1、ITU-T RECMN*Q-541 93 4862593 0584589 357 INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU DIGITAL EXCHANGES Q.541 (03/93) DIGITAL EXCHANGE DESIGN OBJECTIVES - GENERAL ITU-T Recommendation (2.541 (Previously “CCITT Recommendation”) ITU-T RECMN*Q.541 93 = 486
2、2571 0584611 623 FOREWORD The I Telecommunication Standardization Sector (ITU-T) is a permanent organ of the International Telecom- munication Union. The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telec
3、ommunications 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 turn, produce Recommendations on these topics. ITU-T Recommendation Q.541 was revised by the ITU
4、-T Study Group XI (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 CCIT ceased to exist as of 28 February 1993. In its place, the ITU Telecommunication Standardization Secto
5、r (IW-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 Similarly, in this reform process, the CCIR and the IFRB have been replaced by the Radiocommunication Sector.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 Recommendation, no change has been made in the text to references containing the acronyms “CCIT, CCiR or IF
6、RB” 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 telecommunication administration and a recognized operating agency. In this Recommendation, the expression “Admi
7、nistration” 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, including photocopying and microfilm, without permission in writing from the ITU. ITU-T RECMN*Q-54L
8、93 48b259L 0584bL2 5bT CONTENTS 1 General . 2 General design objectives 2.1 Exchange modifications and grow th . 2.2 Service provisioning and records 2.3 Translations and routing information . 2.4 Resource utilization 2.5 Physical design objectives Integrated Digital Network design objectives 3.1 Ex
9、change timing distribution 3.2 Network synchronization 3.3 Slip . 3.5 Synchronization requirements when interworking with a digital satellite system 4 Availability design objectives 4.1 General 4.2 Causes of unavailability 4.3 Intrinsic and operational unavailability 4.4 Planned outages 4.5 Total an
10、d partiai unavailability . 4.6 Statistid basis 4.7 Relevant failure events 4.8 Availability independence 4.9 Intrinsic downtime and unavailability objectives . 4.10 Operational Unavailability objectives . 4.1 1 Initial exchange availability performance . 3 5 Hardware reliability design objectives .
11、Recommendatbn 4.541 (0-3) Page 1 1 1 1 1 1 1 2 2 2 2 3 3 3 4 4 4 4 4 5 5 5 5 5 5 i ITU-T RECMN*Q.541 93 m 4862593 0584633 4Tb m Recommendation 4.541 DIGITAL EXCHANGE DESIGN OBJECTIVES - GENERAL (Melbourne, 1988; moded at Helsinki, 1993) 1 General This Recommendation applies to digital local, combine
12、d, msit and intemational exchanges for telephony in Integrated Digital Networks (IDN) and mixed (analoguddigital) networks, and also to local, combined, transit and international exchanges in an Integrated Services Digital Network (ISDN). The field of application of this Recommendation is more fully
13、 defined in Recommendation QS. Some objectives only apply to a ceriain type (or types) of exchange. Where this occurs, the application is defined in the text. Where no such qualification is made, the objective applies to all exchange applications. 2 General design objectives The exchange and/or any
14、associated operations and maintenance system/centers shaii have the capabilities needed to allow the exchange to be operated and administered efficiently while providing service in accordance with an Administrations performance requirements. 2.1 Exchange modifications and growth Tbe exchange should
15、be capable of having hardware and/or software added or changes made without causing a significant impact on service (see 4.4,4.10.2 - Planned outages). 2.2 Service provisioning and records There should be efficient means of establishing service, testing, discontinuing service and mainiainhg for: rec
16、ords - subscriber lines and services; - interexchange circuits. 2.3 Translations and routing information There should be efficient means of establishing, testing and changing call processing information, such as translation and routing information. 2.4 Resource utilization There should be efficient
17、means of measuring performance and traffic flows and to arrange equipment configuraiions as required to insure efficient use of system resources and to provide a good grade of service to ail subscribers (e.g. load balancing). 25 Physical design objectives The exchange shall have a good physical desi
18、gn that provides: - adequate space for maintenance activities; - conformance with environmental requirements; - - uniform equipment identification (conforming with the Administrations requirements); a limited number of uniform power up/down procedures for all component parts of the exchange. Recomme
19、ndation Q.541 (03/93) 1 ITU-T RECMNxQ-541 93 M 4Bb259L 05B4bL4 332 M 3 Integrated Digital Network design objectives 3.1 Exchange timing distribution The timing distribution system of an exchange will be derived from a highly reliable exchange clock system. The distribution of timing within the excha
20、nge must be designed so that the exchange will maintain synchronism on 64 kbith channel timeslots in a connection through the exchange. 3.2 Network synchronization Within a synchronized IDNDSDN, different methods of providing timing between exchanges may be used. An exchange should be able to be syn
21、chronized: a) by an incoming digital signal at an interface A (or B, if provided) as defined in Recommendation Q.511; this applies only to signals derived rom a primary Reference Source, as defined in Recommendation G.8 1 1 ; b) c) directly by a Primary Reference Source, using an interface complying
22、 with Recommendation G.8 11; optionally, by an analogue signai at one of the frequencies listed in Recommendation G.811. Plesiochronous operation should also be possible. The clock of the local, combined or transit exchange shall be responsible for maintaining the synchronization in the part of the
23、network associated with that exchange. The timing performance of the clocks in local, combined or transit exchanges should comply with Recommen- dation G.811. The timing performance of clocks at subscriber premises, at digital PABXs, in digital concentrators, at muldexes, etc., require further study
24、. Synchronized national networks may be provided with exchange clocks not having the frequency accuracy required for international interworking. However, when these synchronized networks within national boundaries are required to interwork internationally as part of the international IDNDSDN, it wil
25、l be necessary to provide means to operate these national networks to the internationally recommended value of frequency accuracy in Recommendation G.8 1 1. 3.3 Slip The design objective controlled slip rate within a synchronized region (see Note) controlled by the exchange should be zero provided t
26、hat input jitter and wander remain within the limits given in Recommendation G.823 and G.824. The design objective controlled slip rate at a digital exchange in plesiochronous operation (or operating to another synchronized region) shall be not more than one slip in 70 days in any 64 kbit/s channel,
27、 provided that input jitter and wander remain within the limits given in Recommendations G.823 and G.824. The operational performance requirements for the rate of octet slips on an international connection or corresponding bearer channel are covered in Recommendation G.822. The occurrence of a contr
28、olled slip should not cause loss of frame alignment. plesiochronously with other synchronized regions. It may be a continent, country, part of a country or counries. NOTE - A synchronized region is defined as a geographic entity normaiiy synchronized to a single source and operating 3.4 maximum time
29、 interval error (MTIE): the permitted Maximum Time Interval Error (MTE) at the exchange output is defined as the difference in time delay of a given timing signai when compared to a reference timing signal for a given measurement period (see Recommendations G.823 and G.824). 3.4.1 Interface VI The p
30、ermitted maximum time interval error at the exchange output at the interface to the basic access digital section requires further study. 2 Recommendation 4.541 (03193) ITU-T RECMNUQ.543 93 = 4862573 0584635 279 3.4.2 interfaces A, B, V2, V3 and V4 The permitted maximum time interval error at the out
31、put of digital interfaces A, B, Va, V3 and V4 shall conform to limits recommended in Recommendations G.823 and G.824. In the case of synchronous operation the limits are specified on the assumption of an ideal incoming synchronizing signal (no jitter, no wander and no frequency deviation) on the lin
32、e delivering the timing information. In the case of asynchronous operation the limits are specified assuming no frequency deviation of the exchange clock, (this is equivalent to taking the output of the exchange clock as the reference timing signal for the permitted maximum TIE measurements). It is
33、recognized that the approach of using maximum TE to specify the performance of an exchange in the case of synchronous operation in some implementations (e.g. when mutual synchronization methods are used) requires further study. Any internai operation or rearrangement within the synchronization and t
34、iming unit or any other cause should not result in a phase discontinuity greater than 1/8 of a Unit Interval (IJI) on the outgoing digital signal from the exchange. The limits given in RecommendationsG.823 and G.824 may be exceeded in cases of infrequent internal testing or rearrangement operations
35、within the exchange. 3.5 Synchronization requirements when interworking with a digital satellite system On a provisional basis the following should apply: The transfer from the timing of the terrestrial digitai network to the timing of the satellite system, if required (plesiochronous operation), wi
36、ll not be performed by the digital exchange. The earth station will be equipped with buffer memories of suitable size to compensate for the time delay variations due to shifts of the satellite hm its ideal position (and due to any other phenomena with similar effects) and to meet the slip performanc
37、e requirements established in Recornmeendation G.822. 4 Availability design objectives 4.1 General Availability is one aspect of the overali quality of service of an exchange. Availability objectives are important factors to be considered in the design of a switching system and may also be used by A
38、dministrations to judge the performance of a system design and to compare the performance of different system designs. Availability may be determined by collecting and evaluating data from exchanges in operation in accordance with the E-Series Recommendations. Data collection may be facilitated by t
39、he use of the Telecommunications Management Network (TMN). Availability may be expressed as the ratio of the accumulated time during which the exchange (or part of it) is capable of proper operation to a time period of statistically significant duration called the mission time. accumulated up-time a
40、ccumulated up-time mission time accumulated uptime + accumulated down-time - - Availability (A) = Sometimes it is more convenient to use the term unavailability (instead of availability) which is define as: Unavailability 0 = 1 - A. The terms used in this subclause, when they already exist, are in a
41、ccordance with Recommendation G.106. Recommendation Q.541 (03/93) 3 ITU-T RECMN*Q.5YL 93 M 48b259L 0584bLb LO5 m 4.2 Causes of unavailability This Recommendation deais with availability as observed from the exchange termination point of view. Both planned and unplanned outages need to be considered,
42、 and both types need to be minimized. Unplanned outages reflect on the inherent reliability of the exchange and are therefore considered separately from planned outages in this Recommendation. Unplanned unavailability counts all failures that cause unavailability. Thus hardware failure, software mal
43、functions and unintentional outages resulting from craftperson activity are to be counted. 43 Intrinsic and operational unavailability Intrinsic unavailability is the unavailability of an exchange (or part of it) due to exchange (or unit) failure itself, excluding the logistic delay time (e.g. trave
44、l times, unavailability of spare units, etc.) and planned outages. perahonai unavailability is the unavailability of an exchange (or part of it) due to exchange (or unit) failure itself, including the logistic delay time (e.g. travel times, unavailability of spare units, etc.). 4.4 Planned outages P
45、lanned outages are those intentionally induced to facilitate exchange growth or hardware and/or software modifications. The impact of these activities on service depends on their duration, the time of day they are introduced and on the particular system design. 45 Total and partial unavailability Ex
46、change unavailability may be either total or partiai. Total unavailability affects all terminations, and consequently, ail traffic that is offered during the outage is equally affected. A partial outage has an effect only on some terminations. From the point of view of one termination on an exchange
47、 (e.g. a subscriber line termination), the numerical value of mean accumulated downtime (and hence the unavailability) for a specified period of time should not depend on the exchange size or its Wic handling capacity. Similarly, from the point of view of a group of terminations of size n, the mean
48、accumulated downtime for a specified period of time, in case they are simultaneously unavailable, should not depend on exchange size. However, for two groups of terminals of differing size n and m such that n is greater than m (n m), the mean accumulated downtime (and hence the unavailability) for n
49、 will be less than the mean accumulated downtime (ivAJ3T) or the unavailability for m. Thus: MADT(n) m and U(n) .e U(m). The lower limit of m is one termination, and it can be specified as having a mean value of T minutes per year. 4.6 Statistical basis Any estimation of unavailability is of necessity a statistical quantity, because outages are presumed to occur randomly and they are of random duration. Therefore, availability measurements are significant when made over a statistically significant number of exchanges. It follows then, that a single exchange may exceed the unava
