1、 International Telecommunication Union ITU-T G.8263/Y.1363TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU Amendment 2(05/2014) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS Packet over Transport aspects Synchronization, quality and availability targets SERIES Y: GLOBAL INFOR
2、MATION INFRASTRUCTURE, INTERNET PROTOCOL ASPECTS AND NEXT-GENERATION NETWORKS Internet protocol aspects Transport Timing characteristics of packet-based equipment clocks Amendment 2 Recommendation ITU-T G.8263/Y.1363 (2012) Amendment 2 ITU-T G-SERIES RECOMMENDATIONS TRANSMISSION SYSTEMS AND MEDIA, D
3、IGITAL SYSTEMS AND 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
4、 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.450G.499 TRANSMISSION MEDIA AND OPTICAL SYSTEMS CHARACTERISTICS G.600G.699 DIGITAL TERMINAL EQUIPMENTS G.700G.799 DIGITAL
5、NETWORKS G.800G.899 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.8
6、999 Ethernet over Transport aspects G.8000G.8099 MPLS over Transport aspects G.8100G.8199 Synchronization, 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.82
7、63/Y.1363 (2012)/Amd.2 (05/2014) i Recommendation ITU-T G.8263/Y.1363 Timing characteristics of packet-based equipment clocks Amendment 2 Summary Amendment 2 to Recommendation ITU-T G.8263/Y.1363 (2012) adds text to Appendix I, “Packet delay variation noise tolerance testing methodology“, which was
8、previously marked as for further study. It also replaces a sentence in clause 7.1 and adds a note in clause 7.1. History Edition Recommendation Approval Study Group Unique ID*1.0 ITU-T G.8263/Y.1363 2012-02-13 15 11.1002/1000/115241.1 ITU-T G.8263/Y.1363 (2012) Amd. 1 2013-08-29 15 11.1002/1000/1201
9、41.2 ITU-T G.8263/Y.1363 (2012) Amd. 2 2014-05-14 15 11.1002/1000/12191_ *To access the Recommendation, type the URL http:/handle.itu.int/ in the address field 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.826
10、3/Y.1363 (2012)/Amd.2 (05/2014) FOREWORD The International Telecommunication 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 IT
11、U. 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 basis. The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics f
12、or 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 Resolution 1. In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared
13、 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 and a recognized operating agency. Compliance with this Recommendation is voluntary. However, the Recommendation may co
14、ntain certain mandatory provisions (to ensure, e.g., interoperability or applicability) 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 expre
15、ss requirements. The use of such words does not suggest that compliance with 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 Prope
16、rty 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 Recommendation development process. As of the date of approval of this Recommendation, ITU had received notice of inte
17、llectual property, protected by patents, which may be required to implement 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 2014 All ri
18、ghts reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. Rec. ITU-T G.8263/Y.1363 (2012)/Amd.2 (05/2014) 1 Recommendation ITU-T G.8263/Y.1363 Timing characteristics of packet-based equipment clocks Amendment 2 1) Clause 7.1,
19、PEC-S-F Replace the following text: The PEC-S-F must tolerate the noise at the limits specified in clause 8 of ITU-T G.8261.1 (PDV network limits at point C). with: The PEC-S-F must tolerate the noise at the limits specified in clause 8.1.1 of ITU-T G.8261.1 (PDV network limits at point C). Add the
20、following new note, Note 4, after Note 3. NOTE 4 As described in ITU-T G.8261.1, clause 8.1.2, many networks may exhibit lower packet delay variation compared with the network limit for HRM-1 specified in ITU-T G.8261.1, clause 8.1.1. This Recommendation defines one type of packet slave clock that i
21、s suitable for use with HRM-1, as described in ITU-T G.8261.1, clause 8.1.1. Some operators may decide to use a different type of packet slave clock in case their network limits fit with clause 8.1.2 of ITU-T G.8261.1; this alternate type of packet slave clock is for further study. 2) Appendix I Rep
22、lace the whole of Appendix I with the text below. Appendix I Packet delay variation noise tolerance testing methodology (This appendix does not form an integral part of this Recommendation.) While suitable test signals that check conformance to the masks in ITU-T G.8261.1 (Case 3 of ITU-T G.8261.1,
23、as defined in reference point D in Figure 3 of ITU-T G.8261.1) are being studied, the testing methodologies described in clause I.2 below can be used to generate suitable packet delay variation test patterns. As such, the applicable mask is that which is shown in ITU-T G.8261.1, Table 1 and Figure 4
24、; no other masks are appropriate. These methodologies are applicable only to the HRM-1 of ITU-T G.8261.1. Suitable methodologies for the HRM-2 are for further study. Other methodologies for generating suitable test signals that check conformance to the masks in ITU-T G.8261.1 for the HRM-1 are also
25、possible; this is for further study. I.1 Testing set-up for PDV noise tolerance testing The general testing set-up for PDV noise tolerance testing is shown in Figure I.1. 2 Rec. ITU-T G.8263/Y.1363 (2012)/Amd.2 (05/2014) G.8263-Y.1363(12)-Amd.2(14)_FI.1PTP GMTime intervalcounterPacket slaveclockData
26、logFrequencyreferencePacket delaygeneratorStatisticalPDVmodelEthernetPhysical timing signal (e.g., 2048 kHz ITU-T G.703) EthernetPhysical timing signal (e.g., 2048 kHz ITU-T G.703) Figure I.1 PDV noise tolerance testing set-up The whole experiment is timed by a frequency reference clock, e.g., a cae
27、sium primary reference clock (PRC). This generates the input reference for a PTP grandmaster. The Sync messages generated by the PTP GM are delayed by a packet delay generator, and correspondingly the Delay_Request messages generated by the packet slave clock are also delayed using the same packet d
28、elay generator. The delay sequences are generated by means of a statistical model, with the parameters chosen to generate a delay distribution with properties similar to the network limits criterion defined in ITU-T G.8261.1 (i.e., for the HRM-1: 1% of the packets experiencing a delay within 150 s o
29、f the minimum delay in each observation interval of 200 s). Several methodologies for generating suitable PDV patterns are possible, some of which are described for information purposes in clause I.2 below. In general, it is recommended that the two delay sequences for the Sync and Delay_Request mes
30、sages are similar in properties, being generated using the same statistical model, but not identical. This avoids correlation effects where the sequences use similar values at the same time. The packet slave clock generates a physical output timing signal (e.g., a 2048 kHz ITU-T G.703 signal) which
31、is compared back to the timing signal from the frequency reference by a time interval counter. The data log can then be compared to the output mask defined in Figure 4 of ITU-T G.8261.1 to check the compliance of the slave. The test procedure is to be carried out under constant temperature condition
32、s (within 1 K): Any stress testing under a noisy thermal environment is for further study. When measuring PDV tolerance, the PDV test pattern should start to be applied before communication between the packet master clock and the packet slave clock is established. This order of operations will ensur
33、e that all “Event“ packets are impacted by the PDV test pattern. NOTE 1 A stabilization period is required when applying the PDV test patterns to the packet slave clock, before verifying the output signal produced by the packet slave clock is within acceptable limits. The duration of this stabilizat
34、ion period is for further study. NOTE 2 In general, it is the intention of PDV tolerance testing that the packet slave clock does not have prior knowledge of packet master clocks from previous measurements. In a practical test set-up, steps may be taken to ensure that the packet slave clock does not
35、 have this prior knowledge. For example, the packet slave clock may need to be restarted or even power cycled between measurements. However, it should be noted that in the case of power cycling the required stabilization period will likely increase. NOTE 3 The recommended warm-up time of the equipme
36、nt should be followed when performing the PDV tests. Rec. ITU-T G.8263/Y.1363 (2012)/Amd.2 (05/2014) 3 I.2 Test methodologies Three methods for generating suitable PDV test patterns that check conformance to the masks specifying the PDV network limits in ITU-T G.8261.1 for the HRM-1 are described in
37、 this clause: PDV patterns based on flicker noise PDV patterns based on combined sinusoidal waveforms PDV patterns based on a single sinusoidal waveform. Indications about the rationale for using each method are given in each subclause. Table I.1 below summarizes the pros and cons of each method. Ta
38、ble I.1 Comparison of the methods for PDV noise tolerance testing Method Pros Cons Flicker noise Simple test, with limited duration Emulates some typical characteristics of packet networks Does not take into account complex/extreme network scenarios, e.g., with floor delay movements Combined sinusoi
39、dal waveforms Simple test, with limited duration Emulates some typical characteristics of packet networks Includes floor delay movements, emulating moderate variations of traffic load Does not take into account extreme network scenarios, e.g., low-noise delay floor moving full swing, defined by the
40、network limit in a worst-case time frame. Single sinusoidal waveform (optional) Stress test corresponding to worst case scenarios, with extreme variations of the load (e.g., up to 100% of output ports capacity in all the nodes of the network), with important floor delay movements May allow in some c
41、ases determining the bandwidth of the slave clock Long test duration when low frequencies are used Does not emulate typical characteristics of real networks Figure I.2 summarizes the MAFE curves of the delay test patterns using 1% minimum selection and 60 s selection window. The PDV patterns represe
42、nted by curves at higher tau values require higher stability of the clock in order to remain within the clock output limit. 4 Rec. ITU-T G.8263/Y.1363 (2012)/Amd.2 (05/2014) G.8263-Y.1363(12)-Amd.2(14)_FI.2s100 1000 100001E-91E-81E-71E-630 ks20 ks8 ks4 ks2 ks800 s400 s16 ppbCombinedsinusoidsSinusoid
43、s, 150 sFlicker load gamma PDVMAFErelative150s75sNOTE The applicable network limit values are stated in microseconds. Figure I.2 MAFE curves of the test patterns NOTE The generation of PDV patterns that implement the maximum tolerance allowed are artificially generated with these methods. As such, t
44、hey may not represent PDV that may exist or occur in a real deployment or PDV that would be generated as a result of a packet timing master sending packets across a packet network. Some of the methodologies described to generate PDV patterns, especially the third one (single sinusoidal waveform), ma
45、y prevent the full benefits of advanced filtering techniques. As a result, in order to successfully tolerate these artificial PDV patterns a narrower clock bandwidth and a local oscillator which is very stable may be required. I.2.1 PDV patterns based on flicker noise The method for generating PDV p
46、atterns described in this clause consists of a combination of flicker noise with a probability density function given by a gamma distribution as a statistical model of PDV. Purpose and applicability This method is based on a simplified statistical model for a network experiencing bursty traffic. Pre
47、vious studies of Internet traffic (x, y) have shown that the traffic distribution is bursty at many different scales, and that this self-similar behaviour can be represented by using flicker noise to modulate the traffic load. Secondly, it can be shown that the queueing action of a packet switch or
48、router imposes a gamma distribution on the probability density function of the delays through the switch or router. The resulting statistical model can be shown diagrammatically in Figure I.3: Rec. ITU-T G.8263/Y.1363 (2012)/Amd.2 (05/2014) 5 G.8263-Y.1363(12)-Amd.2(14)_FI.3PTPslavePTPGMPacket netwo
49、rkTraffic loadSelf-similar variation, modelled by flicker noiseRegularly spacedtiming packetsTiming packets withflicker noise spacingand gamma distributionFigure I.3 Statistical model of PDV for timing packets It does not include the potential transients which can occur in packet networks, such as floor delay steps or floor delay variations. In addition, it may not model accurately a network which has significant non-bursty traffic (e.g., CBR traffic). This method i
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