1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU$)4!,G0G0.%47/2+3#/.42/,%$G0G03,)0G0G02!4%G0G0/“*%#4)6%3G0G0/.!.G0G0).4%2.!4)/.!,G0G0$)4!,G0G0#/%#4)/.)45G134G0G0RecommendationG0G0 (Extract from the “LUEG0“OOK)NOTES1 ITU-T Recommendation G.822 was published i
2、n Fascicle III.5 of the Blue Book. This file is an extract from theBlue Book. While the presentation and layout of the text might be slightly different from the Blue Book version, thecontents of the file are identical to the Blue Book version and copyright conditions remain unchanged (see below).2 I
3、n this Recommendation, the expression “Administration” is used for conciseness to indicate both atelecommunication administration and a recognized operating agency. ITU 1988, 1993All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or
4、mechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle III.5 - Rec. G.822 1Recommendation G.822CONTROLLED SLIP RATE OBJECTIVES ON AN INTERNATIONAL DIGITAL CONNECTION(Geneva, 1980; further amended)1 GeneralThis Recommendation deals with end-to-end contro
5、lled octet slip rate objectives for 64-kbit/s internationaldigital connections. The objectives are presented for various operational conditions in relation to the evaluation ofconnection quality.Under design conditions for digital network nodes and within defined normal transmission characteristics,
6、 itmay be assumed that there are zero slips in a synchronized digital network. However, the defined transmissioncharacteristics can be exceeded under operating conditions and cause a limited number of slips to occur even in asynchronized network.Under temporary loss of timing control within a partic
7、ular synchronized network, additional slips may beincurred, resulting in a larger number of slips for an end-to-end connection.With plesiochronous operation, the number of slips on the international links will be governed by the sizes ofbuffer stores and the accuracies and the stabilities of the int
8、erconnecting national clocks.2 Scope and considerations2.1 The end-to-end slip rate performance should satisfy the service requirements for telephone and non-telephoneservices on a 64-kbit/s digital connection in an ISDN.2.2 The slip rate objectives for an international end-to-end connection are sta
9、ted with reference to the standarddigital Hypothetical Reference Connection (HRX) of Figure 1/G.801 1 of 27 500 km in length.2.3 It is assumed that international switching centres (ISC) are interconnected by international links which areoperating plesiochronously, using clocks with accuracies as spe
10、cified in Recommendation G.811. It is recognized thatone slip in 70 days per plesiochronous interexchange link is the resulting maximum theoretical slip rate, taking intoaccount clock accuracies according to Recommendation G.811 only, and provided that the performance of thetransmission and switchin
11、g requirements remain within their design limits.2.4 In the case where the connection includes all of the 13 nodes identified in the HRX (Recommendation G.801)and these nodes are all operating together in a plesiochronous mode, the nominal slip performance of a connection couldbe 1 in 70/12 days or
12、1 in 5.8 days. However, since in practice some nodes in such a connection would be part of thesame synchronized network a better nominal slip performance can be expected (e.g. where the National Networks at eachend are synchronized. The Nominal Slip Performance of the connection would be 1 in 70/4 o
13、r 1 in 17.5 days).Note - These calculations assume a maximum of four international links.2.5 In a practical international end-to-end connection containing both international and national portions, the sliprate may significantly exceed the value computed from n plesiochronous interexchange links due
14、to various design,environmental and operational conditions in international and national sections. These include:a) configuration of the international digital network,b) national timing control arrangements,c) wander due to extreme temperature variations,d) operational performance characteristics of
15、 various types of switches and transmission links (includingdiurnal variations of satellite facilities),e) temporary disturbances on transmission and synchronization links (network rearrangements, protectionswitching, human errors, etc.).Note - The maximum number, n, of plesiochronous interexchange
16、links is under study.2 Fascicle III.5 - Rec. G.8222.6 A threshold of slip performance is a suitable compromise between desired service requirements and normallyachievable performance. Slip levels according to category (b) (see Table 1/G.822) exceeding this threshold will begin toaffect performance a
17、nd can cause some services to be considered degraded. In order to ensure that a trend ofperformance has been identified, the threshold rate must be measured over a sufficient period to record a significantnumber of slips. An objective limit is placed on the total time that the threshold is exceeded
18、during the period of oneyear. The performance objectives are intended to represent a uniform set of specifications.Slip is one of several contributing factors to impairment of a digital connection. The performance objectives forthe rate of octet slips on an international connection of 27 500 km in l
19、ength or a corresponding bearer channel are givenin Table 1/G.822. Further study is required to confirm that these values are compatible with other objectives, e.g. theerror performance as listed in Recommendation G.821.TABLE 1/G.822Controlled slip performance on a 64 kbit/sinternational connection
20、or bearer channelPerformance category Mean slip rate Proportion of time(Note 1)(a)(Note 2) 5 slips in 24 hours 98.9 %(b) 5 slips in 24 hoursand 30 slips in 1 hour30 slips in 1 hour 0.1 %Note 1 - Total time 1 year.Note 2 - The nominal slip performance due to plesiochronous operation alone is not expe
21、cted to exceed 1 slipin 5.8 days (see 2.4).3 Allocation of impairments3.1 The probability of more than one section of the network experiencing excessive slips which will simultaneouslyaffect any given connection, is low. Advantage is taken of this factor in the allocation process.3.2 Because the imp
22、act of slips occurring in different parts of a connection will vary in importance depending uponthe type of service and the level of traffic affected, the allocation process includes placing tighter limits on slips detectedat international and national transit exchanges and less stringent limits on
23、small local exchanges.3.3 The recommended allocation process is based on subdividing the percentage of time objectives for performancecategories (b) and (c) (Table 1/G.822). A provisional allocation is made to the various portions of the HRX as shown inTable 2/G.822.Fascicle III.5 - Rec. G.822 3TABL
24、E 2/G.822Allocation of controlled slip performance objectivesPortion of HRX derivedAllocated proportion of eachobjective in Objectives as proportion of total timefrom Figure 1/G.801 1 Table 1/G.822 (b) (c)Internationaltransit portion8.0 % 0.08 % 0.008 %Each nationaltransit portion(Note 2)6.0 % 0.06
25、% 0.006 %Each local portion(Note 2)40.0 % 0.4 % 0.04 %Note 1 - The portions of the HRX are defined in Figure 1/G.822. They are derived from but not identical toRecommendation G.801.Note 2 - The allocation between national transit portion and local portion is given for guidance only. Administrations
26、arefree to adopt a different apportionment provided the total for each national portion (local plus transit) does notexceed 46%.Note 3 - Performance levels are defined in Table 1/G.822.Note 4 - Total time one year.Reference1 CCITT Recommendation Digital transmission models, Vol. III, Rec. G.801, Figure 1/G.801.
