1、INTERNATIONAL TELECOMMUNICATION UNIONITU-TQ.724TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU(11/88)SERIES Q: SWITCHING AND SIGNALLINGSpecifications of Signalling System No. 7 Telephone user partSignalling proceduresITU-T Recommendation Q.724Extract of Blue Book Fascicle VI.8 (1988)NOTES1 ITU-T Recom
2、mendation Q.724 was published in Fascicle VI.8 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 re
3、main 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, 1997All rights reserved. No part of this publication may be reproduced or utilized in any form
4、or by any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle VI.8 Rec. Q.724 1Recommendation Q.724Fascicle VI.8 Rec. Q.724SIGNALLING PROCEDURES1 Normal call set-upIn this Recommendation the signalling procedures are described for
5、 the normal call set-up of an internationalcall. The messages and signals are defined in Recommendation Q.722 and the format and content are given inRecommendation Q.723.1.1 Initial address messageAn initial address message which is sent as the first message of a call set-up generally includes all o
6、f theinformation required by the next international exchange to route the call. The seizing function is implicit in the receptionof this initial address message.The sending sequence of address information will be the country code (not sent to an incoming internationalexchange) followed by the nation
7、al (significant) number. For calls to operator positions (code 11 and code 12), refer toRecommendation Q.107 1.All digits required for routing the call through the international network will be sent in the initial addressmessage. On calls with a country code in the address (except in the case of cal
8、ls to special operators), the initial addressmessage will contain a minimum of 4 digits and should contain as many digits as are available. All digits of the addressmay be included; however, the initial address message can contain one digit in specific circumstances, e.g. nationalapplications.Select
9、ion of the outgoing national circuit normally can start at the incoming international exchange on receiptof the initial address message and signalling can proceed on the first national link.When no echo suppressor or nature-of-circuit indication is received from a preceding circuit using a signallin
10、gsystem with fewer facilities, the indicators will be considered as received no, unless exchange data indicates otherwise.Note When additional signalling information (e.g. related to supplementary services) is to be sent, an initialaddress message with additional information may be used.1.2 Subseque
11、nt address messageThe remaining digits, if any, of the address may be sent individually in one-digit messages or in groups inmultidigit messages. Efficiency can be gained by grouping together as many digits as possible.However, to prevent an increase in post-dialling delay in those cases where overl
12、ap operation with subscribersdialling is used, it may be desirable to send the last few digits individually. With reference to the withholding of digits,sufficient digits should be withheld to avoid the operation at subsequent exchanges of the short 4-6 second timeout whichmay be used in certain cas
13、es to determine the address complete condition. (See Recommendation Q.608, 8.2.1).Subsequent address messages can be sent on the national network as they are received. If a continuity-checkhas to be performed on one or more of the international circuits involved in the connection, appropriate measur
14、es e.g. bywithholding the last digit(s) of the national number must be taken at the last common channel exchange to preventringing the called subscriber or alerting the operator until the continuity of such speech circuits has been verified.Note If in the international network the code 0000 in the n
15、umber of address signals field is received themessage is considered as faulty.1.3 End-of-pulsing (ST) signalThe end-of-pulsing (ST) signal is always sent in the following situations:a) semiautomatic calls,b) test calls, andc) when the end-of-pulsing signal is received from a preceding circuit.2 Fasc
16、icle VI.8 Rec. Q.724In automatic working, the end-of-pulsing signal will be sent whenever the outgoing international exchange is ina position to know, by digit analysis, that the final digit has been sent. Digit analysis may consist of an examination of thecountry code and counting the maximum (or f
17、ixed) number of digits of the national number. In other cases, theend-of-pulsing signal is not sent and the end-of-address information is determined by the receipt of one of theaddress-complete signals from the incoming international exchange.1.4 Continuity-check of the telephone circuitsBecause the
18、 signalling in Signalling System No. 7 does not pass over the speech path, facilities should beprovided for making a continuity-check of the speech path in the circumstances described below.The application of the continuity-check depends on the type of the transmission system used for the telephonec
19、ircuit.For transmission systems having some inherent fault indication features giving an indication to the switchingsystem in case of fault, a continuity check is not required. This situation commonly occurs when fully digital circuits areused. However, a per-call continuity check may be needed on f
20、ully digital circuits when circuits or bundles of circuits inprimary multiplex groups are dropped and inserted en route between switches and alarm indications carried on bits of theprimary multiplex frame structure are lost in passing through an intermediate transmission facility that does not relay
21、them transparently. Typically, per-call continuity checks may be needed when the transmission link between switchescontains a TDMA satellite system, a digital circuit multiplication system or a digital access and crossconnection system,where fault indications are lost.When an initial address message
22、 is received with a request for a continuity-check relating to a digital circuithaving inherent fault indication, one of the following actions is taken:a) the continuity-check request is disregarded; orb) a continuity-check loop is connected and the maintenance system is alerted. In this case the ca
23、ll may failsince no continuity signal may be received from the distant end.Note The reception of such a request could only be caused by an abnormal condition such as administrativeerrors or the occurrence of signalling errors.When the circuit type is unknown to a Signalling System No. 7 exchange, or
24、 in an application where bothanalogue and digital circuits may be served, or when no inherent fault indication is available, a continuity-check loopshould always be connected in the following cases:i) when the exchange has the capability to process initial address messages with continuity-check requ
25、est andsuch messages are received;ii) when continuity-check requests are received.For analogue circuits with pilot supervision it is sufficient to perform the continuity-check on a statistical basisor by test calls (see 7.5)1). For analogue circuits not using pilot supervision and for mixed circuits
26、, i.e. analogue anddigital circuits, the continuity-check should be performed on a per call basis. Within mixed connections, i.e. connectionscomposed of circuits with and without continuity-check on a per call basis, it shall be ensured that the continuity signal beforwarded to the destination point
27、 although no continuity-check may have been performed on one or more parts of theend-to-end connection.The continuity-check is not intended to eliminate the need for routine testing of the transmission path.The continuity-check of the speech circuit will be done, link-by-link, on a per call basis or
28、 by a statisticalmethod prior to the commencement of conversation. Procedures and requirements are specified in 7.The actions to be taken when pilot supervision is used are described in 9._1)The application to the international circuits and the quantitative aspects (in particular, the frequency of p
29、erforming thecontinuity-check) are for further study.Fascicle VI.8 Rec. Q.724 31.5 Cross-office checkFor digital exchanges the requirements mentioned in Recommendation Q.504 2 shall be met. For otherexchanges Administrations shall ensure the reliability of a connection through a switching machine (c
30、ross-office check)either on a per call basis or by a statistical method. With either method, the probability of the connection beingestablished with an unacceptable speech path transmission quality should not exceed 105as the long-term average.1.6 Address-complete signalsAn address-complete signal w
31、ill not be sent until the continuity signal has been received and the cross-officecheck made, if they are applicable.If the succeeding network does not provide electrical called-partys-line-condition signals, the last SignallingSystem No. 7 exchange shall originate and send an address-complete signa
32、l when the end of address signalling has beendetermined and a possible GRQ/GSM cycle has been completed:a) by receipt of an end-of-pulsing signal;b) by receipt of the maximum number of digits used in the national numbering plan;c) by analysis of the national (significant) number to indicate that a s
33、ufficient number of digits has beenreceived to route the call to the called party;d) by receipt of an end-of-selection signal from the succeeding network (e.g. number received signal inSignalling System No. 4); ore) exceptionally, if the succeeding network uses overlap signalling and number analysis
34、 is not possible, byobserving that 4 to 6 seconds have elapsed since the last digit was received, and that no fresh informationhas been received; in such circumstances, transmission to the national network of the last digit receivedmust be prevented until the end of the waiting period which causes a
35、n address-complete signal to be sentover the international circuit. In this way, it is ensured that no national answer signal can arrive before anaddress-complete signal has been sent.Specifically, in cases d) and e) above, the address-complete charge signal should be sent.Note If the succeeding net
36、work provides electrical called-partys-line-condition signals, the last SignallingSystem No. 7 exchange shall originate and send address-complet signal when that condition has been received from thesucceeding network and a possible GRQ/GSM cycle has been completed.If in normal operation, delay in th
37、e receipt of an address-complete or equivalent signal from the succeedingnetwork is expected, the last common channel signalling exchange will originate and send an address-complete signal 15to 20 seconds after receiving the latest address message. This time-out condition is an upper limit consideri
38、ng the clausesof 6.4.1 (20 to 30 seconds for outgoing international exchanges in abnormal release conditions).On receipt of an address-complete signal, the first Signalling System No. 7 exchange will through-connect thespeech path of the interconnected circuit2).After an address-complete signal, onl
39、y the following signals relating to the call set-up may be sent in thebackward direction:a) in normal operation, one of the answer or release-guard signals;b) call-failure signal; orc) the national network congestion signal; ord) the circuit group congestion signal.Note Cases b), c) and d) can only
40、occur after an address complete signal without subscriber free.Any further information about the called-partys-line-condition will be transmitted to the calling subscriber oroperator as audible tones or announcements._2)It is envisaged that in the future evolution of the Telephone User Part (e.g. in
41、 the context of an integrated services digital network)the through-connection immediately after sending of the initial address message may become a mandatory requirement.4 Fascicle VI.8 Rec. Q.724The address-complete signal with the subscriber-free indication is sent when it is known that the called
42、subscribers line is free (not busy). It must be originated in the called subscribers exchange, and therefore cannot befollowed by one of the unsuccessful backward set-up information signals.If an incoming international exchange has sent a general request message, then an address complete messagemust
43、 not be sent until a general forward set-up information message has been received in response to that general forwardset-up information message.1.7 Address-incomplete signalThe determination that the proper number of digits has not been received can be made at once if theend-of-pulsing signal is rec
44、eived or by receipt of an address-incomplete signal (or equivalent) from the national network.When overlap working is used and the end-of-pulsing signal has not been received, the address-incomplete signal will besent by the last common channel Signalling exchange 15 to 20 seconds after receipt of t
45、he latest digit.Each Signalling System No. 7 exchange on receipt of the address-incomplete signal will send the signal to thepreceding Signalling System No. 7 exchange, if any, and clear forward the connection. The first Signalling System No. 7exchange will send a suitable signal on the preceding ci
46、rcuit if the related signalling system permits to do so; otherwisethe appropriate tone or announcement for the national network concerned will be sent to the calling party.1.8 Congestion signalsAs soon as the congestion condition is detected one of the congestion signals (see Recommenda-tion Q.722,
47、3.4) is sent without waiting for the completion of a possible continuity-check sequence.Reception of a congestion signal at any Signalling System No. 7 exchange will cause the clear-forward signal tobe sent and cause an appropriate signal to be sent to the preceding exchange if the signalling system
48、 allows this or anappropriate tone or announcement to be sent to the originating subscriber or operator.1.9 Called-partys-line-condition signalsThe called-partys-line-condition signals (see Recommendation Q.722, 3.4) will be sent when the appropriateelectrical signals are received at the incoming in
49、ternational exchange from the national network.The called-partys-line-condition signals will be sent without waiting for the completion of a possible conti-nuity check. On receipt of one of these signals, the first Signalling System No. 7 exchange (or the outgoing internationalexchange) will clear forward the connection and cause an appropriate signal to be sent to the preceding exchange if thesignalling system allows this or an appropriate tone or announcement to be sent to the originating subscriber or operator.E
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