ITU-T E 550-1993 Grade-of-Service and New Performance Criteria under Failure Conditions in International Telephone Exchanges - Telephone Network and ISDN - Quality of Service Netwo.pdf

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1、ITU-T RECMN*E.SSO 73 4862571 0591473 235 INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU E.550 (03/93) TELEPHONE NETWORK AND ISDN QUALITY OF SERVICE, NETWORK MANAGEMENT AND TRAFFIC ENGINEERING GRADE-OF-SERVICE AND NEW PERFORMANCE CRITERIA UNDER FAILURE CON

2、DITIONS IN INTERNATIONAL TELEPHONE EXCHANGES ITU-T Recommendation E.550 (Previously “CCITT Recommendation”) FOREWORD The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of the International Telecom- munication Union. The ITU-T is responsible for studying technical, operatin

3、g and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Conference (WTSC), which meets every four years, established the topics for study by the ITU-T Study Groups which, in their tur

4、n, produce Recommendations on these topics. ITU-T Recommendation E.550 was revised by the ITU-T Study Group II (1988-1993) and was approved by the WTSC (Helsinki, March 1-12, 1993). NOTES 1 As a consequence of a reform process within the International Telecommunication Union (ITU), the CCITT ceased

5、to exist as of 28 February 1993. In its place, the IT Telecommunication Standardization Sector (ITU-T) was created as of 1 March 1993. Similarly, in this reform process, the CCIR and the IFRB have been replaced by the Radiocommunication Sector. In order not to delay publication of this Recommendatio

6、n, no change has been made in the text to references containing the acronyms “CCITT, CCIR or IFREi” or their associated entities such as Plenary Assembly, Secretariat, etc. Future editions of this Recommendation will contain the proper terminology related to the new ITU structure. 2 telecommunicatio

7、n administration and a recognized operating agency. In this Recommendation, the expression “Administration” is used for conciseness to indicate both a o ITU 1994 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, incl

8、uding photocopying and microfilm, without permission in writing from the ITU. ITU-T RECMNrE.550 93 4862593 0591475 008 CONTENTS 1 Introduction 2 General considerations . 3 Exchange performance characteristics under fault situations 4 GOS and applicable models . 5 6 Performance monitoring . Annex A -

9、 A model for mean exchange inaccessibility Annex B - Evaluation of instantaneous exchange service inaccessibility GOS standards and inaccessibility . Recommendation ES50 (03/93) Page 1 1 2 2 4 5 5 8 1 Recommendation ES50 GRADE-OF-SERVICE AND NEW PERFORMANCE CRITERIA UNDER FAILURE CONDITIONS IN INTER

10、NATIONAL TELEPHONE EXCHANGES (Modified at Helsinki, 1993) 1 Introduction 1.1 exchange - network impacts are not covered in these Recommendations. This Recommendation is confined to failures in a single exchange and their impact on calls within that 1.2 This Recommendation from the viewpoint of excha

11、nge Grade of Service (GOS) has been established. 1.3 In conformity with Recommendation E.543 for transit exchanges under normal operation, this Recommendation applies primarily to international digital exchanges. However, Administrations may consider these Recommendations for their national networks

12、. 1.4 The GOS seen by a subscriber (blocking and/or delay in establishing calls) is not only affected by the variations in traffic loads but also by the partial or complete faults of network components. The concept of customer-perceived GOS is not restricted to specific fault and restoration conditi

13、ons. For example, the customer is usually not aware of the fact that a network problem has occurred, and he is unable to distinguish a failure condition from a number of other conditions such as peak traffic demands or equipment shortages due to routine maintenance activity. It is therefore necessar

14、y that suitable performance criteria and GOS objectives for international telephone exchanges be formulated that take account of the impact of partial and total failures of the exchange. Further, appropriate definitions, models and measurement and calculation methods need to be developed as part of

15、this activity. 1.5 From the subscribers point of view, the GOS should not only be defined by the level of unsatisfactory service but also by the duration of the intervals in which the GOS is unsatisfactory and by the frequency with which it occurs. Thus, in its most general form the performance crit

16、eria should take into account such factors as: intensity of failures and duration of resulting faults, traffic demand at time of failures, number of subscribers affected by the failures and the distortions in traffic patterns caused by the failures. However, from a practical viewpoint, it will be de

17、sirable to start with simpler criteria that could be gradually developed to account for all the factors mentioned above. 1.6 Total or partial failures within the international part of the network have a much more severe effect than similar failures in the national networks because the failed compone

18、nts in the national networks can be isolated and affected traffic can be rerouted. Failures in the international part of the network may therefore lead to degraded service in terms of increased blocking delays and even complete denial of service for some time. The purpose of this Recommendation is t

19、o set some service objectives for international exchanges so that the subscribers demanding international connections are assured a certain level of service. It should be noted however that where there are multi-gateway exchanges providing access to and from a country, with diversity of circuits and

20、 provision for restoration, the actual GOS will be better than that for the single exchange. 2 General considerations 2.1 The new performance criteria being sought involve concepts from the field of “availability” (intensity of failures and duration of faults) and “traffic congestion” (levels of blo

21、cking and/or delay). It is therefore necessary that the terminology, definitions and models considered should be consistent with the appropriate Recommendations on terminology and vocabulary. Recommendation ES50 (03/93) 1 ITU-T RECMN*E-550 93 m 4862591 0591477 980 2.2 During periods of heavy congest

22、ion, caused either by traffic peaks or due to malfunction in the exchange, a significant increase in repeated attempts is likely to occur. Further, it is expected that due to accumulated demands during a period of complete faults, the exchange will experience a heavy traffic load immediately after a

23、 failure condition has been removed and service restored. The potential effects of these phenomena on the proposed GOS under failure conditions should be taken into account (for further study). 3 Exchange performance characteristics under fault situations 3.1 The exchange is considered to be in a fa

24、ult situation if any failure in the exchange (hardware, software, human errors) reduces its throughput when it is needed to handle traffk. The following four classes of exchange faults are included in this Recommendation: a) complete exchange faults; b) partial faults resulting in capacity reduction

25、 in all traffic flows to the same extent; c) partial faults in which traffic flows to or from a particular point are restricted or totally isolated from their intended route; d) intermittent fault affecting a certain proportion of calls. 3.2 To the extent practical, an exchange should be designed so

26、 that the failure of a unit (or units) within the exchange should have as little as possible adverse affect on its throughput. In addition, the exchange should be able to take measures within itself to lessen the impact of any overload resulting from failure of any of its units. Units within an exch

27、ange whose failure reduces the exchange throughput by greater amounts than other units should have proportionally higher availability (see 4/4.504). 3.3 When a failure reduces exchange throughput and congestion occurs, the exchange should be able to initiate congestion control indications to other e

28、xchanges and network management systems so as to help control the offered load to the exchange (see Recommendations E.410 and Q.506). 4 GOS and applicable models 4.1 In this subclause, the terms “accessible” and “inaccessible” are used in the sense defined in Recomrnenda- tionG.106 (Red Book). The G

29、OS for exchanges under failure conditions can be formulated at the following two conceptual levels from a subscribers viewpoint: 4.1.1 Instantaneous service accessibility (inaccessibility) At this level, one focuses on the probability that the service is accessible (not accessible) to the subscriber

30、 at the instant he places a demand. 4.1.2 Mean service accessibility (inaccessibility) At this level, one extends the concept of “downtime” used in availability specifications for exchanges to include the effects of partial failures and traffic overloads over a long period of time. 4.2 defined as fo

31、llows: Based on the GOS concept outlined in 4.1, the GOS parameters for exchanges under failure conditions are 4.2.1 instantaneous exchange inaccessibility is the probability that the exchange in question cannot perform the required function (i.e. cannot successfully process calls) under stated cond

32、itions at the time a request for service is placed. 4.2.2 prespecified observation period (e.g. one year). mean exchange service inaccessibility is the average of instantaneous exchange service inaccessibility over a 2 Recommendation ES50 (03/93) ITU-T RECMNtE.550 73 = 4862571 0593478 A37 4.2.3 NOTE

33、S 1 The GOS model in the case of instantaneous exchange inaccessibility parallels the concept of the call congestion in traffic theory and needs to be extended to include the call congestion caused by exchange failures classified in 3.1. The GOS value can then be assigned on a basis similar to Recom

34、mendation ES43 for transit exchanges under normal operation. A model for estimating the mean exchange inaccessibility is provided in Annex A. Though the model provides a simple and hence attractive approach, some practical issues related to measurement and monitoring and the potential effects of net

35、work management controls and scheduled maintenance on the GOS need further study. 2 4.3 The model in Figure 1 outlines the change in the nature of traffic offered under failure conditions. where A, is the offered traffic; At is the total traffic; B N are the resources; Y is the carried traffic; W is

36、 the congestion factor (call attempts not processed) which may include the effects of network management controis; is the proportion of blocked call attempts that reattemp. FIGURE 1E.550 Model for traffic offered under failure conditions In normal conditions the congestion factor B is low and there

37、should be few repeat attempts: as a consequence the traffic A, approximates A,. Under failure conditions there is a reduction in resources and the congestion factor B increases. This provokes the phenomenon of repeat attempts and hence the load A, on the exchange becomes greater than the original A,

38、. Therefore it is necessary to evaluate the congestion with the new load A, assuming system stability exists, which may not always be the case. Recommendation ES01 furnishes the appropriate models to detect the traffic offered from the carried traffic taking into account the repeat attempts. 4.4 The

39、 impact on the GOS for each of the exchange fault modes can be characterized by: - - inaccessibility (instantaneous and mean), congestion and delay parameters (call Set-up, load in Erlangs (A,) and busy hour call attempts (BHCA); through-connection, etc.); - fault duration; - failure intensity. Reco

40、mmendation ES50 (03/93) 3 ITU-T RECMN*E-550 93 4862593 0593479 753 5 GOS standards and inaccessibility 5.1 fault free conditions. Exchange fault situations can create similar effects to overload traffk conditions applied to an exchange under In general, digital exchanges operating in the network sho

41、uld be capable of taking action to ensure maximum throughput when they encounter an overload condition, including any that have been caused by a fault condition within the exchange. Calls that have been accepted for processing by the exchange should continue to be processed as expeditiously as possi

42、ble, consistent with the overload protection strategies recommended in 3/Q.543. 5.2 One of the actions the exchange may take to preserve call processing capacity is to initiate congestion controls and/or other network management actions, to control the load offered to the exchange (see Recommendatio

43、ns E.410, E.413 and 4.506). The most obvious impact from the callers viewpoint may be a lowering of the probability that the network as a whole will be able to complete some portion of the call attempts that the exchange is unable to accept during the failure condition. 5.3 International exchanges o

44、ccupy a prominent place in the network and it is important that their processing capacity have high availability. There are likely to be many variations in exchange architectures and sizes that will have different impacts in the categories of failure and the resulting loss of capacity. In general, f

45、ailures that cause large proportions of exchange capacity to be lost must have a low probability of occurring and a short downtime. It is important that maintenance procedures to achieve appropriate exchange availability performance be adopted. 5.4 The formal expression of the criterion of mean exch

46、ange service inaccessibility is as follows: Let: y(r): Intensity of call attempts gaining access through the exchange assuming no failures. s(t): Intensity of call attempts actually given access through the exchange, taking into account the fault conditions which occur in the exchange. Then the mean

47、 exchange service inaccessibility during a period of time Tis given by O Annex A describes a practical implementation of this criterion. For periods in which the exchange experiences a complete fault, i.e. s(?) = O, the expression: The contribution of such periods to the total criterion P may then b

48、e expressed simply as the fraction Ptotul of the evaluation period T during which complete exchange outage due to failure occurred. The objective for Ptotd is given as Prora not more than 0.4 hours per year. For the period of partial failure, it is convenient to also express the objective as equival

49、ent hours per year - the term equivalent is used because the duration of partial faults is weighted by the fraction: of call attempts denied access. The objectives for the contribution of period of partial exchange faults to the total criterion P is given by: Ppuriial not more than 1 .O equivalent hours per year. 4 Recommendation ES50 (03/93) ITU-T RECMNxE.550 93 4862591 0593480 475 Note that by definition P = Pioral + Ppartial The inaccessibility criterion does not cover: - planned outages - faults with duration of less than 10 seconds - - accidental damage to equipment during maint