1、 INTERNATIONAL TELECOMMUNICATION UNION ITU-T G.812TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (06/2004) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS Digital networks Design objectives for digital networks Timing requirements of slave clocks suitable for use as node cloc
2、ks in synchronization networksITU-T Recommendation G.812 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 INDI
3、VIDUAL 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.4
4、50G.499 TESTING EQUIPMENTS G.500G.599 TRANSMISSION MEDIA CHARACTERISTICS G.600G.699 DIGITAL TERMINAL EQUIPMENTS G.700G.799 DIGITAL NETWORKS G.800G.899 General aspects G.800G.809 Design objectives for digital networks G.810G.819 Quality and availability targets G.820G.829 Network capabilities and fun
5、ctions G.830G.839 SDH network characteristics G.840G.849 Management of transport network G.850G.859 SDH radio and satellite systems integration G.860G.869 Optical transport networks G.870G.879 DIGITAL SECTIONS AND DIGITAL LINE SYSTEM G.900G.999 QUALITY OF SERVICE AND PERFORMANCE - GENERIC AND USER-R
6、ELATED ASPECTS G.1000G.1999 TRANSMISSION MEDIA CHARACTERISTICS G.6000G.6999 DIGITAL TERMINAL EQUIPMENTS G.7000G.7999 DIGITAL NETWORKS G.8000G.8999 For further details, please refer to the list of ITU-T Recommendations. ITU-T Rec. G.812 (06/2004) i ITU-T Recommendation G.812 Timing requirements of sl
7、ave clocks suitable for use as node clocks in synchronization networks Summary This Recommendation outlines minimum requirements for timing devices used as node clocks in synchronization networks. The requirements relate to frequency deviation; pull-in, hold-in and pull-out range; noise generation,
8、tolerance and transfer; transient response and holdover performances. The node clocks are suitable for use in SDH and PSTN network applications. This Recommendation includes specifications for three types of clocks. The Type I clock is primarily intended for use in networks optimized for the 2048 kb
9、it/s hierarchy. The Types II and III clocks are primarily intended for use in networks optimized for the particular 1544 kbit/s hierarchy that includes the rates 1544 kbit/s, 6312 kbit/s and 44 736 kbit/s. Additionally, this Recommendation includes specifications for three other clocks in Annex A. T
10、ype IV is typically deployed in existing networks that support the 1544 kbit/s. The Types V and VI clocks were defined for transit and local node applications in the 1988 version of this Recommendation. Source ITU-T Recommendation G.812 was approved on 13 June 2004 by ITU-T Study Group 15 (2001-2004
11、) under the ITU-T Recommendation A.8 procedure. The typo corrected by G.812 (2004) Erratum 1 is included in the electronic version of this Recommendation. Keywords Clock performance objectives, clock performance parameters, jitter performance, node clock, wander performance. ii ITU-T Rec. G.812 (06/
12、2004) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical, operating and tariff question
13、s 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 on
14、 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, the
15、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 applic
16、ability) 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 th
17、e 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 applicabi
18、lity 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 not received notice of intellectual property, protected by patents, which may be required to implement
19、 this Recommendation. However, implementors are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. ITU 2004 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior wr
20、itten permission of ITU. ITU-T Rec. G.812 (06/2004) iii CONTENTS Page 1 Scope 1 2 Node clock types and applications . 1 2.1 Synchronization network considerations 1 2.2 Node clock types and applications . 2 3 References. 3 4 Definitions 3 5 Abbreviations 3 6 Frequency accuracy 4 7 Pull-in, hold-in,
21、and pull-out ranges. 4 8 Noise generation . 4 8.1 Wander in locked mode 5 8.2 Non-locked wander 7 8.3 Jitter 7 9 Noise tolerance . 8 9.1 Wander tolerance 9 9.2 Jitter tolerance 12 10 Noise transfer 14 11 Transient response and hold-over performance 16 11.1 Short-term phase transient response. 16 11.
22、2 Long-term phase transient response (hold-over) 17 11.3 Phase response to input signal interruptions 19 11.4 Phase discontinuity. 20 12 Interfaces. 21 Annex A Specifications for Types IV, V and VI clocks. 21 A.1 Frequency accuracy 21 A.2 Pull-in, hold-in and pull-out ranges 21 A.3 Noise generation.
23、 22 A.4 Noise tolerance . 25 A.5 Noise transfer 29 A.6 Transient response and hold-over performance . 30 A.7 Interfaces 34 Appendix I Relationship between TDEV and power spectral density 35 Appendix II Measurement method for noise transfer . 37 II.1 Measurement set-up 37 II.2 Functional model of TDE
24、V noise generator 38 ITU-T Rec. G.812 (06/2004) 1 ITU-T Recommendation G.812 Timing requirements of slave clocks suitable for use as node clocks in synchronization networks 1 Scope This Recommendation outlines minimum requirements for timing devices used as node clocks in synchronization networks. T
25、he function of a node clock is to select one of the external synchronization links coming into a telecommunication station as the active synchronization reference, to attenuate its jitter and wander and subsequently to distribute the reference to the telecommunication equipment in the station. The r
26、equirements in this Recommendation apply under the normal environmental conditions specified for telecommunications equipment. This Recommendation specifies node clocks suitable for use in SDH and PSTN network applications. They may provide acceptable performance for other applications, but that has
27、 to be investigated for each case individually. In normal operation, a node clock is operating as a slave clock, traceable to a primary reference clock. For purposes of redundancy, a node clock will in general have multiple reference inputs. In the event that all links between the master(s) and the
28、node clock fail, the node clock should be capable of maintaining operation within prescribed performance limits (the hold-over mode of operation). A node clock can be a separate piece of equipment called a Stand Alone Synchronization Equipment (SASE) or it can be a part of another equipment such as
29、a telephony exchange or an SDH cross-connect. This Recommendation defines six Types of clocks. Applications for each clock Type are described in clause 2. 2 Node clock types and applications 2.1 Synchronization network considerations Design of synchronization networks in general is not standardized.
30、 Principles for synchronization of SDH networks are outlined in ITU-T Rec. G.803. Some of these principles can be taken as general design rules for synchronization networks. Synchronization requirements for node clocks in a PSTN environment can in principle be derived from the controlled slip rate o
31、bjectives in ITU-T Rec. G.822. There are three elements that determine whether G.822 objectives can be achieved: the hold-over stability of the clocks; the network topology (i.e., length, routing and redundancy of the references); and the operational practices of the operator (i.e., mean time to rep
32、air). Only the first element is covered in this Recommendation. Network providers can choose to use particular clock Types in their synchronization planning in order to match their network topology and operational practices. Synchronization requirements for node clocks in an SDH environment are main
33、ly driven by the jitter and wander specifications in ITU-T Recs G.823, G.824 and G.825. Given that SDH equipment clocks (as specified in ITU-T Rec. G.813) have only limited filtering capabilities and at the same time need to maintain their STM-N and PDH outputs in conformance with ITU-T Recs G.823,
34、2 ITU-T Rec. G.812 (06/2004) G.824 and G.825, the phase transients at the output of node clocks must meet specifications that are more stringent than what is strictly needed for pure PDH transport equipment. NOTE Since the bandwidths of SDH equipment clocks are wider in networks based on the 2048 kb
35、it/s hierarchy compared to those based on the 1544 kbit/s hierarchy (as described in ITU-T Rec. G.813 option 1 and option 2, respectively), the output phase transient requirements are correspondingly more stringent for node clocks deployed in 2048 kbit/s hierarchy environments. 2.2 Node clock types
36、and applications This Recommendation includes specifications for three clocks. The Type I clock is primarily intended for use in networks optimized for the 2048 kbit/s hierarchy. The Types II and III clocks are primarily intended for use in networks optimized for the particular 1544 kbit/s hierarchy
37、 that includes the rates 1544 kbit/s, 6312 kbit/s and 44 736 kbit/s. The Type I clock can be used at all levels of the synchronization hierarchy in 2048 kbit/s based networks. The wander generation and bandwidth of Type I clocks are limited to values that allow the deployment of the maximum number o
38、f node clocks according to the synchronization network reference chain as defined in ITU-T Rec. G.803. Although a Type I clock is primarily intended for use in networks supporting the 2048 kbit/s hierarchy, a Type I clock can also be deployed in 1544 kbit/s based networks as long as at least its pul
39、l-in range, noise generation and noise tolerance (see clauses 7, 8 and 9) comply with the more stringent requirements that apply to Types II and III clocks in order to be compatible with SDH equipment clocks built according to ITU-T Rec. G.813 option 2. The Type II clock has a more stringent hold-ov
40、er stability specification than a Type I clock. It is typically deployed at distribution hubs in networks that support the 1544 kbit/s hierarchy mentioned above. Type II clocks have a hold-over stability specification sufficient to operate with a single reference at the highest levels of the synchro
41、nization hierarchy. Although a Type II clock is primarily intended for use in networks supporting the 1544 kbit/s hierarchy, a clock with Type II hold-over specifications can also be deployed in 2048 kbit/s based networks as long as at least its noise generation, noise tolerance and transient behavi
42、our (see clauses 8, 9, 11.1 and 11.4) comply with the more stringent requirements that apply to Type I clocks in order to be compatible with SDH equipment clocks built according to ITU-T Rec. G.813 option 1. The Type III clock has a less stringent hold-over stability requirement than Type I and Type
43、 II. It is typically deployed in end offices in networks that support the 1544 kbit/s hierarchy mentioned above. Like a Type II clock, a clock with Type III hold-over stability may also be deployed in 2048 kbit/s based networks as long as at least its noise generation, noise tolerance and transient
44、behaviour (see clauses 8, 9, 11.1 and 11.4) comply to the more stringent requirements that apply to Type I when it is used to synchronize SDH equipment. Additionally, this Recommendation includes specifications for three clocks in Annex A. The Type IV clock is typically deployed in existing networks
45、 that support the 1544 kbit/s hierarchy mentioned above. If clocks with Type IV hold-over performance are embedded in SDH equipment in this hierarchy, the requirements in ITU-T Rec. G.813 option 2 should be met as well. The Type V clock is typically deployed in existing transit nodes of networks bas
46、ed on both 1544 kbit/s and 2048 kbit/s hierarchies according to the specifications of the 1988 version of this Recommendation. It should be noted that these clocks are perfectly suitable for the synchronization of 2048 kbit/s based SDH networks provided they conform at least to the noise generation
47、and short-term stability requirements (clauses 8, 11.1 and 11.4) of Type I clocks. The Type VI clock is typically deployed in existing local nodes of networks based on the 2048 kbit/s hierarchy according to the specifications of the 1988 version of this Recommendation. Like Type V, clocks with Type
48、VI hold-over stability characteristics may be used for the ITU-T Rec. G.812 (06/2004) 3 synchronization of SDH networks as long as the clock at least meets the noise generation and short-term stability requirements (clauses 8, 11.1 and 11.4) of the Type I clock. 3 References The following ITU-T Reco
49、mmendations and other 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 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 thi
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