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2、/2+3)45G134G0G0RecommendationG0G08 (Extract from the “LUEG0“OOK)NOTES1 ITU-T Recommendation X.71 was published in Fascicle VIII.3 of the Blue Book. This file is an extract fromthe Blue Book. While the presentation and layout of the text might be slightly different from the Blue Book version, thecont
3、ents of the file are identical to the Blue Book version and copyright conditions remain unchanged (see below).2 In this Recommendation, the expression “Administration” is used for conciseness to indicate both atelecommunication administration and a recognized operating agency. ITU 1988, 1993All righ
4、ts reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle VIII.3 - Rec. X.71 1Recommendation X.71Fascicle VIII.3 - Rec. X.71DECENTRALIZED TERMI
5、NAL AND TRANSIT CONTROL SIGNALLING SYSTEMON INTERNATIONAL CIRCUITS BETWEEN SYNCHRONOUS DATA NETWORKS(Geneva, 1976, amended at Geneva, 1980;Malaga-Torremolinos, 1984, and Melbourne, 1988)With the appearance of public data networks in various countries it becomes necessary to establish theappropriate
6、international control signalling schemes for interworking in order to facilitate the introduction of suchnetworks as much as possible. The main objective of public data networks is to offer to the user a great range of datasignalling rates with a minimum of restrictions, very short call set-up and c
7、lear-down times and a variety of newservice facilities. These requirements can be fulfilled only by specially conceived signalling systems which cater for allforeseeable needs and which are flexible enough to provide also for new, not yet defined, services.For these reasons, the CCITTunanimously rec
8、ommendsfor interworking between synchronous data networks utilizing decentralized control signalling techniques thescheme given below should be used on international circuits.Note 1 - The synchronous user classes of service are as specified in Recommendation X.1.Note 2 - The signalling on links betw
9、een synchronous and anisochronous networks is the subject of furtherstudy.Note 3 - The interworking between common channel and decentralized signalling is the subject ofRecommendation X.80.ScopeThis Recommendation defines a decentralized control signalling system for use in setting up terminal andtr
10、ansit calls on international circuits between synchronous data networks.1 General switching and signalling principles1.1 Signalling will be at bearer rates appropriate to the synchronous user classes of data only. It is expected thatstart-stop user classes of data, telex, etc., will be assembled and
11、 transmitted in accordance with Recommendation X.52.1.2 The control signalling should employ bits transmitted at the maximum data signalling rate of the linksprovided.1.3 Decentralized signalling will apply, the same channel being used for control signalling and data transmission.1.4 Both terminal a
12、nd transit operation will be required. Due to the inclusion of transit operation, link-by-linksignalling control of calls will be adopted.2 Fascicle VIII.3 - Rec. X.71The data network identification code (DNIC) (see Recommendation X.121), and network or serviceidentification signals will be transmit
13、ted on both transit and terminal calls. However, the data country code (DCC)portion of the DNIC may be suppressed and only the network or service digit forwarded on terminal calls if requestedby the incoming network.Onward selection from transit and incoming terminal centres should be arranged in or
14、der to commence assoon as possible.Selection signals will be transmitted by the originating or transit country, or network, in a single block.1.5 The numbering scheme that will be applied to networks accessed by this signalling system is defined inRecommendation X.121.1.6 Alternative routing will be
15、 permitted. The principle of high-usage circuits will be adopted, with overflowonto adequately provided routes between centres.Overflow onto higher speed circuits will not be permitted.In order to prevent repeated alternative routing causing traffic to circulate back to the originating point,alterna
16、tive routing will be restricted to once per call.1.7 Both-way operation will be assumed and inverse order of testing of circuits on both-way routes or a closeapproximation to it by testing the route in small groups in fixed order always starting the search from the sameposition, will be specified in
17、 order to minimize head-on collisions.1.8 It is assumed that the gathering of information required for charging and accounting should normally be theresponsibility of the calling Administration (see Recommendation D.10). Other arrangements for gathering informationare for further study.1.9 The grade
18、 of service to apply for the provision of circuits for links between public data networks of thesynchronous type which carry traffic overflowing from other routes, or from which overflow was not permitted, wouldnot be worse than one call lost in 50.For high-usage direct links circuits would be provi
19、ded at a grade of service of not worse than one lost callin 10.1.10 Sufficient switching equipment will be provided to ensure that congestion will not be signalled by return of areception-congestion signal or absence of a proceed-to-select signal on more than 0.4% of calls in the busy hour and,in th
20、e first case, only then when congestion has been positively identified.1.11 The target setting-up time for the user classes of service applicable to these types of data networks is forfurther study.2 Specific signalling characteristicsNotes applicable to 2.Note 1 - X denotes the international centre
21、 which originates the call under consideration on the internationallink concerned. Y denotes the international centre which receives the call under consideration on the international link.Note 2 - Timings shown are within the centre concerned, excluding propagation and other transmission delays.Note
22、 3 - The signalling plan will employ 8-bit signalling characters and continuous 0s1)and 1s._1)The impact of the all zeros pattern is left for further study.Fascicle VIII.3 - Rec. X.71 3During the control signalling stage, the status bits are 0s. Upon the final through-connection in the originatingex
23、change, the status bits on both signalling paths are 1s.For the case of signalling characters, the parity of the characters will be odd, and hence will be consistent withRecommendation X.4 for links and connections using end-to-end synchronous operation, and with RecommendationX.21. For the case of
24、signals being continuous 0s, or continuous 1s, parity is undefined inasmuch as no characters areemployed. Moreover, character synchronization is not maintained over a period of continuous 0s or 1s, but must be re-established when further signalling characters are sent.All groups of contiguous charac
25、ters will be preceded by at least two repetitions of International AlphabetNo. 5 (IA5) character 1/6 (SYN). The term “at least“ means two SYN characters for the 600 bit/s user class. For thehigher speed user classes, the number of SYN characters could be two or more but the total number of SYN chara
26、ctersshould not unnecessarily prolong the setting-up time. If two signalling groups are combined to form one group ofcontiguous characters, the SYN characters may be omitted from within this group.The end-of-selection signal will be the IA5 character 2/11 (+). The call confirmation and proceed-to-se
27、lectsignals will use IA5 character 2/10 (*).Apart from the above mentioned signals (namely, continuous 0s, continuous 1s, 1/6, 2/10 and 2/11), all signalswill be characters chosen from column 3 of IA5 (see Table 1/X.71). This choice helps ensure that the synchronizationand other characters specified
28、 above are uniquely separable from the IA5 column 3 signalling characters.An example of three successive signalling characters within five octets of one channel of the RecommendationX.50 multiplex structure is shown in Appendix V. In the Recommendation X.51 multiplex structure, the signallingcharact
29、ers will be aligned with the 8 + 2 envelope.2.1 The signals between two data networks of synchronous type are described in Table 1/X.71. There are twoprotocols, the Call Confirmation Protocol (CCP) and the Proceed-To-Select Protocol (PTSP). The CCP is the basicmethod of this Recommendation and the P
30、TSP is an option for an interim period at the discretion of the incomingnetwork.2.2 The incoming equipment may release the connection as follows:2.2.1 Call Confirmation ProtocolIf the calling signal exceeds the specified maximum period, but not before at least one call confirmationcharacter has been
31、 transmitted.2.2.2 Proceed-to-select ProtocolIf the first selection signal is not received within 2 seconds after having sent the proceed-to-select signal.2.3 A head-on collision is detected by the fact that exchange X receives calling signal (repetitions of 1s) followedby SYN characters, instead of
32、 call confirmation or proceed-to-select signal (SYN characters followed by repetitions of2/10) or reception-congestion signal (repetitions of 1s followed by clearing signal).When a head-on collision is detected, the switching equipment at each end of the circuit should make anotherattempt to select
33、a free circuit, either on the same group of circuits or on a group of overflow circuits, if facilities foralternative routing exist and there are no free circuits on the primary route. In the event of a further head-on collisionon the second attempt, no further attempt will be made and the call will
34、 be cleared down. In the case of a transit centre,the call progress signal No. 20 is returned to the preceding centre within a sequence of signals ordered as follows: callconfirmation or proceed-to-select, network or service identification, the call progress signal and clearing.2.4 Failure to receiv
35、e reception-congestion, call confirmation or proceed-to-select signal within 4 seconds fromthe start of the calling signal, the reception of a spurious signal as indicated by a signal other than reception-congestion, call confirmation or proceed-to-select signal, or by a head-on collision, can initi
36、ate the automatic retestsignal on the circuit concerned.4 Fascicle VIII.3 - Rec. X.71The need for an automatic retest signal may not be so great in a digital environment, its purpose being met byalternative methods. If an automatic retest signal is used, however, it will conform to 2.16.In the case
37、of failure to receive reception-congestion, call confirmation or proceed-to-select signal, an attemptto select another circuit should be made (once only). In the case of transit calls, if the second attempt is unsuccessful,the call progress signal No. 20 is returned to the preceding centre within a
38、sequence of signals ordered as follows: callconfirmation or proceed-to-select, network or service identification, the call progress signal and clearing.Fascicle VIII.3 - Rec. X.71 5TABLE 1/X.71Decentralized signalling between synchronous data networksSignal or function Forward path(X towards Y)Backw
39、ard path(Y towards X)RemarksFree line S = 0, continuousrepetitions of 0sS = 0, continuousrepetitions of 0sS = 0, continuousrepetitions of 1sCalling signal- - - - - - - - - - - - - - - - - -For CC protocol, this signalis continuous for aminimum period of 10 msor 16 information bits,whichever is the g
40、reater intime, and for a maximumperiod of 15 ms or 24information bits, whicheveris the greater in timea)- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -The equipment at exchange Y should beready to receive selection signals withina period of 10 ms or 16 infor
41、mation bits,whichever is the greater in time, fromthe start of the received calling signal.- - - - - - - - - - - - - - - - - -For PTS protocol, thissignal is continuous untilthe proceed-to-selectsignal is received- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
42、 -The proceed-to-select signal should bereturned when the equipment is ready toreceive selection signals.Call confirmation signal(CC protocol)S = 0, continuous repeti-tions of IA5 character 2/10maintained until the firstcall-of-traffic character isrecognized and alwayspreceded by at least 2 SYNchara
43、cters (1/6).Returned within 10 ms or 16 informa-tion bits of receipt of the calling signal,whichever is the greater in time.The call confirmation signal shall befollowed by the network or serviceidentification signal within 50 ms ofreceipt of the first class-of-trafficcharacter, followed by the wait
44、ing signalif no other characters followcontiguously.The call confirmation signal will have tobe absorbed at centre X and should notbe able to go through the equipment toarrive at the preceding centre.Proceed-to-select signal(PTS protocol)S = 0, continuous repeti-tions of IA5 character 2/10maintained
45、 until the firstclass-of-traffic character isrecognized and alwayspreceded by at least 2 SYNcharacters (1/6).Returned within 3 seconds from the startof the received calling signal.The proceed-to-select signal shall befollowed by the network or serviceidentification signal within 50 ms ofreceipt of t
46、he first class-of-trafficcharacter, followed by the waiting signalif no other signalling characters followcontiguously.The proceed-to-select signal will have tobe absorbed at centre X and should notbe able to go through the equipment toarrive at the preceding centre.6 Fascicle VIII.3 - Rec. X.71TABL
47、E 1/X.71 (continued)Signal or function Forward path(X towards Y)Backward path(Y towards X)RemarksSelection signals S = 0, at least one (firstclass-of-traffic characteronly) and possibly severalnetwork selection signalsdepending on the networkrequirement (see AppendixI), the digits of the DNICof the
48、called network, thedigits of the called terminalnumber, and as end-of-selection character (2/11)and then followed by thewaiting signalThe selection signals are transmitted atthe maximum data signalling rate of thelinks provided.The data country code (DCC) may beomitted on terminal calls at the reque
49、stof the incoming country.For CC protocol, these signals, precededby at least two SYN characters, aretransmitted immediately after the callingsignal without awaiting the reception atX of the call-confirmation signal.For PTS protocol, these signals,preceded by at least two SYN characters,are transmitted immediately afterreception at X of the proceed-to-selectsignal.Network or serviceidentification signalsS = 0, IA5 character 3/11followed by the DNIC ofthe network followed bythe waiting signal if noother signalling charactersfollow contiguouslyThe characters 3/11 and
