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.8271/Y.1366 TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU Amendment 2 (01/2015) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS Packet over Transport aspects Synchronization, quality and availabil
2、ity targets SERIES Y: GLOBAL INFORMATION INFRASTRUCTURE, INTERNET PROTOCOL ASPECTS AND NEXT-GENERATION NETWORKS Internet protocol aspects Transport Time and phase synchronization aspects of packet networks Amendment 2 Recommendation ITU-T G.8271/Y.1366 (2012) Amendment 2 ITU-T G-SERIES RECOMMENDATIO
3、NS 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 INDIVIDUAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON METALLIC LINES G
4、.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.450G.499 TRANSMISSION MEDIA AND OPTICAL SYSTEMS CHARACTERISTICS G.600G.699 DIGITAL TER
5、MINAL 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.1000G.1999 TRANSMISSION MEDIA CHARACTERISTICS G.6000G.6999 DATA OVER TRANSPORT GENERIC ASPECTS G.7000G.7999 PAC
6、KET OVER TRANSPORT ASPECTS G.8000G.8999 Ethernet over Transport aspects G.8000G.8099 MPLS over Transport aspects G.8100G.8199 Synchronization, quality and availability targets G.8200G.8299 Service Management G.8600G.8699 ACCESS NETWORKS G.9000G.9999 For further details, please refer to the list of I
7、TU-T Recommendations. Rec. ITU-T G.8271/Y.1366 (2012)/Amd.2 (01/2015) i Recommendation ITU-T G.8271/Y.1366 Time and phase synchronization aspects of packet networks Amendment 2 Summary Amendment 2 to Recommendation ITU-T G.8271/Y.1366 (2012) provides the following update: Additional information on t
8、he delay asymmetry and asymmetry compensation aspects. History Edition Recommendation Approval Study Group Unique ID* 1.0 ITU-T G.8271/Y.1366 2012-02-13 15 11.1002/1000/11527 1.1 ITU-T G.8271/Y.1366 (2012) Amd. 1 2013-08-29 15 11.1002/1000/12033 1.2 ITU-T G.8271/Y.1366 (2012) Amd. 2 2015-01-13 15 11
9、.1002/1000/12391 _ * 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.8271/Y.1366 (2012)/Amd.2 (01/2015) FOREWORD The Internat
10、ional 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 responsible for studying technical, opera
11、ting 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 the ITU-T study groups which, in turn, pr
12、oduce 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 collaborative basis with ISO and IEC. NOTE In t
13、his 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 certain mandatory provisions (to ensure, e.g.,
14、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 requirements. The use of such words does not sugge
15、st that compliance with the Recommendation is required of any party. INTELLECTUAL PROPERTY RIGHTSITU 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 eviden
16、ce, 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 not received notice of intellectual property, protected by patents, which may
17、 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 2015 All rights reserved. No part of this publication may be
18、reproduced, by any means whatsoever, without the prior written permission of ITU. Rec. ITU-T G.8271/Y.1366 (2012)/Amd.2 (01/2015) 1 Recommendation ITU-T G.8271/Y.1366 Time and phase synchronization aspects of packet networks 1) Clause I.6, Derivation of delay asymmetry Replace clause I.6 with the fo
19、llowing: I.6 Derivation of delay asymmetry Figure I.1 illustrates the delays between a packet slave clock function, or requestor (denoted as slave throughout this clause), and a packet master clock function, or responder (denoted as master throughout this clause). The mean propagation delay is measu
20、red at the slave after exchange of event messages. If the Delay Request and the Delay Response mechanism is used (see IEEE 1588-2008), the slave sends Delay_Req and the master sends Delay_Resp and, separately, Sync and Follow_Up (i.e., the sending of Sync and Follow_Up are not part of the Delay_Req/
21、Delay_Resp exchange; the Follow_Up message is sent if, and only if, the clock is two-step). If the Peer Delay mechanism is used (see IEEE 1588-2008), the slave sends Pdelay_Req and the master sends Pdelay_Resp and, if the clock is two-step, Pdelay_Resp_Follow_Up. The figure shows the effective point
22、s in the protocol stack of each clock where timestamps are generated, after any corrections for ingress and egress latencies are made (see section 7.3.4 and Figure 19 of IEEE 1588-2008). These points would ideally be at the reference plane, i.e., the boundary point between the PHY and the network ph
23、ysical medium. However, in practice, the corrections for ingress and egress latencies are not perfect, and the effective points at which the timestamps are generated differ from the reference plane. The delays between the effective points where timestamps are taken and the reference plane are denote
24、d dtxPHY,M and drxPHY,M for egress and ingress, respectively, at the master, and dtxPHY,S and drxPHY,S for egress and ingress, respectively, at the slave. In this notation, the subscript t (transmit) is used for egress and the subscript r (receive) is used for ingress. In general, these four quantit
25、ies can all be different. The figure also shows the link delays, which are measured from the reference plane of one clock to the reference plane of the other clock. The delay from the master to the slave is denoted dmslink, and the delay from the slave to the master is denoted dsmlink. 2 Rec. ITU-T
26、G.8271/Y.1366 (2012)/Amd.2 (01/2015) Figure I.1 Illustration of delays between a packet slave clock function, or requestor, and a packet master clock function, or responder The total delay from the master to the slave, tms, is the sum of the delays in that direction: SP H Yrxl i n kmsMP H Ytxms dddt
27、 , (I-1) Similarly, the total delay from the slave to the master, tsm, is the sum of the delays in that direction: MP H Yrxl i n ksmSP H Ytxsm dddt , (I-2) For the sign convention for the delay asymmetry, the same convention as in section 7.4.2 of IEEE 1588-2008 is adopted. Let Dmean denote the meas
28、ured mean path delay (i.e., the measured result of the exchange of Delay_Req and Delay_Resp or of Peer Delay messages), and Dasym denote the total delay asymmetry. Then, Dasym is defined to be positive when the delay from the master to the slave is larger than the delay from the slave to the master.
29、 Likewise, Dasym is defined to be negative when the delay from the master to the slave is smaller than the delay from the slave to the master. Then: a s y mm e a nsma s y mm e a nms DDt DDt (I-3) Equations (I-3) imply that: 2 smmsm ea n ttD (I-4) as required. Substituting equations (I-1) and (I-2) i
30、nto equation (I-4) gives: 2 )()( , MP H Yrxl i n ksmSP H YtxSP H Yrxl i n kmsMP H Ytxm e a n ddddddD (I-5) Rec. ITU-T G.8271/Y.1366 (2012)/Amd.2 (01/2015) 3 Either of the two equations (I-3) may be used with equation (I-4) to obtain the delay asymmetry in terms of the component delays. Using the fir
31、st of equations (I-3) produces: Sphya s y ml i n kMphySPHYtxSPHYrxl i n ksml i n kmsMPHYrxMPHYtxMPHYrxl i n ksmSPHYtxSPHYrxl i n kmsMPHYtxSPHYrxl i n kmsMPHYtxm e a nmsa s y meeedddddddddddddddDtD 2222)()()( ,(I-6) where: 2 , MP H YrxMP H YtxMphy dde (I-7) 2 l i n ksml i n kmsa s y ml i n k dde (I-8
32、) 2 , SP H YrxSP H YtxSphy dde (I-9) Equations (I-7) and (I-9) are the errors due to PHY latency asymmetry at the master and slave respectively. Equation (I-8) is the error due to link asymmetry. Equation (I-6) indicates that, in computing the total asymmetry, the errors due to PHY latency at the ma
33、ster and due to the link are added, while the error due to PHY latency at the slave is subtracted. 2) Appendix IV, Link and network asymmetry compensation Add the following new Appendix: 4 Rec. ITU-T G.8271/Y.1366 (2012)/Amd.2 (01/2015) Appendix IV Link and network asymmetry compensation (This appen
34、dix does not form an integral part of this Recommendation.) In order to compensate for link delay asymmetry, it might be desirable to have in place some automatic link asymmetry calibration procedure. This could be based on calculating the propagation delays by means of two-way measurements made on
35、the fibres used by the traffic. The procedure can be done separately on both fibres (in the fibre used in the forward direction and in the fibre used for the reverse direction) providing the forward propagation delay df and the reverse propagation delay dr. This is shown in Figure IV.1. Figure IV.1
36、Link asymmetry calibration process (performed separately on both fibres) Alternatively the round trip measurement could be done in two steps on both fibres by reversing the direction of transmission. This is shown in Figure IV.2. Figure IV.2 Link asymmetry calibration process (performed on both fibr
37、es at the same time) NOTE 1 In the case of the connection between master and slave, as shown in Figure I.1, the following would apply: df = dms dr = dsm The link asymmetry calibration mechanism must meet an accuracy objective for df and dr estimations. This limit is for further study. Rec. ITU-T G.8
38、271/Y.1366 (2012)/Amd.2 (01/2015) 5 NOTE 2 In the case during the asymmetry calculation procedure where one node enters holdover (e.g., caused by the fibres-swapping if this is required by the procedure), the effect of the frequency holdover needs to be taken into account as it might impact the accu
39、racy of the measurement. Several implementations are possible, e.g., based on optical switches or fixed or tunable add drop filters. Depending on the implementation, it may not be required to interrupt the traffic during the calibration process and hence in-service operation might be possible. Howev
40、er, the asymmetry compensation is a process that is only required at start-up or during rearrangements in the network. This measurement is applicable for WDM systems (including OTN) and non-WDM systems. In the case of wavelength-division-multiplexing (WDM) systems, this measurement should also take
41、into account possible delay due to dispersion-compensating fibre (DCF). NOTE 3 In the case of WDM systems, the asymmetry due to the use of different wavelengths in the two directions should also be taken into account. Indeed, the use of different wavelengths on the two fibres, (or in a single fibre
42、in the case of a transmission system using a single fibre), would result in different delays even if the fibres have the same length. Note also that a compensation related to the same aspect would be required if the wavelength used during the link asymmetry calibration process is different from the
43、wavelength used by the traffic. Suitable methodologies to address this point are introduced in Appendix III. The difference (df dr) can be used in the evaluation of the delay asymmetry to be used in the time recovering process. In particular the delayAsymmetry parameter as defined in section 7.4 of
44、IEEE 1588-2008 would be half of that difference. NOTE 4 If a T-BC is implemented in every node, the compensation can be triggered directly by the T-BC, which would know the difference (df dr). If this is not the case, some means have to be provided in order to make the difference (df dr) available a
45、t the points in the network where the precision time protocol (PTP) messages are processed. This is for further study. NOTE 5 In the case of a time synchronization carried by PTP, the PTP connection may have asymmetry due to a variety of reasons, including network paths, loading levels or cable leng
46、ths. The asymmetry of a PTP connection may be evaluated at a PTP network element, if the network element has access to a second time synchronization source that is not significantly impacted by asymmetry (such as a GNSS receiver, or a time synchronization reference carried via timing protocols such
47、as PTP with proper accuracy) as shown in Figure IV.3. If the asymmetry of the PTP connection is evaluated using such a second time synchronization source, then the offset caused by the asymmetry may be compensated by the network element. The same principle could be applied between network elements i
48、n a chain. Figure IV.3 PTP slave evaluating PTP connection asymmetry ITU-T Y-SERIES RECOMMENDATIONS GLOBAL INFORMATION INFRASTRUCTURE, INTERNET PROTOCOL ASPECTS AND NEXT-GENERATION NETWORKS GLOBAL INFORMATION INFRASTRUCTURE General Y.100Y.199 Services, applications and middleware Y.200Y.299 Network
49、aspects Y.300Y.399 Interfaces and protocols Y.400Y.499 Numbering, addressing and naming Y.500Y.599 Operation, administration and maintenance Y.600Y.699 Security Y.700Y.799 Performances Y.800Y.899 INTERNET PROTOCOL ASPECTS General Y.1000Y.1099 Services and applications Y.1100Y.1199 Architecture, access, network capabilities and resource management Y.1200Y.1299 Transport Y.1300Y.1399 Interworking Y.1400Y.1499 Quali
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