1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 5 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(G0G037)4#().%.%2!,(90/4(%4)#!,G0G02%(b) the overall subscriber-to-subscriber performance objectives;(c) the technical provisions in Recommendations R.57 and R.58 concerning standard limits oftransmiss
2、ion quality;(d) the need to standardize the signalling functions in international/intercontinental transit exchanges;(e) the telex signalling specified in Recommendations U.1 (types A and B), U.11 (type C) and U.12(type D);(f) the level differences existing among the type A, B, C and D signalling fu
3、nctions,unanimously recommendsthe use of the hypothetical reference connections contained in this Recommendation.1 General1.1 The hypothetical reference connections set down in the present Recommendation (see Figure 1/U.8) areintended for assessing the overall subscriber-to-subscriber performance, f
4、or determining answerback return delay,signal transfer delay and other characteristics and setting-up delays related to the hypothetical reference circuit.1.2 The hypothetical reference connections concerning signalling aspects set down in the presentRecommendation(see Figure 2/U.8 and Tables 1/U.8
5、to 3/U.8) are intended for specifying the transit environmentwhere the signalling functions should be considered.2 Signalling levels2.1 There will be two levels of signalling:a) low level (type A or B);b) high level (type C or D). High level in this case indicates the ability of the signalling syste
6、m to signaladditional customer facilities and/or additional network facilities, such as alternative routing.2.2 Only high level signalling will be used in a transit connection where alternative routing is possible because ofthe need to indicate changes of routing for accounting purposes.2.3 Routing
7、may be on the basis of all high level, all low level or one transition from low to high and then backfrom high to low if required.2.4 To restrict the call set-up delay to a reasonable period,a) low level signalling types, because of their slower compelled nature, will not be used for transitswitchin
8、g on routes with long propagation delays, e.g., satellite links;b) dial selection will not be used for transit switching.2.5 Only Recommendation F.69 codes will be used for routing purposes in transit switching,2 Fascicle VII.2 - Rec. U.82.6 It is noted that as an interim solution, transit traffic i
9、s at present being switched on a fixed routing basis usingonly low level signalling.2.7 Connections using ARQ radio circuits and signalling according to Recommendation U.20 have been excluded.Fascicle VII.2 - Rec. U.8 3TABLE 1/U.8Signalling combinations for two transit linksCombination No. Link Excluded120123AABCD4567BABCD891011CABCD12131415DABCDTABLE 2/U.8Signalling combinations for three transit linksCombination No.Link Excluded Combination No. LinkExcluded123 1230123ACABCD16171819CCABCD4567DABCD20212223DABCD891011BCABCD24252627DCABCDX12131415DABCD28293031DABCD4Fascicle VII.2 - Rec. U.8
