1、INTERNATIONAL TELECOMMUNICATION UNION ITU=T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU 4.703 (07/96) SERIES Q: SWITCHING AND SIGNALLING Specifications of Signalling System No. 7 - Message transfer part Signalling link ITU-T Recommendation Q.703 (Previously CCITT Recommendation) COPYRIGHT Intern
2、ational Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesITU-T Q-SERIES RECOMMENDATIONS SWITCHING AND SIGNALLING SIGNALLING IN THE INTERNATIONAL MANUAL SERVICE =UNCTIONS AND INFORMATION FLOWS FOR SERVICES IN THE ISDN SPECIFICATION OF SIGNALLING SYSTEMS No. 4 AN
3、D No. 5 SPECIFICATIONS OF SIGNALLING SYSTEM No. 6 SPECIFICATIONS OF SIGNALLING SYSTEM RI SPECIFICATIONS OF SIGNALLING SYSTEM R2 DIGITAL EXCHANGES INTERWORKING OF SIGNALLING SYSTEMS SPECIFICATiONS OF SIGNALLING SYSTEM No. 7 NTERNATIONAL AUTOMATIC AND SEMI-AUTOMATIC WORKING ZLAUSES APPLICABLE TO ITU-T
4、 STANDARD SYSTEMS General ge transfer part Simplified message transfer part Signalling connection control part Telephone user part ISDN supplementary services Data user part Signalling System No. 7 management ISDN user part Transaction capabilities application part Test specification Q3 interface DI
5、GITAL SUBSCRIBER SIGNALLING SYSTEM No. I PUBLIC LAND MOBILE NETWORK INTERWORKING WITH SATELLITE MOBILE SYSTEMS INTELLIGENT NETWORK BROADBAND ISDN Q. I -Q. 3 Q.4-Q.59 Q.60-Q.99 Q. I OO-Q. 1 1 9 Q.120-Q.249 Q.250-Q.309 Q.310-Q.399 Q.400-Q.499 Q.500-Q.599 Q.600-Q.699 Q.700-Q.849 Q.700 Q.701-Q.709 Q.710
6、 Q.711-Q.719 Q. 720-Q. 729 Q.730-Q.739 Q.740-Q ,749 Q.750-Q.759 Q.760-Q.769 Q.770-Q.779 Q.780-Q.799 Q ,800-Q . 849 Q. 850-Q. 999 Q.1000-Q.1099 Q.ll OO-Q.1 I99 Q.1200-Q.1999 Q.2000-Q.2999 Forjrther details, please refer to ITU-T List of Recommendations. . COPYRIGHT International Telecommunications Un
7、ion/ITU TelecommunicationsLicensed by Information Handling Services STDmITU-T RECMN Q-703-ENGL L77b W 4Bb257L Ob270b2 821 9 ITU-T RECOMMENDATION Q.703 SIGNALLING LINK Summary This Recommendation describes the functions and procedures for and relating to the transfer of messages over one signalling d
8、ata link. Annex A has been added to support the use of data rates of 1.5 and 2.0 Mbit/s as a national option. In addition some errors in the SDL diagrams have been corrected. Source ITU-T Recommendation Q.703 was revised by ITU-T Study Group 11 (1993-1996) and was approved under the WTSC Resolution
9、No. 1 procedure on the 9th of July 1996. COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesSTD-ITU-T RECMN Q.703-ENGL L77b m 4b257L Ub27Ub3 7b8 FOREWORD ITU (International Telecommunication Union) is the United Nations Specialized Agency
10、in the field of telecommunications. The IT Telecommunication Standardization Sector (ITU-T) is a permanent organ of the ITU. The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldw
11、ide basis. The World Telecommunication Standardization Conference (WTSC), which meets every four years, establishes the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. The approval of Recommendations by the Members of the IT-T is covered by t
12、he procedure laid down in WTSC Resolution No. 1 (Helsinki, March 1-12, 1993). In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with IS0 and IEC. NOTE In this Recommendation, the expression “Administration“ is used
13、 for conciseness. to indicate both a telecommu.nication administration and a recognized operating agency. O ITU 1997 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without per
14、mission in writing from the ITU. 11 Recommendation Q.703 (07/96) COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling Services STD-ITU-T RECMN Q.703-ENGL L77b i8b259L Ob270b4 bT4 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 2 2.1 2.2 2.3 2.4 3 3.1 3.2 4 4.1 4.
15、2 5 5.1 5.2 CONTENTS General Introduction Signal unit delimitation and alignment . Error detection Error correction . Initial alignment Signalling link error monitoring . Link state control functions Flow control Basic signal unit format General Signal unit format . Function and codes of the signal
16、unit fields . 2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 2.3.6 2.3.7 2.3.8 2.3.9 2.3.10 General Flag Length indicator Service information octet Sequence numbering . Indicator bits Check bits Signalling information field Spare fields Status field . Order of bit transmission Signal unit delimitation Flags . Zero i
17、nsertion and deletion . Acceptance procedure . Acceptance of alignment Error detection Basic error correction method General Acknowledgements (positive acknowledgement and negative acknowledgement) . 5.2.1 Sequence numbering . Recommendation 4.703 (07196) Page 1 1 1 1 2 3 3 3 3 4 4 6 6 6 6 7 7 7 7 7
18、 7 7 7 8 8 8 8 9 9 10 10 . 111 COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesSTD-ITU-T RECMN Q-703-ENGL L77b m 48b2.572 Ub270b5 530 9 5.3 6 6.1 6.2 6.3 6.4 7 7.1 7.2 7.3 7.4 8 9 9.1 9.2 9.3 9.4 10 10.1 10.2 10.3 11 iv 5.2.2 Signal uni
19、t sequence control . 5.2.3 Positive acknowledgement 5.2.4 Negative acknowledgement Retransmission . 5.3.1 Response to a positive acknowledgement . 5.3.2 Response to a negative acknowledgement 5.3.3 Repetition of message signal units Error correction by preventive cyclic retransmission . General Ackn
20、owledgements 6.2.1 Sequence numbering . 6.2.2 Signal unit sequence control . 6.2.3 Positive acknowledgement Preventive cyclic retransmission 6.3.1 Response to a positive acknowledgement . 6.3.2 Preventive cyclic retransmission procedure Forced retransmission . 6.4.1 Forced retransmission procedure .
21、 6.4.2 Limitation of the values N, and N2 Initial alignment procedure . General Initial alignment status indications . Initial alignment procedure . Proving periods . Processor outage . Level 2 flow control . General Detection of congestion Congestion abatement procedure Procedure in the congestion
22、situation . Signalling link error monitoring . General Signal unit error rate monitor . Alignment error rate monitor Page 10 11 11 11 11 12 13 13 13 13 13 13 14 14 14 15 15 15 15 16 16 16 16 18 19 20 20 20 20 20 21 21 21 22 Level 2 codes and priorities 22 Recommendation Q.703 (07/96) COPYRIGHT Inter
23、national Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesSTD-ITU-T RECMN Q-703-ENGL L77b 48b257L Ob270bb 977 U Page 1 1.1 11.2 Link status signal unit . Transmission priorities within level 2 12 12.2 Abbreviations . State transition diagrams. abbreviations and
24、 timers . 12.3 Timers . Annex A -Additions for a national option for high speed signalling links . A.l Introduction A . 1 . 1 Procedures for 1.5 and 2.0 Mbit/s data rate signalling links . A.4.1 Acceptance of alignment . A.4 Acceptance procedure . A.lO.l General A . 10.2 Errored interval monitor for
25、 1.5 Mbit/s and 2.0 Mbits links A . 10.3 Alignment error rate monitor A.12.3 Timers 22 23 24 24 26 81 81 82 83 83 83 83 84 84 Recommendation 4.703 (07/96) V COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesSTD-ITU-T RECMN Q.703-ENGL 1776
26、 48b2571 Ob270b7 303 Recommendation 4.703 SIGNALLING LINK (Geneva 1980; modiJied at Helsinki, I993, revised in I996) 1 General 1.1 Introduction 1.1.1 This Recommendation describes the functions and procedures for and relating to the transfer of signalling messages over one signalling data link. The
27、signalling link functions, together with a signalling data link as bearer, provide a signalling link for reliable transfer of signalling messages between two directly connected signallingpoints. Signalling messages delivered by superior hierarchical levels are transferred over the signalling link in
28、 variable length signal units. The signal units include transfer control information for proper operation of the signalling link in addition to the signalling information. 1.1.2 The signalling link functions comprise: 4 signal unit delimitation; b) signal unit alignment; c) error detection; d) error
29、 correction; e) initial alignment; 0 signalling link error monitoring; g) flow control. All these functions are coordinated by the link state control (see Figure 1). 1.2 The beginning and end of a signal unit are indicated by a unique %bit pattern, called the )ag. Measures are taken to ensure that t
30、he pattern cannot be imitated elsewhere in the unit. Loss of alignment occurs when a bit pattern disallowed by the delimitation procedure (more than six consecutive 1 s) is received, or when a certain maximum length of signal unit is exceeded. Loss of alignment will cause a change in the mode of ope
31、ration of the signal unit error rate monitor. Signal unit delimitation and alignment 1.3 Error detection The error detection function is performed by means of 16 check bits provided at the end of each signal unit. The check bits are generated by the transmitting signalling link terminal by operating
32、 on the preceding bits of the signal unit following a specified algorithm. At the receiving signalling link terminal, the received check bits are operated on using specified rules which correspond to that algorithm. 1 A signalling link terminal refers to the means of performing ail of the functions
33、defined at level 2 regardless of their implementation. Recommendation 4.703 (07/96) 1 COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesSTD-ITU-T RECMN Q-703-ENGL L97b M LiBb2592 Ob270bA 2LiT If consistency is not found between the receiv
34、ed check bits and the preceding bits of the signal unit, according to the algorithm, then the presence of errors is indicated and the signal unit is discarded. Signalling network functions (level 3) MSUa) Signalling link control (level 2) Reception LSSU.) A . B . Link state control part Congestion c
35、ontrol . Transmission , 4 Part Retrieved MSU ai MSUa) I I Signalling message flows Controls and indications MSU Message Signal Unit SU Signal Unit LSSU Link Status Signal Units *) These signal units do not include all error control information. Error detection delimitation and alignment Transmitted
36、and received bits - Signalling data link (level 1) T1156520-93 Figure VQ.703 - Interactions of the functional specification blocks for signalling link control 1.4 Error correction . 1.4.1 Two forms of error correction are provided, the basic method and the preventive cyclic retransmission method. Th
37、e following criteria should be used for determining the international fields of application for the two methods: a) the basic method applies for signalling links using non-intercontinental terrestrial transmission means and for intercontinental signalling links where the one-way propagation delay is
38、 less than 15 ms; the preventive cyclic retransmission method applies for intercontinental signalling links where the one-way propagation delay is greater than or equal to 15 ms and for all signalling links established via satellite. In cases where one signalling link within an international link se
39、t is established via satellite, the preventive cyclic retransmission method should be used for all signalling links of that link set (combined linkset). 1.4.2 The basic method is a non-compelled, positivehegative acknowledgement, retransmission error correction system. A signal unit which has been t
40、ransmitted is retained at the transmitting signalling link terminal until a positive acknowledgement for that signal unit is received. If a negative acknowledgement is received, then the transmission of new signal units is interrupted and those signal units which have been transmitted but not yet po
41、sitively acknowledged starting with that b) 2 Recommendation 4.703 (07/96) COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling Servicesindicated by the negative acknowledgement will be retransmitted once, in the order in which they were first trans
42、mitted. 1.4.3 The preventive cyclic retransmission method is a non-compelled, positive acknowledgement, cyclic retransmission, forward error correction system. A signal unit which has been transmitted is retained at the transmitting signalling link terminal until a positive acknowledgement for that
43、signal unit is received. During the period when there are no new signal units to be transmitted, all the signal units which have not yet been positively acknowledged are retransmitted cyclically. The forced retransmission procedure is defined to ensure that forward error correction occurs in adverse
44、 conditions (e.g. high error rate and/or high traffic loading). When a predetermined number of retained, unacknowledged signal units exists, the transmission of new signal units is interrupted and the retained signal units are retransmitted cyclically until the number of unacknowledged signal units
45、is reduced. 1.5 Initial alignment The initial alignment procedure is appropriate to both first time initialization (e.g. after “switch-on“) and alignment in association with restoration after a link failure. The procedure is based on the compelled exchange of status information between the two signa
46、lling points concerned and the provision of a proving period. No other signalling link is involved in the initial alignment of any particular link, the exchange occurs only on the link to be aligned. 1.6 Signalling link error monitoring Two signalling link error rate monitor functions are provided:
47、one which is employed whilst a signalling link is in service and which provides one of the criteria for taking the link out of service, and one which is employed whilst a link is in the proving state of the initial alignment procedure. These are called the signal unit error rate monitor and the alig
48、nment error rate monitor respectively. The characteristics of the signal unit error rate monitor are based on a signal unit error count, incremented and decremented using the “leaky bucket“ principle whilst the alignment error rate monitor is a linear count of signal unit errors. During loss of alig
49、nment, the signal unit error rate monitor error count is incremented in proportion to the period of the loss of alignment. 1.7 Link state control functions Link state control is a function of the signalling link which provides directives to the other signalling link functions. The interfaces with link state control are shown in Figures 1 and 7. The split into the functional blocks shown in the figures is made to facilitate description of the signalling link procedures and should not be taken to imply any particular implementation. The link state control function is shown in
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