1、 I n t e r n a t i o n a l T e l e c o m m u n i c a t i o n U n i o n ITU-T G.811.1 TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (08/2017) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS Digital networks Design objectives for digital networks Timing characteristics of enha
2、nced primary reference clocks Recommendation ITU-T G.811.1 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 IN
3、DIVIDUAL 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、.450G.499 TRANSMISSION MEDIA AND OPTICAL SYSTEMS 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 Synchronization, quality and availability targets G.820G.829 Network capabilitie
5、s and functions 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 MULTIMEDIA QUALITY OF SERVICE AND PERFORMANCE
6、GENERIC AND USER-RELATED ASPECTS G.1000G.1999 TRANSMISSION MEDIA CHARACTERISTICS G.6000G.6999 DATA OVER TRANSPORT GENERIC ASPECTS G.7000G.7999 PACKET OVER TRANSPORT ASPECTS G.8000G.8999 ACCESS NETWORKS G.9000G.9999 For further details, please refer to the list of ITU-T Recommendations. Rec. ITU-T G.
7、811.1 (08/2017) i Recommendation ITU-T G.811.1 Timing characteristics of enhanced primary reference clocks Summary Recommendation ITU-T G.811.1 outlines the requirements for enhanced primary reference clocks (ePRCs) suitable for frequency synchronization. These requirements apply under normal enviro
8、nmental conditions specified for the equipment. History Edition Recommendation Approval Study Group Unique ID* 1.0 ITU-T G.811.1 2017-08-13 15 11.1002/1000/13301 Keywords ePRC, ePRTC, frequency synchronization. * To access the Recommendation, type the URL http:/handle.itu.int/ in the address field o
9、f your web browser, followed by the Recommendations unique ID. For example, http:/handle.itu.int/11.1002/1000/11830-en. ii Rec. ITU-T G.811.1 (08/2017) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications, information an
10、d 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 questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide ba
11、sis. 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. The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Reso
12、lution 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 expression “Administration“ is used for conciseness to indicate both a telecommunication administration
13、 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 compliance with the Recommendation is achieved when all of these mandatory provisions ar
14、e 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 is required of any party. INTELLECTUAL PROPERTY RIGHTSITU draws attention to the pos
15、sibility 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 Intellectual Property Rights, whether asserted by ITU members or others outside of the
16、 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 Recommendation. However, implementers are cautioned that this may not represent the latest infor
17、mation and are therefore strongly urged to consult the TSB patent database at http:/www.itu.int/ITU-T/ipr/. ITU 2017 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. Rec. ITU-T G.811.1 (08/2017) iii Table of Con
18、tents Page 1 Scope . 1 2 References . 1 3 Definitions 1 3.1 Terms defined elsewhere 1 3.2 Terms defined in this Recommendation . 2 4 Abbreviations and acronyms 2 5 Conventions 2 6 Frequency accuracy 2 7 Noise generation . 2 7.1 Wander . 3 7.2 Jitter 4 8 Phase discontinuity . 4 9 Degradation of the p
19、erformance of an ePRC 5 10 Interfaces . 5 Bibliography. 6 Rec. ITU-T G.811.1 (08/2017) 1 Recommendation ITU-T G.811.1 Timing characteristics of enhanced primary reference clocks 1 Scope This Recommendation outlines the requirements for enhanced primary reference clocks (ePRCs) suitable for frequency
20、 synchronization. These requirements apply under normal environmental conditions specified for the equipment. The ePRC provides the reference signal for the timing or synchronization of other clocks within a network or section of a network. In particular, the ePRC in its autonomous form can provide
21、a frequency input to an enhanced primary reference time clock (ePRTC). This Recommendation defines the ePRC output, but it does not apply to the output of a slave clock directly fed by an ePRC. The long-term accuracy of the ePRC should be maintained at 1 part in 1012 or better, with verification to
22、coordinated universal time (UTC). An ePRC may be realized as an autonomous clock, operating independently of other sources. Alternatively, the ePRC may be realized as a non-autonomous clock which is disciplined by UTC-derived precision signals received from a radio or satellite system. In either cas
23、e, the requirements for long-term accuracy and short-term stability, as specified in this Recommendation, apply. Several classes of ePRCs are being considered. This version specifies ePRC class A. Other classes are for further study. The long-term accuracy specified in this Recommendation is adequat
24、e when a single ePRC provides the reference synchronization signal to all other clocks within a network (synchronous mode of operation). In the pseudo-synchronous mode, i.e., not all clocks in the network have timing traceable to the same ePRC, the long-term accuracy is dependent on the number of eP
25、RCs in that network. When more than one ePRC is used in a network, a statistical approach is needed to determine the long-term accuracy of each ePRC in that network. 2 References The following ITU-T Recommendations and other references contain provisions which, through reference in this text, consti
26、tute 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 Rec
27、ommendations 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, the status of a Recommendation. ITU-T G.703 Recommendation ITU-T G.703 (2016
28、), Physical/electrical characteristics of hierarchical digital interfaces. ITU-T G.810 Recommendation ITU-T G.810 (1996), Definitions and terminology for synchronization networks. ITU-T G.8260 Recommendation ITU-T G.8260 (2015), Definitions and terminology for synchronization in packet networks. 3 D
29、efinitions 3.1 Terms defined elsewhere The terms and definitions used in this Recommendation are contained in ITU-T G.810 and ITU-T G.8260. 2 Rec. ITU-T G.811.1 (08/2017) 3.2 Terms defined in this Recommendation None. 4 Abbreviations and acronyms This Recommendation uses the following abbreviations
30、and acronyms: CMI Coded Mark Inversion ePRC enhanced Primary Reference Clock ePRTC enhanced Primary Reference Time Clock FPM Flicker Phase Modulation MTIE Maximum Time Interval Error NE Network Element OAM Operations and Maintenance PLL Phase Locked Loop PRC Primary Reference Clock PRTC Primary Refe
31、rence Time Clock SDH Synchronous Digital Hierarchy SEC SDH Equipment Clock SSMB Synchronization Status Message Byte STM Synchronous Transport Module TDEV Time Deviation UI Unit Interval UTC Coordinated Universal Time WFM White Frequency Modulation 5 Conventions None. 6 Frequency accuracy The maximum
32、 allowable fractional frequency offset for observation times greater than one week is 1 part in 1012, over all applicable operational conditions. 7 Noise generation The noise generation of an ePRC represents the amount of phase noise produced at its output. A suitable reference, for practical testin
33、g purposes, implies a performance level that is more stable than the output requirements. The ability of the clock to limit this noise is described by its frequency stability. The measures, maximum time interval error (MTIE) and time deviation (TDEV) are useful for characterization of noise generati
34、on performance. Both MTIE and TDEV are measured through an equivalent 10 Hz, first-order, low-pass measurement filter, at a maximum sampling time, 0, of 1/30 seconds. The minimum measurement period, T, for Rec. ITU-T G.811.1 (08/2017) 3 TDEV is twelve times the integration period, (T = 12). For meas
35、urements over longer observation periods, alternative filter bandwidth and sampling time may be required for practical considerations. 7.1 Wander The wander, expressed in MTIE and measured using the independent clock configuration defined in Figure 2a of ITU-T G.810, should have the limits as shown
36、in Table 1 and Figure 1: Table 1 Wander generation (MTIE) MTIE limit s Observation interval s .004 0.1 1000 Figure 1 MTIE as a function of an observation (integration) period The wander, expressed in TDEV and measured using the independent clock configuration defined in Figure 2a of ITU-T G.810, sho
37、uld have the limits as shown in Table 2 and shown in Figure 2: Table 2 Wander generation (TDEV) TDEV limit ns Observation interval s 1 0.1 10000 4 Rec. ITU-T G.811.1 (08/2017) Figure 2 TDEV as a function of an observation (integration) period NOTE Longer observation time and corresponding TDEV mask
38、is for further study. 7.2 Jitter While most specifications in this Recommendation are independent of the output interface at which they are measured, this is not the case for jitter production; jitter generation specifications must utilize existing specifications that are currently specified differe
39、ntly for different interface rates. These requirements are stated separately for the interfaces identified in clause 8. To be consistent with other jitter requirements, the specifications are in UIpp, where the unit interval (UI) corresponds to the reciprocal of the bit rate of the interface. The in
40、trinsic jitter at 2048 kHz and 2048 kbit/s output interfaces as measured over a 60-second interval shall not exceed 0.05 UIpp when measured through a single pole band-pass filter with corner frequencies at 20 Hz and 100 kHz. The intrinsic jitter at 1544 kbit/s output interfaces as measured over a 60
41、-second interval shall not exceed 0.015 UIpp when measured through a single pole band-pass filter with corner frequencies at 10 Hz and 40 kHz. The intrinsic jitter at 10 MHz output interfaces as measured over a 60-second interval shall not exceed 0.01 UIpp when measured through a single pole band-pa
42、ss filter with corner frequencies at 20 Hz and 100 kHz. 8 Phase discontinuity Primary reference clocks need a very high level of reliability and are likely to include replication of the equipment to ensure the continuity of the output. However, any phase discontinuity, due to internal operations wit
43、hin the clock, should only result in a lengthening or shortening of the timing signal interval and must not, at the clock output, cause a phase discontinuity in excess of 1/8 UI over a period of time up to 60 seconds (this refers to output signals at 1544 kbit/s, 2048 kbit/s, 2048 kHz, or 10 MHz). R
44、ec. ITU-T G.811.1 (08/2017) 5 9 Degradation of the performance of an ePRC If redundancy is applied and a clock frequency departs significantly from its nominal value, this should be detected and switching to an undegraded oscillator should then be effected. This switching should be accomplished befo
45、re the MTIE or TDEV specification is exceeded. 10 Interfaces The requirements in this Recommendation are related to reference points, which may be internal to the equipment or network element (NE) in which the ePRC is embedded and are, therefore, not necessarily available for measurement or analysis
46、 by the user. Consequently, the performance of the ePRC is not specified at these internal reference points, but rather at the external interfaces of the equipment. The output interfaces specified for the equipment in which the ePRC clock may be contained are: 2048 kHz interfaces according to clause
47、 15 of ITU-T G.703 with additional jitter and wander requirements as specified herein; 1544 kbit/s interfaces according to clause 7 of ITU-T G.703 with additional jitter and wander requirements as specified herein; 2048 kbit/s interfaces according to clause 11 of ITU-T G.703 with additional jitter a
48、nd wander requirements as specified herein; 10 MHz interfaces according to ITU-T G.703; NOTE 10 MHz interfaces may be available on autonomous ePRCs. 10 MHz interfaces may be required in the future when evolving an existing autonomous ePRC into an ePRTC. Such interfaces may provide an improvement in
49、phase noise performance. other interfaces (such as sine wave 8 kHz to 5 MHz) are for further study. All of the above interfaces may be implemented on all equipment. 6 Rec. ITU-T G.811.1 (08/2017) Bibliography b-ITU-T G.783 Recommendation ITU-T G.783 (1997), Characteristics of synchronous digital hierarchy (SDH) equipment functional blocks. b-ITU-T G.811 Recommendation ITU-T G.811 (1997), Timing characteristics of primary reference clocks. b-ITU-T G.812 Recommendation ITU-T G.8
