1、 International Telecommunication Union ITU-T G.8260TELECOMMUNICATION 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 Definitions and terminology for synchronizatio
2、n in packet networks Amendment 1 Recommendation ITU-T G.8260 (2012) Amendment 1ITU-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 S
3、YSTEMS G.200G.299 INDIVIDUAL 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
4、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.899 DIGITAL SECTIONS AND DIGITAL LINE SYSTEM G.900G.999 MULTIMEDIA QUALITY OF SERVICE AND PERFORMANCE GENERIC AND USER-RELATED ASPECTS G.1000
5、G.1999 TRANSMISSION MEDIA CHARACTERISTICS G.6000G.6999 DATA OVER TRANSPORT GENERIC ASPECTS G.7000G.7999 PACKET OVER TRANSPORT ASPECTS G.8000G.8999 Ethernet over Transport aspects G.8000G.8099 MPLS over Transport aspects G.8100G.8199 Quality and availability targets G.8200G.8299Service Management G.8
6、600G.8699 ACCESS NETWORKS G.9000G.9999 For further details, please refer to the list of ITU-T Recommendations. Rec. ITU-T G.8260 (2012)/Amd.1 (08/2013) i Recommendation ITU-T G.8260 Definitions and terminology for synchronization in packet networks Amendment 1 Summary Amendment 1 to Recommendation I
7、TU-T G.8260 (2012): Adds a reference in clause 2. Modifies definition 3.1.8, “packet-based method with timing support from the network“. Adds a number of definitions in clause 3.1. Adds new clause I.5.1 to Appendix I, describing the determination of floor delay and discussion of the impact of re-rou
8、te events. Adds new clause I.5.2 to Appendix I, describing the impact of exceptional events on packet network limit. History Edition Recommendation Approval Study Group 1.0 ITU-T G.8260 2010-08-12 15 2.0 ITU-T G.8260 2012-02-13 15 2.1 ITU-T G.8260 (2012) Amd. 1 2013-08-29 15 ii Rec. ITU-T G.8260 (20
9、12)/Amd.1 (08/2013) 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 ITU. ITU-T is
10、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 for study by
11、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 on a collab
12、orative 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 contain certai
13、n 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 express requireme
14、nts. 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 Property Right. I
15、TU 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 intellectual pro
16、perty, 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 2013 All rights reserve
17、d. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. Rec. ITU-T G.8260 (2012)/Amd.1 (08/2013) iii Table of Contents Page 1) Clause 2, References . 1 2) Clause 3.1, Terms defined in this Recommendation 1 3) Clause I.3.2, Packet select
18、ion methods . 2 4) Clause I.5, PDV metrics studying floor delay packet population. 3 Rec. ITU-T G.8260 (2012)/Amd.1 (08/2013) 1 Recommendation ITU-T G.8260 Definitions and terminology for synchronization in packet networks Amendment 1 1) Clause 2, References Add the following reference: ITU-T G.823
19、Recommendation ITU-T G.823 (2000), The control of jitter and wander within digital networks which are based on the 2048 kbit/s hierarchy. 2) Clause 3.1, Terms defined in this Recommendation a) Replace definition 3.1.8: 3.1.8 packet-based method with timing support from the network: Packet-based meth
20、od (frequency or time-phase synchronization) requiring that all the network nodes on the path of the synchronization flow implement one of the two following types of functional support: termination and regeneration of the timing (e.g., NTP stratum clocks, PTP boundary clock); a mechanism to measure
21、the delay introduced by the network node and/or the connected links (e.g., PTP transparent clock) so that the delay variation can be compensated using this information. with the following: 3.1.8 packet-based method with full timing support to the protocol level from the network: Packet-based method
22、(frequency or time-phase synchronization) requiring that all the network nodes on the path of the synchronization flow implement one of the two following types of timing support: termination and regeneration of the timing (e.g., NTP stratum clocks, PTP boundary clocks); a mechanism to correct for th
23、e delay introduced by the network node and/or the connected links (e.g., PTP transparent clocks). b) Add the following additional definitions at the end of clause 3.1: 3.1.18 packet-based method with partial timing support to the protocol level from the network: Packet-based method (frequency or tim
24、e-phase synchronization) where not all of the network nodes on the path of the synchronization flow implement timing support. 3.1.19 packet timing monitor: A device capable of analysing the packet flow (e.g., PTP) including precise measurement of the sending times and arrival times of timing event m
25、essages utilizing an accurate, stable clock. A tapped monitor does not substantively impact the transmission of packets between the communicating clocks; an in-line monitor introduces a fixed, symmetric, delay for packets in the two directions of transmission and thereby does not substantively impac
26、t the transfer of timing between the communicating clocks. 2 Rec. ITU-T G.8260 (2012)/Amd.1 (08/2013) 3.1.20 time error (Based on ITU-T G.810): Constant time error: With reference to the time error model provided in ITU-T G.810, the constant time error is the term x0. Constant time error estimate: G
27、iven a time error sequence x(n); n = 0,1,(N1), an estimate of the constant time error is the average of the first M samples of the time error sequence. M is obtained from the observation interval providing the least value for TDEV as computed for the given time error sequence. If a frequency offset
28、is present then a linear regression method in accordance with Appendix II of ITU-T G.823 can be applied. Considerations for measurement data containing transients is for further study. NOTE In some cases due to the frequency components of the noise of the signal being measured it might be difficult
29、to identify a stable, consistent observation interval. These cases must be addressed case by case. 3) Clause I.3.2, Packet selection methods Replace clauses I.3.2 through I.3.2.3 with the following: I.3.2 Packet selection methods Four examples of packet selection methods are described in the clauses
30、 that follow. The first two, minimum packet selection and percentile average packet selection, focus on packet data at the floor. The second two, band average packet selection and cluster range packet selection, can be applied either at the floor or at some other region. I.3.2.1 Minimum packet selec
31、tion method The minimum packet selection method involves selecting a minimum within a section of data. This can be represented as follows: () ( )1for minmin+= nijixixj(I-2) I.3.2.2 Percentile average packet selection method The percentile average packet selection method is related to the minimum pac
32、ket selection method, except that instead of selecting the minimum, some number (or some percentage) of minimum values are chosen and averaged together. It is a special case of the band average packet selection method described below with the lower index set to zero. I.3.2.3 Band average packet sele
33、ction method The band average packet selection method can be used to select a section of packet data at the floor or from some other region such as the ceiling or somewhere else above the floor. To perform the band average packet selection, it is first necessary to represent the sorted packet time-e
34、rror sequence. Let x represent this sorted phase sequence from minimum to maximum over the range i j i + n 1. Next, it is necessary to represent the indices which are themselves set based on the selection of two percentile levels. Let a and b represent indices for the two selected percentile levels.
35、 The averaging is then applied to the x variable indexed by a and b. The number of averaged points m is related to a and b: m = b a + 1. ()=+=bajijmavgbandxix1_(I-3) Rec. ITU-T G.8260 (2012)/Amd.1 (08/2013) 3 A percentile level is selected by using rounding to find the closest index from the desired
36、 percentile value. The additional constraint is that the index value has a minimum of the first index and a maximum of the last index. Thus, for example, a set of ten points with a percentile set to 2% (0.02) would be set to the minimum index so that at least a single point would be selected. 4) Cla
37、use I.5, PDV metrics studying floor delay packet population Add the following text at the end of clause I.5: I.5.1 Determination of observed floor delay When calculating the floor population metrics, it is first necessary to determine the value of the “observed floor delay“. Whereas it is permissibl
38、e for the user to specify a suitable value for the floor delay, two data-driven methods of determining this value are described here. The first method, called the overall minimum method, is to use the minimum delay observed over the entire measurement period. The second method, called the progressiv
39、e minimum method, is to use the minimum observed delay in the measurement period up to the time window over which the individual floor population metric value is calculated. Refer to clause I.5.1.3 for information concerning the impact of packet network re-route events on the determination of observ
40、ed floor delay. I.5.1.1 Minimum floor delay over the entire measurement In the overall minimum method, the “observed floor delay“ used in computing the floor population metrics is the minimum delay value over the entire measurement data set according to clause I.5, equation (I-33). As the overall mi
41、nimum delay for a measurement period may not be known until the end of the period, calculation of floor population metric values over a given time window may depend upon delay values that have not yet been observed. This dependency upon future observations makes it more difficult to provide an early
42、 indication of floor population conformance for long-term tests. I.5.1.2 Progressive determination of floor delay In applications where it is not practical to wait till the end of the measurement period to determine the observed floor delay, the following causal estimation procedure can be used. At
43、each floor population metric computation point n, the observed floor delay is estimated as the smallest delay value in the measurement period up to (and including) the window over which the metric is computed. This running estimate of floor delay is then used when calculating the floor population me
44、tric. This enables calculation of the floor population metric value at any given time depends only upon delay values that have already been observed. To accommodate the dynamic notion of “observed floor delay“, the floor population metrics defined in clause I.5 are modified to use the current retros
45、pective estimate of the floor delay rather than the minimum over the whole data set. The terminology used is FPxM(n,W,dmin(n) where “x“ represents the metric (“count“, “rate“, “percent“), the subscript M indicates that the formula used is a modified form of the ITU-T G.8260 definition, and dmin(n) i
46、s the current running estimate of the floor delay. In terms of equation (I-33) above, the observed floor delay at time n (where n is always a sample index at the end of an observation window) can be estimated as: min)(0minixndni= (I-40) The value of the floor packet metrics in equations (I-35) to (I
47、-37) at time n are then calculated using dmin(n) instead of dminin equation (I-33). 4 Rec. ITU-T G.8260 (2012)/Amd.1 (08/2013) As an example for a specific implementation, the floor delay at time n can be iteratively estimated as according to the following algorithm: 1) Denote d(n) as the minimum pa
48、cket delay of the most recent observation window; 2) Compare d(n) to the current estimate of the “observed floor delay“ dmin(n) a) If d(n) dmin(n) dmin(n) = d(n) b= Otherwise dmin(n) remains unchanged (I-41) The progressive floor determination method continually refines the estimate of the observed
49、floor delay value during the measurement period. At each Floor Population metric computation point n, the observed floor delay is estimated as the smallest delay value, dmin(n), in the measurement period up to (and including) the window over which the metric is computed. The running value of this estimate is then used when calculating the (estimated) Floor Population metric. The windows could be sliding, overlapping, or jumping. Starting from the first observation window and for each subsequent win
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