, a portion of network. 2.2.2 Mielsfiir senlice integrity mid retiiitiahility emliratioti For the evaluation of the service integrity and retainability the relevant models are indicated in Figures A-1E.850 and B-1E.855. Recommendation E.830 (10/92) 3
