1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 8 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU05“,)#G0G0$!4!G0G0.%47/2+342!.3-)33)/. G03).!,).G0!.$G037)4#().4%2-).!,G0G0!.$G0G042!.3)4G0G0#/.42/,3).!,).G0G03934%-G0G0Malaga-Torremolinos, 1984)With the appearance of public data networks in various countr
2、ies it becomes necessary to establish theappropriate 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
3、minimum of restrictions, very short call set-up and clear-down times and a variety of new servicefacilities. These requirements can be fulfilled only by a specially conceived signalling system which caters for allforeseeable needs and which is flexible enough to provide also for new, not yet defined
4、 services.For these reasons, the CCITT,unanimously recommendsfor interworking between anisochronous data networks the control signalling scheme given below should beused on international circuits.Note 1 - The start-stop user classes of service are specified in Recommendation X.1.Note 2 - The signall
5、ing for synchronous user classes of service provided on anisochronous networks is thesubject of further study.Note 3 - The signalling on links between synchronous and anisochronous networks is the subject of furtherstudy.ScopeThis Recommendation defines a decentralized terminal and transit control s
6、ignalling system for start-stopservices on international circuits between anisochronous data networks.1 General switching and signalling principles1.1 The two classes, namely user class 1 and user class 2, which are considered applicable to anisochronous types ofdata network require a control signal
7、ling rate of 300 bit/s and 200 bit/s respectively.Telex service based upon 50-baud trunks does not form part of this Recommendation1).1.2 Decentralized signalling will apply, the same channel being used for control signalling and data transmission.1.3 Both terminal and transit operation will be requ
8、ired. Due to the inclusion of transit operation, link-by-linksignalling control of calls will be adopted.Onward selection from transit and incoming terminal centres should be arranged to overlap the receipt ofselection signals, this in order to minimize call set-up times.1)See Recommendation U.12 fo
9、r telex and similar telegraph services.2 Fascicle VIII.3 - Rec. X.70Selection signals will be transmitted by the originating country at automatic speed in a single block.1.4 The numbering scheme that will be applied to networks accessed by this signalling system is defined inRecommendation X.121.The
10、 data network identification code (DNIC) (see Recommendation X.121), and network or serviceidentification signals will be transmitted on both transit and terminal calls. However, the data country code (DCC)portion of the DNIC may be suppressed in the selection signals and only the network or service
11、 digit forwarded onterminal calls if requested by the incoming network.1.5 Alternative routing will be permitted. The principle of high-usage circuits will be adopted, with overflow on toadequately provided routes between centres. Overflow on to higher speed circuits will not be permitted.In order t
12、o prevent repeated alternative routing causing traffic to circulate back to the originating point,alternative routing will be restricted to once per call.1.6 Both-way operation will be assumed and inverse order testing of circuits on both-way routes, or a closeapproximation to it by testing the rout
13、e in small groups in fixed order always starting the search from the same positionwill be specified in order to minimize head-on collisions.1.7 It is assumed that the gathering of information required for charging and accounting should normally be theresponsibility of the originating Administration
14、(see Recommendation D.10). Other arrangements for gatheringinformation are for further study.1.8 The grade of service to apply for the provision of circuits for links between public data networks ofanisochronous type which carry traffic overflowed from other routes or from which overflow was not per
15、mitted wouldnot be worse than one lost call in 50.For high-usage direct links, circuits would be provided at a grade of service of not worse than one lost callin 10.1.9 Sufficient switching equipment will be provided to ensure that congestion will not be signalled on more than0.4% of calls in the bu
16、sy hour, and only then when congestion has been positively identified.1.10 The target setting-up time for the user classes of service applicable to these types of data networks will be onesecond.2 Specific signalling characteristicsNotes applicable to 2.Note 1 - X denotes the international centre th
17、at originates the call under consideration on the international linkconcerned. Y denotes the international centre that receives the call under consideration on the international link.Note 2 - Timings shown are within the centre concerned with no allowance being made for propagation andother delays,
18、such as slow sending of selection signals from the originating terminal.Note 3 - The times for permanent start polarity (A) and stop polarity (Z) are generally indicated in the followingsignal descriptions as integral multiples of a character (see Note 4).Note 4 - For user class 1 the control signal
19、ling code (CSC) will employ 7-unit signalling characters with oneparity bit, one start and two stop elements (see Table 8/X.70). The parity of the characters will be even and hence will beconsistent with Recommendation X.4. The individual bits should be transmitted at the nominal modulation rate(300
20、 bit/s) with the low order bit (i.e. b1) first and completed by the parity bit (b8).Fascicle VIII.3 - Rec. X.70 3The end-of-selection signal will be the International Alphabet (IA5) character 2/11(+). The receptionconfirmation will use IA5 character 2/10 (*). All other signals will be characters cho
21、sen from column 3 of IA5 (seeTable 1/X.70). This choice helps ensure that the end of selection and reception confirmation signals are uniquelyseparable from the other signalling characters.For user class 2 the CSC will employ 4-unit signalling characters with one parity bit, one start and two stopel
22、ements (see Table 8/X.70). The parity of the characters will be even with regard to elements of Z polarity. Theindividual bit should be transmitted at the nominal modulation rate (200 bit/s) with the low order bit (i.e. b1) first andcompleted by the parity bit (b5).2.1 The signalling system between
23、two data networks of anisochronous type is described in Table 1/X.70.2.2 The incoming equipment may release the connection if the calling signal exceeds the specified maximum period(see Remarks column of Table 1/X.70). Start polarity will be maintained on the backward signalling path from centre Yto
24、 centre X.2.3 The first forward path signal following the calling signal (class-of-traffic character) is distinctive from the firstbackward path signal to provide a guard against head-on collisions in the case of both-way operation.A head-on collision is detected by the fact that centre X receives a
25、 first class-of-traffic character instead of thereception-confirmation signal or reception-congestion signal.When a head-on collision is detected, the switching equipment at each end of the circuit should make anotherattempt to select a free circuit, either on the same group of circuits or on a grou
26、p 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 collision onthe second attempt, no further attempt will be made and the call will be cleared down. In the case of a transit centre, thecall progre
27、ss signal No. 20 followed immediately by the clearing signal will be returned to the preceding centre after thereception-confirmation signal and the network or service identification signals.2.4 Failure to receive the reception-confirmation or reception-congestion signal within 4 seconds from the st
28、art ofthe calling signal or the reception of a spurious signal, as indicated by a character other than a first class-of-trafficcharacters the reception-confirmation signal or reception-congestion signal, should initiate the automatic retest signal onthe circuit concerned.In the case of failure to re
29、ceive the correct reception-confirmation or reception-congestion signal, anotherattempt to select a circuit should be made (once only). In the case of transit calls, if the second attempt is unsuccessful,the call progress signal No. 20 followed immediately by the clearing signal, will be returned to
30、 the preceding centre afterthe reception-confirmation signal and the network or service identification signals.2.5 Selection signals can be divided into two parts. The first part, designated as the network selection signals,contains information regarding network and user requirements and may be comp
31、osed of one to nine (or possibly more)characters (see Tables 2/X.70, 3/X.70, 4/X.70, 4a/X.70, 5/X.70 and 5a/X.70). The second part comprises the addresssignals (the called network terminal number which is preceded by the DNIC always in the case of a transit call and alsofor terminal calls unless the
32、 destination country requests omission of the data country code portion, see Tables 6/X.70and 6a/X.70).The network selection signals used in the forward direction (see also Appendix II) are further subdivided andassembled as follows (see 2.5.1 to 2.5.4 below) for signalling purposes.Note that the te
33、rm “user class of service“ is abbreviated in the following to “user class“.4 Fascicle VIII.3 - Rec. X.702.5.1 First class-of-traffic character (see Table 2/X.70)The calling signal is always followed by at least one class-of-traffic character. The bit functions of this characterwere so chosen that no
34、 further characters are needed for most connections.If there is a need for indication of further requirements, a second class-of-traffic character (see 2.5.3) may beused. Whether a second class-of-traffic or user class characters follow or not, will be indicated by the bits b3and b4ofthe first class
35、-of-traffic character.Fascicle VIII.3 - Rec. X.70 5TABLE 1/X.70Decentralized signalling between anisochronous data networksSignal or function Forward path(X towards Y)Backward path(Y towards X)RemarksFree line Start polarity (polarity A) Start polarity (polarity A)Calling signal Stop polarity (polar
36、ity Z) fora minimum period of onecharacter and a maximumperiod of two charactersfollowed immediately byselection signalsThe equipment at centre Y should beready to receive selection signals withinone character period.The minimum and consequently themaximum periods will be lengthened atthe request of
37、 the incoming country.(Note - The duration of the calling signalmay require review in the light of falsecalling signals.)Reception confirmationsignalStop polarity followed byCSC character No. 14 (userclass 2) or by IA5 characterNo. 2/10 (user class 1)Stop polarity returned within threecharacter peri
38、ods after the end of receiptof the first class-of-traffic character.The return of CSC character No 14 orIA5 character No 2/10 shall becommenced within one to two characterperiods after the inversion to stoppolarity.The reception-confirmation signal willhave to be absorbed by the switchingequipment o
39、f X and should not be ableto go through that equipment to arrive atthe preceding centre.Selection signals At least one (first class-of-traffic character only) orpossibly several networkselection signals dependingon the network requirement(see Appendix I), the digitsof the DNIC of the callednetwork,
40、the digits of thecalled terminal number, andan end-of-selection signalThese signal are transmitted immediatelyafter the calling signal without awaitingthe reception at X of the reception-confirmation signal.The selection signals are transmittedaccording to the control signalling codeat the appropria
41、te data signalling rate forthe user class-of-service concerned andat automatic speed in a single blockwhich included an end-of-selectionsignal.For the user class 1 the end-of-selectionsignal will be IA5 character No 2/11 .For user class 2 the end-of-selectionsignal will be CSC character No. 11.The d
42、ata country code (DCC) may beomitted on terminal calls at the requestof the incoming country.6 Fascicle VIII.3 - Rec. X.70TABLE 1/X.70 (continued)Signal or function Forward path(X towards Y)Backward path(Y towards X)RemarksNetwork or serviceidentification signalsCSC No. 12 followed by thedata networ
43、k identificationcode (DNIC) of the networkThe character CSC No. 12 and DNICfollow the reception-confirmation signalat automatic speed within one to twocharacter periods. These signals must gothrough centre X and arrive at theoriginating network.In all cases, the country or networkidentity shall cons
44、ist of four decimaldigits. The value of the fourth digitshould, in the case when it is notexplicity defined by the numbering plan,be at the discretion of the country inquestion within the limits allowed by thenumbering plan.Reception-congestion signal Stop polarity for a period ofone or two characte
45、rsfollowed by the clearingsignalThis signal is returned within 0-5character periodsafter the start of receiptof the calling signal when the selectionsignal cannot be received.This signal should be absorbed by centreX and not allowed to be received by apreceding exchange.Call progress signalswithout
46、clearingCSC No. 11 followed by 2digits (see Table 7d/X.70)Examples would be redirected call orterminal-called call progress signal (forfurther details see Appendix III.)Call connected signal One CSC character (seeTable 7/X.70)See 2.14 of the text and for furtherdetails see Appendix III.Strat of tran
47、sit through-connect signal (STTC)CSC No. 15 (see Table7/X.70)The signal always precedes the transitthrough-connect signal.Transit through-connectsignal (TTC)One CSC character (seeTable 7b/X.70)This signal will always be prefaced bythe start of transit through-connectsignal and will be returned prece
48、ding acall progress signal without clearingwhen this has to be sent. It will also betransmitted when the calling and/orcalled line identification is required (forfurther details see Appendix III).Transit centres through-connected signal (TTD)CSC No. 11 (see Table6/X.70)The signal will be transmitted
49、 within 40to 120 ms after the receipt of the transitthrough-connect signal (TTC) when nocalling line identification is required (forfurther details see Appendix III).Called line identification(if applicable)Combinations of the calledline identification signalstransmitted at automaticspeed within 120 ms of thereceipt of the TTD signal orthe first character of thecalling line identificationsignalsThe called line identification signalconsists of the data network or serviceidentification code (DNIC) followed bythe digits of the network terminalnumber and CSC No. 12. Where noidenti
