1、 International Telecommunication Union ITU-T G.8272/Y.1367TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU Amendment 1(08/2013) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS Packet over Transport aspects Quality and availability targets SERIES Y: GLOBAL INFORMATION INFRASTRUC
2、TURE, INTERNET PROTOCOL ASPECTS AND NEXT-GENERATION NETWORKS Internet protocol aspects Transport Timing characteristics of primary reference time clocks Amendment 1 Recommendation ITU-T G.8272/Y.1367 (2012) Amendment 1 ITU-T G-SERIES RECOMMENDATIONS TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AN
3、D NETWORKS INTERNATIONAL TELEPHONE CONNECTIONS AND CIRCUITS G.100G.199 GENERAL CHARACTERISTICS COMMON TO ALL ANALOGUE CARRIER-TRANSMISSION SYSTEMS G.200G.299 INDIVIDUAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON METALLIC LINES G.300G.399 GENERAL CHARACTERISTICS OF INTERNATIONAL CA
4、RRIER TELEPHONE SYSTEMS ON RADIO-RELAY OR SATELLITE LINKS AND INTERCONNECTION WITH METALLIC LINES G.400G.449 COORDINATION OF RADIOTELEPHONY AND LINE TELEPHONY G.450G.499 TRANSMISSION MEDIA AND OPTICAL SYSTEMS CHARACTERISTICS G.600G.699 DIGITAL TERMINAL EQUIPMENTS G.700G.799 DIGITAL NETWORKS G.800G.8
5、99 DIGITAL SECTIONS AND DIGITAL LINE SYSTEM G.900G.999 MULTIMEDIA QUALITY OF SERVICE AND PERFORMANCE 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 Ethernet over
6、 Transport aspects G.8000G.8099 MPLS over Transport aspects G.8100G.8199 Quality and availability targets G.8200G.8299Service Management G.8600G.8699 ACCESS NETWORKS G.9000G.9999 For further details, please refer to the list of ITU-T Recommendations. Rec. ITU-T G.8272/Y.1367 (2012)/Amd.1 (08/2013) i
7、 Recommendation ITU-T G.8272/Y.1367 Timing characteristics of primary reference time clocks Amendment 1 Summary Amendment 1 to Recommendation ITU-T G.8272/Y.1367 (2012) covers the case where the PRTC is integrated with a telecom grand master (T-GM) clock, therefore several clauses have been amended
8、to reflect this. History Edition Recommendation Approval Study Group Unique ID*1.0 ITU-T G.8272/Y.1367 2012-10-29 15 11.1002/1000/11817-en 1.1 ITU-T G.8272/Y.1367 (2012) Amd. 1 2013-08-29 15 11.1002/1000/12013-en _ *To access the Recommendation, type the URL http:/handle.itu.int/ in the address fiel
9、d of 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.8272/Y.1367 (2012)/Amd.1 (08/2013) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunic
10、ations, 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 questions and issuing Recommendations on them with a view to standardizing telecommunicat
11、ions 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. The approval of ITU-T Recommendations is covered by the procedure
12、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 expression “Administration“ is used for conciseness to indicate both a telecommun
13、ication 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 compliance with the Recommendation is achieved when all of these m
14、andatory 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 is required of any party. INTELLECTUAL PROPERTY RIGHTS ITU dra
15、ws 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 Intellectual Property Rights, whether asserted by ITU members o
16、r 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 Recommendation. However, implementers are cautioned that this may not rep
17、resent the latest information and are therefore strongly urged to consult the TSB patent database at http:/www.itu.int/ITU-T/ipr/. ITU 2014 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.8272/Y.13
18、67 (2012)/Amd.1 (08/2013) iii Table of Contents Page 1 Clause 1 1 2 Clause 2 1 3 Clause 4 1 4 Clause 6 1 5 Clause 6.1 . 1 6 Clause 6.2 . 2 7 Clause 9.1 . 2 8 Clause 9.2 . 2 9 Appendix I 2 Rec. ITU-T G.8272/Y.1367 (2012)/Amd.1 (08/2013) 1 Recommendation ITU-T G.8272/Y.1367 Timing characteristics of p
19、rimary reference time clocks Amendment 1 1 Clause 1 Add the following sentence after the last paragraph of the scope: This Recommendation also covers the case where a PRTC is integrated with a telecom grand master (T-GM) clock. In this case it defines the performance at the output of the combined PR
20、TC and T-GM function, i.e., the PTP messages. 2 Clause 2 Add the following reference to clause 2: ITU-T G.8273 Recommendation ITU-T G.8273/Y.1368 (2013), Framework of phase and time clocks. 3 Clause 4 Add the following abbreviations: PTP Precision Time Protocol T-GM Telecom Grand Master 4 Clause 6 A
21、dd the following paragraph and note after the last paragraph of clause 6: The performance specified in clauses 6.1 and 6.2 also applies to the output of the combined PRTC and T-GM function when integrated into a single piece of equipment. Therefore, there is no additional allowance for the inclusion
22、 of the T-GM function. NOTE Optimization of the noise inside the equipment is possible by combining the two functions. Therefore, the total noise of equipment that integrates the PRTC and T-GM can be the same as equipment that only contains the PRTC. 5 Clause 6.1 Replace the following paragraph: Und
23、er normal, locked operating conditions, the time output of the PRTC should be accurate to within 100 ns or better when verified against of the applicable primary time standard (e.g., UTC). This value includes all the noise components, i.e., the constant time error (time offset) and the phase error (
24、wander and jitter) of the PRTC. With: Under normal, locked operating conditions, the time output of the PRTC or combined PRTC and T-GM function should be accurate to within 100 ns or better when verified against of the applicable primary time standard (e.g., UTC). For the PRTC this value includes al
25、l the noise components, i.e., the constant time error (time offset) and the phase error (wander and jitter) of the PRTC. For the 2 Rec. ITU-T G.8272/Y.1367 (2012)/Amd.1 (08/2013) combined PRTC and T-GM function the same applies, except the samples are processed using the method specified in ITU-T G.
26、8273 to address timestamp quantization. 6 Clause 6.2 Add the following paragraph after the last paragraph of clause 6.2: The applicable MTIE and TDEV requirements for an Ethernet interface carrying PTP messages are applied after averaging over at least 100 consecutive samples, to avoid errors caused
27、 by timestamp quantization, or any quantization of packet position in the test equipment. 7 Clause 9.1 Add the following bullet and note after the first bullet: Ethernet interface carrying PTP messages NOTE Ethernet interfaces can combine synchronous Ethernet for frequency and PTP messages. 8 Clause
28、 9.2 Add the following bullet and note after the second to last bullet: synchronous Ethernet interfaces; NOTE Ethernet interfaces can combine PTP messages and synchronous Ethernet. 9 Appendix I Replace Appendix I with the following appendix: Appendix I Measuring the performance of a PRTC and a PRTC
29、combined with T-GM (This appendix does not form an integral part of this Recommendation.) The time error of a PRTC output is difficult to measure because time is a relative quantity. Unlike frequency, there is no such thing as a “time generator“; it always has to be compared back to a standard such
30、as UTC. Even UTC itself is only known in retrospect, by comparing the outputs of many national time standards over a period of time. NOTE 1 The accuracy of the PRTC performance test is for further study. NOTE 2 The test details of a PRTC combined with T-GM is in ITU-T G.8273. I.1 Factors influencing
31、 the performance of a GNSS-based PRTC The most common type of PRTC is one that distributes the time using radio signals from a GNSS system. However, the performance of a GNSS system is dependent on a range of issues outside the control of the equipment vendor. Therefore, any vendor specification can
32、 only indicate what the equipment is capable of, rather than what performance the equipment will actually deliver in any given installation. In measuring the performance of a GNSS-based PRTC, the following conditions should be verified as far as possible: Rec. ITU-T G.8272/Y.1367 (2012)/Amd.1 (08/20
33、13) 3 The equipment is properly commissioned and calibrated for fixed offsets such as antenna cable length and cable amplifiers. For example, an antenna cable will produce a delay of approximately 4 ns/m, depending on the cable type. Any 1PPS output signal asymmetry compensation contained within the
34、 PRTC (such as that described in clause A.1.2 of ITU-T G.8271) is stable. The antenna has a clear view of the sky with minimal multipath distortion. This may be verified by recording the number of satellites visible throughout the measurement. The GNSS or radio distribution system is properly mainta
35、ined and operated by the relevant authorities. This may be verified by checking the operational status bulletins issued by the relevant operating authorities. In addition to these primary factors, there are some secondary conditions which may cause errors in the time measured by a GNSS system. These
36、 factors may be more difficult to quantify or mitigate against. Secondary factors may include: interference from ground level transmissions. While filters may be used to remove some ground level interference, this may not protect against local jamming. The presence of jamming may be verified by usin
37、g interference detection equipment; atmospheric conditions such as thunderstorms and heavy rain or fog; solar interference such as sunspots and flares, affecting ionosphere delay. I.2 Phase wander measurement It is possible to measure the phase wander of a PRTC relative to a PRC-quality frequency re
38、ference, such as a caesium clock. A time interval counter is used to compare the phase of a 1 pulse-per-second (1PPS) output signal from the PRTC against that of a PRC. The experimental set-up is shown in Figure I.1: G.8272-Y.1367(12)-Amd.1(13)_FI.1Radio distributed PRTC signal (e.g., GNSS)PRTCunder
39、 test1PPS1PPSe.g., CsclockTime intervalcounterMeasurementagainst additionalPRC to verify driftagainst referenceTime intervalcounterTime intervalcounterPRCFreq. Ref.PRCFreq. Ref.Data logFigure I.1 Measuring phase wander of a PRTC Where a combined PRTC and T-GM function is to be tested, the time inter
40、val counter can be replaced by a packet timing monitor device, as shown in Figure I.2: 4 Rec. ITU-T G.8272/Y.1367 (2012)/Amd.1 (08/2013) G.8272-Y.1367(12)-Amd.1(13)_FI.2Radio distributed PRTC signal (e.g., GNSS)PRTC+T-GMunder test1PPSPTPe.g., CsclockTime intervalcounterMeasurementagainst additionalP
41、RC to verify driftagainst referencePacket timingmonitorPacket timingmonitorPRCFreq. Ref.PRCFreq. Ref.Data logFigure I.2 Measuring phase wander of a combined PRTC and T-GM The wander of a caesium reference clock is extremely low, although it may have a slight offset to UTC frequency. For a PRC, this
42、is guaranteed to be within 1 part in 1011, but typical caesium references have much better performance. This frequency offset causes a tilt in the phase plot, which must be removed to reveal the wander performance of the PRTC. In order to distinguish between wander of the PRTC and that of the PRC, a
43、 second PRC can be used to make a three-way comparison. This is shown in Figures I.1 and I.2 by the components in the shaded boxes. This additional check may be omitted if not required. Since the caesium reference is only a source of frequency and not time, this experiment only indicates phase wande
44、r, and cannot measure the time error from the GNSS system time. However, it does indicate that if the static error can be measured and calibrated out, the PRTC is capable of maintaining time within certain limits. I.3 Time error measurements In order to determine the maximum time error of a PRTC, it
45、 is necessary to compare it to another source of accurate time. I.3.1 Comparison against a reference receiver In the laboratory context, an accurate source of time might be another GNSS receiver of known uncertainty, or a “reference receiver“. The experimental set-up is very similar to the wander me
46、asurement, but substituting the reference receiver for the caesium PRC. A time interval counter is used to compare the time difference of a 1 pulse-per-second (1PPS) output signal from the PRTC against that of the reference receiver. The experimental set-up is shown in Figure I.3: Rec. ITU-T G.8272/
47、Y.1367 (2012)/Amd.1 (08/2013) 5 G.8272-Y.1367(12)-Amd.1(13)_FI.3Radio distributed PRTC signal (e.g., GNSS)PRTCunder test1PPS1PPSTime intervalcounterReferencereceiverData logFigure I.3 Comparing time accuracy against a reference receiver As before, for a combined PRTC and T-GM function, the time inte
48、rval counter can be replaced by a packet timing monitor, as shown in Figure I.4 below: G.8272-Y.1367(12)-Amd.1(13)_FI.4Radio distributed PRTC signal (e.g., GNSS)PRTC/T-GMunder test1PPSPTPPacket timingmonitorReferencereceiverData logFigure I.4 Comparing time accuracy of a PRTC and T-GM against a refe
49、rence receiver In this set-up, the reference receiver should ideally have a significantly better performance than the PRTC in order for the results to be valid. Since the PRTC time error specification is approaching the limits of what is possible using a GNSS system, this type of measurement is able to give an indication that the time accuracy is in the right area, rather than prove that the accuracy specification has been met. The reference receiver approach may be improved by using a collec
