1、 INTERNATIONAL TELECOMMUNICATION UNION I.357TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (11/2000) SERIES I: INTEGRATED SERVICES DIGITAL NETWORK Overall network aspects and functions Performance objectives B-ISDN semi-permanent connection availability ITU-T Recommendation I.357 (Formerly CCITT Re
2、commendation) ITU-T I-SERIES RECOMMENDATIONS INTEGRATED SERVICES DIGITAL NETWORK GENERAL STRUCTURE Terminology I.110I.119 Description of ISDNs I.120I.129 General modelling methods I.130I.139 Telecommunication network and service attributes I.140I.149 General description of asynchronous transfer mode
3、 I.150I.199 SERVICE CAPABILITIES Scope I.200I.209 General aspects of services in ISDN I.210I.219 Common aspects of services in the ISDN I.220I.229 Bearer services supported by an ISDN I.230I.239 Teleservices supported by an ISDN I.240I.249 Supplementary services in ISDN I.250I.299 OVERALL NETWORK AS
4、PECTS AND FUNCTIONS Network functional principles I.310I.319 Reference models I.320I.329 Numbering, addressing and routing I.330I.339 Connection types I.340I.349 Performance objectives I.350I.359 Protocol layer requirements I.360I.369 General network requirements and functions I.370I.399 ISDN USER-N
5、ETWORK INTERFACES Application of I-series Recommendations to ISDN user-network interfaces I.420I.429 Layer 1 Recommendations I.430I.439 Layer 2 Recommendations I.440I.449 Layer 3 Recommendations I.450I.459 Multiplexing, rate adaption and support of existing interfaces I.460I.469 Aspects of ISDN affe
6、cting terminal requirements I.470I.499 INTERNETWORK INTERFACES I.500I.599 MAINTENANCE PRINCIPLES I.600I.699 B-ISDN EQUIPMENT ASPECTS ATM equipment I.730I.739 Transport functions I.740I.749 Management of ATM equipment I.750I.759 Multiplexing aspects I.760I.769 For further details, please refer to the
7、 list of ITU-T Recommendations. ITU-T I.357 (11/2000) i ITU-T Recommendation I.357 B-ISDN semi-permanent connection availability Summary This Recommendation defines network performance parameters, objectives and measurement methods for describing B-ISDN ATM semi-permanent connection availability. Th
8、e specified parameters and objectives apply to international ATM semi-permanent connection portions delimited by measurement points: National Portions, International Transit Portions and International Interoperator Portions. The objectives, which are worst-case values, are intended to assist provide
9、rs in network planning by limiting the aggregate effect of network impairments, including congestion, equipment failures and transmission errors. Guidance on determining expected end-to-end performance is provided in Annex C. A two-state availability model is defined along with criteria for entry in
10、to and exit from the unavailable state. An estimation procedure is also defined, providing a means of estimating availability performance using sampling techniques. Source ITU-T Recommendation I.357 was revised by ITU-T Study Group 13 (2001-2004) and approved under the WTSA Resolution 1 procedure on
11、 24 November 2000. Keywords ATM semi-permanent connection portion, availability, availability objectives, availability performance, availability ratio, mean time between outages, severely errored second on the ATM layer, unavailability ratio. ii ITU-T I.357 (11/2000) FOREWORD The International Telec
12、ommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them
13、with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics. The approval of ITU-T R
14、ecommendations is covered by the procedure laid down in WTSA Resolution 1. In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with ISO and IEC. NOTE In this Recommendation, the expression “Administration“ is used fo
15、r conciseness to indicate both a telecommunication administration and a recognized operating agency. INTELLECTUAL PROPERTY RIGHTS ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes
16、no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation, ITU had not received notice of intellectual property
17、, protected by patents, which may be required to implement this Recommendation. However, implementors are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. Ge3 ITU 2001 All rights reserved. No part of this publication ma
18、y be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from ITU. ITU-T I.357 (11/2000) iii CONTENTS Page 1 Introduction. 1 1.1 Purpose 1 1.2 Scope. 1 1.3 Related Recommendations . 1 2 References. 2 3 Abbr
19、eviations 2 4 Method for availability specification 3 4.1 Definition of Availability 3 4.2 Availability model 3 4.3 Definition of Unavailability Entry/Exit Criteria . 3 4.4 Availability parameters. 4 4.4.1 Availability Ratio. 4 4.4.2 Mean Time Between Outages 4 5 B-ISDN ATM semi-permanent connection
20、 portions 4 6 Availability performance objectives . 6 6.1 Availability Ratio 6 6.2 Mean Time Between Outages. 6 Annex A In-service estimation of B-ISDN ATM semi-permanent availability parameters. 6 A.1 In-service estimation of SESATM6 A.1.1 Near-end estimation of SESATM7 A.1.2 Bidirectional availabi
21、lity estimation of an ATM connection 8 A.2 In-service estimation of availability parameters . 10 Annex B Related availability parameters 10 B.1 Unavailability Ratio 10 B.2 Outage Intensity 11 Annex C Calculation of end-to-end availability performance 11 C.1 Purpose 11 C.2 End-to-end availability cal
22、culations 11 ITU-T I.357 (11/2000) 1 ITU-T Recommendation I.357 B-ISDN semi-permanent connection availability 1 Introduction 1.1 Purpose The purpose of this Recommendation is to define network performance parameters, worst-case objectives and measurement methods for describing B-ISDN ATM semi-perman
23、ent connection availability. The specified parameters and objectives apply to ATM semi-permanent connection portions delimited by measurement points (MPT or MPI) as defined in ITU-T I.356: National Portions, International Transit Portions and International Interoperator Portions. An international B-
24、ISDN ATM semi-permanent connection consists of two national portions, each delimited by a MPT and MPI, and one international portion delimited by two MPIs. The international portion may also be subdivided into a number of connection portions delimited by MPIs. Using this Recommendation, worst-case p
25、erformance objectives can be derived for the national portions and the international portion of an international B-ISDN ATM semi-permanent connection. Methods for estimating end-to-end availability performance are also provided. The worst-case objectives specified in this Recommendation are intended
26、 to assist providers in network planning by limiting the aggregate effect of network impairments, including congestion, equipment failures and transmission errors, on B-ISDN ATM semi-permanent connection availability. They do not directly correspond to the level of Quality of Service to be expected
27、by customers. 1.2 Scope This Recommendation currently specifies availability parameters and objectives for B-ISDN ATM semi-permanent connections only. Characterization of the performance of B-ISDN ATM switched connections will be the subject of a future Recommendation. This Recommendation defines th
28、e availability of a semi-permanent connection in a way that is independent of user behaviour (i.e. a semi-permanent connection can be declared unavailable even if the user is not transmitting ATM cells at a given time). Availability objectives need not be met on any connection that a network provide
29、r has determined to be non-compliant. 1.3 Related Recommendations In characterizing availability performance, this Recommendation applies concepts and definitions provided in related ISDN performance Recommendations. These include ITU-T I.353 and ITU-T I.356. I.353 defines the following: Measurement
30、 points (MPs) at which ITU-T recommended ISDN protocols may be observed. Particular MPs (designated MPT and MPI) that delimit portions of an end-to-end ISDN connection for which performance objectives may be specified. A set of performance-significant Cell Reference Events (CREs), each of which corr
31、esponds to the transfer of a cell of control or user information across an MP in accordance with an ITU-T recommended protocol. Rules for identifying the time of occurrence of any CRE at any MP. 2 ITU-T I.357 (11/2000) ITU-T I.356 defines the set of primary performance parameters which will be used
32、as a basis for defining availability criteria. 2 References The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendati
33、ons and other references are subject to revision; all users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly pu
34、blished. 1 ITU-T I.353 (1996), Reference events for defining ISDN and B-ISDN performance parameters. 2 ITU-T I.356 (2000), B-ISDN ATM layer cell transfer performance. 3 ITU-T I.610 (1999), B-ISDN operation and maintenance principles and functions. 4 ITU-T I.361 (1999), B-ISDN ATM layer specification
35、. 3 Abbreviations This Recommendation uses the following abbreviations: AIS Alarm Indication Signal AR Availability Ratio B-ISDN Broadband-Integrated Services Digital Network CC Continuity Check CLR Cell Loss Ratio CRE Cell Reference Event FPM Forward Performance Monitoring FS Frontier Station ISC I
36、nternational Switching Centre ISDN Integrated Services Digital Network LE Local Exchange MP Measurement Point MPI Measurement Point I MPT Measurement Point T MTBO Mean Time Between Outages RDI Remote Defect Indication SECBR Severely Errored Cell Block Ratio SESATM Severely Errored Second in the ATM
37、layer UR Unavailability Ratio ITU-T I.357 (11/2000) 3 4 Method for availability specification 4.1 Definition of Availability From a dependability point of view, a portion of an international B-ISDN ATM semi-permanent connection should have the following properties: The fraction of time during which
38、it is in a down state (i.e. unable to support a transaction) should be as low as possible. Once a transaction has been established, it should have low probability of being either terminated (because of insufficient data transfer performance) or prematurely released (due to the failure of a network c
39、omponent) before the intended end of transaction. Availability of a B-ISDN ATM semi-permanent connection portion is defined as the fraction of time during which the portion is able to support a transaction. Conversely, unavailability of a portion is the fraction of time during which the portion is u
40、nable to support a transaction (i.e. it is in the down state). Annex B specifies other commonly used availability definitions and their relationships. 4.2 Availability model A common availability model is used within this Recommendation which applies to any semi-permanent connection type. The model
41、uses two states corresponding to the ability or inability of the network to sustain a connection in the available state. Transitions between the states of the model are governed by the occurrence of patterns of Severely Errored Seconds in the ATM layer (SESATM). This Recommendation views availabilit
42、y from the network perspective, where availability performance is characterized independently of user behaviour. 4.3 Definition of Unavailability Entry/Exit Criteria In order to define the availability of an ATM semi-permanent connection portion, a criterion is defined for entry into the unavailable
43、 state. This criterion is applicable to any ATM semi-permanent connection portion, whether the user continuously transmits cells or not. This is achieved by defining a cell transfer outcome, the Severely Errored Second in the ATM Layer (SESATM). A given second is considered to be an SESATMif: a) use
44、r information cells are presented during this period of time to the connection portion and either the Cell Loss Ratio (CLR) 1/1024 or the Severely Errored Cell Block Ratio (SECBR) 1/32, where CLR and SECBR are computed over the considered period of time; NOTE The above CLR threshold is intended to s
45、upport QOS classes in which the CLR objective is 105. Appropriate CLR thresholds for other QOS classes are for further study. b) user information cells are not presented during this period of time to the connection portion, but the ATM connection is considered to be unable to provide acceptable cell
46、 transfer performance, because an interruption has occurred within the connection portion. This interruption prevents cells from being transmitted on the connection portion during the considered one-second period of time, should the user attempt to transmit cells. An interruption corresponds to a fa
47、ilure occurring within the connection portion, either of the physical layer or of the ATM layer. Annex A provides means of estimating the occurrence of an SESATM. The methods for estimating the occurrence of an SESATMare taken from the set of cell transfer parameters defined in ITU-T I.356 and the O
48、AM facilities defined in ITU-T I.610. 4 ITU-T I.357 (11/2000) The onset of unavailability begins with the occurrence of ten consecutive SESATM. These ten seconds are part of unavailable time. A period of unavailability ends with the occurrence of ten consecutive seconds, none of which are SESATM. Th
49、ese ten seconds are part of available time. The ten-second criteria are supported using a sliding window with one-second granularity. A portion of a bidirectional B-ISDN connection is available if, and only if, both directions are available. It is recognized that in-service measurement of availability as defined above may not be practicable in many cases. A number of estimation methods that can be employed for in-service estimation can be found in Annex A. 4.4 Availability parameters Performance objectives are defined in this Recommendation f
