1、 International Telecommunication Union ITU-T G.781TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (09/2008) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS Digital terminal equipments Principal characteristics of multiplexing equipment for the synchronous digital hierarchy Syn
2、chronization layer functions Recommendation ITU-T G.781 ITU-T G-SERIES RECOMMENDATIONS TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS INTERNATIONAL TELEPHONE CONNECTIONS AND CIRCUITS G.100G.199 GENERAL CHARACTERISTICS COMMON TO ALL ANALOGUE CARRIER-TRANSMISSION SYSTEMS G.200G.299 INDIV
3、IDUAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON METALLIC LINES G.300G.399 GENERAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON RADIO-RELAY OR SATELLITE LINKS AND INTERCONNECTION WITH METALLIC LINES G.400G.449 COORDINATION OF RADIOTELEPHONY AND LINE TELEPHONY G.45
4、0G.499 TRANSMISSION MEDIA AND OPTICAL SYSTEMS CHARACTERISTICS G.600G.699 DIGITAL TERMINAL EQUIPMENTS G.700G.799 General G.700G.709 Coding of voice and audio signals G.710G.729 Principal characteristics of primary multiplex equipment G.730G.739 Principal characteristics of second order multiplex equi
5、pment G.740G.749 Principal characteristics of higher order multiplex equipment G.750G.759 Principal characteristics of transcoder and digital multiplication equipment G.760G.769 Operations, administration and maintenance features of transmission equipment G.770G.779 Principal characteristics of mult
6、iplexing equipment for the synchronous digital hierarchy G.780G.789 Other terminal equipment G.790G.799 DIGITAL NETWORKS G.800G.899 DIGITAL SECTIONS AND DIGITAL LINE SYSTEM G.900G.999 QUALITY OF SERVICE AND PERFORMANCE GENERIC AND USER-RELATED ASPECTS G.1000G.1999 TRANSMISSION MEDIA CHARACTERISTICS
7、G.6000G.6999 DATA OVER TRANSPORT GENERIC ASPECTS G.7000G.7999 PACKET OVER TRANSPORT ASPECTS G.8000G.8999ACCESS NETWORKS G.9000G.9999 For further details, please refer to the list of ITU-T Recommendations. Rec. ITU-T G.781 (09/2008) i Recommendation ITU-T G.781 Synchronization layer functions Summary
8、 Recommendation ITU-T G.781 defines the atomic functions that are part of the two synchronization layers, the synchronization distribution (SD) layer and the network synchronization (NS) layer. It also defines some atomic functions, part of the transport layer, which are related to synchronization.
9、These functions describe the synchronization of SDH NEs and how SDH NEs are involved in Network Synchronization. The specifications in this Recommendation are the superset of functionality of three regional standards bodies. Care should be taken when selecting from this Recommendation. Not every ato
10、mic function defined in this Recommendation is required for every application. Different subsets of atomic functions may be assembled in different ways according to the combination rules given in Recommendation ITU-T G.783 to provide a variety of different capabilities. Network operators and equipme
11、nt suppliers may choose which functions must be implemented for each application. Source Recommendation ITU-T G.781 was approved on 22 September 2008 by ITU-T Study Group 15 (2005-2008) under Recommendation ITU-T A.8 procedure. ii Rec. ITU-T G.781 (09/2008) FOREWORD The International Telecommunicati
12、on Union (ITU) is the United Nations specialized agency in the field of telecommunications, information and communication technologies (ICTs). The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical, operating and tariff quest
13、ions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations
14、 on these topics. The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1. In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with ISO and IEC. NOTE In this Recommendation, t
15、he expression “Administration“ is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain mandatory provisions (to ensure e.g. interoperability or app
16、licability) and compliance with the Recommendation is achieved when all of these mandatory provisions are met. The words “shall“ or some other obligatory language such as “must“ and the negative equivalents are used to express requirements. The use of such words does not suggest that compliance with
17、 the Recommendation is required of any party. INTELLECTUAL PROPERTY RIGHTS ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence, validity or applic
18、ability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation, ITU had received notice of intellectual property, protected by patents, which may be required to implement
19、this Recommendation. However, implementers are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database at http:/www.itu.int/ITU-T/ipr/. ITU 2009 All rights reserved. No part of this publication may be reproduced, by any means w
20、hatsoever, without the prior written permission of ITU. Rec. ITU-T G.781 (09/2008) iii CONTENTS Page 1 Scope 1 2 References. 1 3 Terms and definitions . 2 4 Abbreviations and acronyms 3 5 Synchronization principles . 5 5.1 Network synchronization 5 5.2 Synchronization distribution trails . 7 5.3 Syn
21、chronization interfaces . 8 5.4 Clock-source quality-level 11 5.5 Synchronization status messages (SSMs) and timing marker (TM) channels 15 5.6 Selection process 22 5.7 Signal fail 23 5.8 Hold-off time 23 5.9 Wait to restore time 24 5.10 Synchronization source priorities. 24 5.11 External commands 2
22、5 5.12 Automatic reference selection process . 26 5.13 Timing loop prevention 26 5.14 Delay times for NEs with SEC/ST3/SIC 33 5.15 Delay times for NEs with SSU/ST2 or for SASE/BITS. 34 5.16 Synchronization layer functions. 34 5.17 Overview of the processes performed within the atomic functions . 35
23、5.18 Interworking between option I, II and III synchronization networks. 36 6 Synchronization distribution layer atomic functions 36 6.1 SD connection function (SD_C). 37 6.2 SD trail termination functions 38 6.3 SD adaptation functions . 42 7 Network synchronization layer atomic functions. 47 7.1 N
24、S_connection functions (NS_C) 48 8 Transport layer to SD layer atomic functions. 50 8.1 STM-n multiplex section adaptation functions 50 8.2 Pqs adaptation functions. 54 8.3 P12s layer adaptation functions 57 8.4 T12 layer adaptation functions . 64 8.5 P11s layer adaptation functions 66 8.6 T01 layer
25、 adaptation functions . 66 8.7 T02 layer adaptation functions . 68 iv Rec. ITU-T G.781 (09/2008) Page 8.8 T21 layer adaptation functions . 70 8.9 ETH layer adaptation functions 71 9 Equipment clock to transport layers clock adaptation functions 74 9.1 STM-N layer. 74 9.2 VC layers 75 9.3 Pxx layers
26、. 77 9.4 T12 layer. 79 9.5 T01 layer. 79 9.6 T02 layer. 79 9.7 T21 layer. 80 9.8 ETYn layer . 81 10 T12 section layer functions. 82 10.1 T12 connection function T12_C. 82 10.2 T12 trail termination functions. 82 11 T01 section layer functions. 84 11.1 T01 connection function T01_C. 85 11.2 T01 trail
27、 termination functions. 85 12 T02 section layer functions. 87 12.1 T02 connection function T02_C. 87 12.2 T02 trail termination functions. 87 13 T21 section layer functions. 88 13.1 T21 connection function T21_C. 89 13.2 T21 trail termination functions. 89 Annex A Synchronization selection process.
28、90 Appendix I Transport layer models for synchronization information 101 Appendix II Examples of synchronization functionality in the NE 105 II.1 Examples of synchronization functionality in the NE for option I networks . 105 II.2 Examples of synchronization functionality in the NE for option II net
29、works . 106 II.3 Examples of synchronization functionality in the NE for option III networks . 107 Appendix III Delay time allocation for option I and III networks 108 III.1 Delay and processing times for the synchronization selection process 108 III.2 Non-switching message delay TNSM. 108 III.3 Swi
30、tching message delay TSM109 III.4 Holdover message delay THM. 110 III.5 Wait to restore time TWTR. 110 Rec. ITU-T G.781 (09/2008) v Page Appendix IV Interworking of option II equipment supporting second generation SSM and first generation SSM using a translation function . 112 Bibliography. 114 Rec.
31、 ITU-T G.781 (09/2008) 1 Recommendation ITU-T G.781 Synchronization layer functions 1 Scope This Recommendation specifies a library of basic synchronization distribution building blocks, referred to as “atomic functions“ and a set of rules by which they are combined in order to describe a digital tr
32、ansmission equipments synchronization functionality. The library defined in this Recommendation forms part of the set of libraries defined furthermore in Recommendations ITU-T G.783, b-ITU-T G.705, b-ITU-T G.798, b-ITU-T G.8021 and b-ITU-T G.8121. The library comprises the functional building blocks
33、 needed to completely specify the generic functional structure of the ITU-T digital transmission hierarchy. Equipment that is compliant with these Recommendations should be describable as an interconnection of a subset of these functional blocks contained within these Recommendations. The interconne
34、ction of these blocks should obey the combination rules given in ITU-T G.806. The generic functionality is described in ITU-T G.806. The specifications in this Recommendation are the superset of functionality of three regional standards bodies. When different processing within a single atomic functi
35、on is required to support the specific regional processing, this is identified by means of options I, II and III processing. “Option I“ applies to networks optimized for the 2048 kbit/s hierarchy. “Option II“ applies to networks optimized for the 1544 kbit/s hierarchy that include the rates 1544 kbi
36、t/s, 6312 kbit/s and 44 736 kbit/s. “Option III“ applies to networks optimized for the 1544 kbit/s hierarchy that include the rates 1544 kbit/s, 6312 kbit/s, 33 064 kbit/s, 44 736 kbit/s and 97 728 kbit/s. As a general rule, this Recommendation does not specify the atomic functions that are specific
37、 to PRC (ITU-T G.811) and SSU (ITU-T G.812) clock equipment; the SSM selection algorithm specified in this Recommendation is only generally applicable to SEC (ITU-T G.813 and ITU-T G.8262) clock equipment. However, strictly for option II networks, there are some specific topologies listed containing
38、 both SEC and SSU clock equipment, for which the application of the SSM selection algorithm is described. The general case of using the SSM selection algorithm simultaneously in SSU and SEC clock equipment is for further study for all options. 2 References The following ITU-T Recommendations and oth
39、er references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision; users of this Recommendation are therefore encouraged
40、 to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. The reference to a document within this Recommendation does not give it, as a stand-alone document
41、, the status of a Recommendation. ITU-T G.703 Recommendation ITU-T G.703 (2001), Physical/electrical characteristics of hierarchical digital interfaces. ITU-T G.704 Recommendation ITU-T G.704 (1998), Synchronous frame structures used at 1544, 6312, 2048, 8448 and 44 736 kbit/s hierarchical levels. 2
42、 Rec. ITU-T G.781 (09/2008) ITU-T G.706 Recommendation ITU-T G.706 (1991), Frame alignment and cyclic redundancy check (CRC) procedures relating to basic frame structures defined in Recommendation G.704. ITU-T G.707 Recommendation ITU-T G.707/Y.1322 (2007), Network node interface for the synchronous
43、 digital hierarchy (SDH). ITU-T G.775 Recommendation ITU-T G.775 (1998), Loss of Signal (LOS), Alarm Indication Signal (AIS) and Remote Defect Indication (RDI) defect detection and clearance criteria for PDH signals. ITU-T G.783 Recommendation ITU-T G.783 (1997), Characteristics of synchronous digit
44、al hierarchy (SDH) equipment functional blocks. ITU-T G.803 Recommendation ITU-T G.803 (2000), Architecture of transport networks based on the synchronous digital hierarchy (SDH). ITU-T G.806 Recommendation ITU-T G.806 (2006), Characteristics of transport equipment Description methodology and generi
45、c functionality. ITU-T G.810 Recommendation ITU-T G.810 (1996), Definitions and terminology for synchronization networks. ITU-T G.811 Recommendation ITU-T G.811 (1997), Timing characteristics of primary reference clocks. ITU-T G.812 Recommendation ITU-T G.812 (2004), Timing requirements of slave clo
46、cks suitable for use as node clocks in synchronization networks. ITU-T G.813 Recommendation ITU-T G.813 (2003), Timing characteristics of SDH equipment slave clocks (SEC). ITU-T G.8261 Recommendation ITU-T G.8261/Y.1361 (2008), Timing and synchronization aspects in packet networks. ITU-T G.8262 Reco
47、mmendation ITU-T G.8262/Y.1362 (2007), Timing characteristics of synchronous Ethernet equipment slave clock (EEC). ITU-T G.8264 Recommendation ITU-T G.8264/Y.1364 (2008), Distribution of timing information through packet networks. 3 Terms and definitions This Recommendation defines the following ter
48、ms: 3.1 timing loop: This is a network condition where a slave clock providing synchronization becomes locked to its own timing signal. It is generally created when the slave clock timing information is looped back to its own input, either directly or via other network equipments. Timing loops shoul
49、d be prevented in networks by careful network design. Rec. ITU-T G.781 (09/2008) 3 3.2 QL_minimum: QL_minimum is a configurable parameter used in the squelching of clock output signals. If the quality level of the signal used to derive the output falls below QL_minimum then the output will be squelched (cut-off or set to AIS). 3.3 clock-source quality-level: The clock-source quality-level of a SEC or SASE is defined as the grade of clock to which it is ultimately traceable, i.e., the grade of cloc
