1、COVERING NOTE Maximum bandwidth ( pull-in, hold-in and pull- out range; noise generation, 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 cl
2、ocks. 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 that includes the rates 1544 kbit/s, 63 12 kbit/s and 44 736 kbit/s. Add
3、itionally, this Recommendation includes specifications for three other clocks in Annex A. Type 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
4、 ITU-T Recommendation G.812 was approved on 13 June 2004 by ITU-T Study Group 15 (200 1-2004) under the ITU-T Recommendation A.8 procedure. Keywords Clock performance objectives, clock performance parameters, jitter performance, node clock, wander performance. ITU-T Rec. G.812 (06/2004) i FOREWORD T
5、he 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 questions and issuing Re
6、commendations 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 these topics. T
7、he 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 IS0 and IEC. NOTE In this Recommendation, the expression “Admi
8、nistration“ 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 applicability) and com
9、pliance 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 the Recommendation
10、 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 applicability of claimed
11、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 this Recommenda
12、tion. However, implementors are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. O ITU 2004 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permissi
13、on of ITU. ITU-T Rec. G.812 (06/2004) 11 CONTENTS 9 10 11 12 Scope 2.1 Synchronization network considerations Node clock types and applications . Node clock types and applications . 2.2 References . Definitions Abbreviations Frequency accuracy Pull-in, hold.in, and pull-out ranges . Noise generation
14、 . 8.1 Wander in locked mode 8.2 Non-locked wander 8.3 Jitter Noise tolerance . 9.1 Wander tolerance 9.2 Jitter tolerance Noise transfer Transient response and hold-over performance Short-term phase transient response . Long-term phase transient response (hold-over) Phase response to input signal in
15、temptions 1 1.1 1 1.2 1 1.3 11.4 Phase discontinuity . Interfaces . Annex A . Specifications for Types IV. V and VI clocks . A.1 Frequency accuracy A.3 Noise generation . A.2 Pul1.in. hold-in and pull-out ranges A.4 Noise tolerance . A.5 Noise transfer A.6 A.7 Interfaces Appendix I - Relationship be
16、tween TDEV and power spectral density Appendix II - Measurement method for noise transfer . Measurement set-up Transient response and hold-over performance . 11 . 1 11.2 Functional model of TDEV noise generator ITU-T Rec . G.812 (06/2004) Page 1 1 1 2 3 3 3 4 4 4 5 7 7 8 9 12 14 16 16 17 19 20 21 21
17、 21 21 22 25 29 30 34 35 37 37 38 . 111 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. The functio
18、n 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 requirement
19、s 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 to be inv
20、estigated 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 node clock
21、 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 a telephon
22、y 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. Principle
23、s 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 objectives
24、in ITU-T Rec. G.822. There are three elements that determine whether G.822 objectives can be achieved: o 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 repair). o
25、o 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 mainly d
26、riven 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, ITU-
27、T Rec. G.812 (06/2004) 1 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 kbit
28、/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 This Recommendation
29、 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 that includes the rates 1544 kbit/s
30、, 63 12 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 of node clocks according to the sync
31、hronization 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 pull-in range, noise generation and no
32、ise 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-over stability specification than a T
33、ype 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 synchronization hierarchy. Although a Type
34、 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 behaviour (see clauses 8, 9, 1 1.1 and 1
35、1.4) 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.8 13 option 1. The Type III clock has a less stringent hold-over stability requirement than Type I and Type II. It is typically deployed in
36、 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 behaviour (see clauses 8, 9, 1 1
37、.1 and 1 1.4) 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 that support the 1544 kbit/s
38、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.8 13 option 2 should be met as well. The Type V clock is typically deployed in existing transit nodes of networks based on both 1544 kbit/s and 20
39、48 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 and short-term stability requ
40、irements (clauses 8, 1 1.1 and 1 1.4) 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 VI hold-over stability char
41、acteristics may be used for the Node clock types and applications 2 ITU-T Rec. G.812 (06/2004) synchronization of SDH networks as long as the clock at least meets the noise generation and short-term stability requirements (clauses 8, 1 1.1 and 1 1.4) of the Type I clock. 3 References The following I
42、TU-T Recommendations 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 Recommendat
43、ion 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 this Recommendation does not give i
44、t, as a stand-alone document, the status of a Recommendation. ITU-T Recommendation G.703 (200 I), Physical/electrical characteristics of hierarchical digital interfaces. ITU-T Recommendation G.783 (2004), Characteristics of synchronous digital hierarchy (SDH) equipment functional blocks. ITU-T Recom
45、mendation G.80 1 (1 988), Digital transmission models. ITU-T Recommendation G.803 (2000), Architecture of transport networks based on the synchronous digital hierarchy (SDH). ITU-T Recommendation G.8 1 O (1 996), Definitions and terminology for synchronization networks. ITU-T Recommendation G.8 1 1
46、(1997), Timing requirements ofprimary reference clocks. ITU-T Recommendation G.8 13 (2003), Timing requirements of SDH equipment slave clocks (SEC). ITU-T Recommendation G.822 (1 988), Controlled slip rate objectives on an international digital connection. ITU-T Recommendation G.823 (2000), The cont
47、rol ofjitter and wander within digital networks which are based on the 2048 kbith hierarchy. ITU-T Recommendation G.824 (2000), The control ofjitter and wander within digital networks which are based on the 1544 kbith hierarchy. ITU-T Recommendation G.825 (2000), The control ofjitter and wander with
48、in digital networks which are based on the synchronous digital hierarchy (SDH). ITU-T Recommendation Q.55 1 (2002), Transmission characteristics of digital exchanges. Definitions The terms and definitions used in this Recommendation are contained in ITU-T Rec. G.810. 5 Abbreviations This Recommendat
49、ion uses the following abbreviations. CMI Coded Mark Inversion MTIE Maximum Time Interval Error NE Network Element PDH Plesiochronous Digital Hierarchy PPm parts per million ITU-T Rec. G.812 (06/2004) 3 PRBS PRC PSD PSTN SASE SDH SEC ssu STM TDEV u1 Accuracy Pseudo Random Binary Sequence Primary Reference Clock Power Spectral Density Public Switched Telephone Network Stand Alone Synchronization Equipment Synchronous Digital Hierarchy SDH Equipment Clock Synchronization Supply Unit Synchronous Transport Module Time Deviation Unit Interval Type 1 Type II Type III NA 1.6 x lo- 4.6 x 6 Frequency
