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.8260 TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (08/2015) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS Packet over Transport aspects Synchronization, quality and availability targets Definit
2、ions and terminology for synchronization in packet networks Recommendation ITU-T G.8260 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-TRANS
3、MISSION SYSTEMS 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 RADIOT
4、ELEPHONY 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 ASPEC
5、TS 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 Transport aspects G.8000G.8099 MPLS over Transport aspects G.8100G.8199 Synchronization, quality and availability targets G.8200G.
6、8299 Service 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.8260 (08/2015) i Recommendation ITU-T G.8260 Definitions and terminology for synchronization in packet networks Summary Recommendation ITU-T G.8260 p
7、rovides the definitions, terminology and abbreviations used in ITU-T Recommendations on timing and synchronization in packet networks. History Edition Recommendation Approval Study Group Unique ID* 1.0 ITU-T G.8260 2010-08-12 15 11.1002/1000/10907 2.0 ITU-T G.8260 2012-02-13 15 11.1002/1000/11521 2.
8、1 ITU-T G.8260 (2012) Amd. 1 2013-08-29 15 11.1002/1000/12016 2.2 ITU-T G.8260 (2012) Amd. 2 2014-05-14 15 11.1002/1000/12189 3.0 ITU-T G.8260 2015-08-13 15 11.1002/1000/12545 3.1 ITU-T G.8260 (2015) Amd. 1 2016-04-13 15 11.1002/1000/12808 Keywords Frequency, phase and time, packet delay variation,
9、synchronization definitions. _ * 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.8260 (08/2015) FOREWORD The International Te
10、lecommunication 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, operating and
11、 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, produce Re
12、commendations 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 this Reco
13、mmendation, 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., interope
14、rability 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 suggest that
15、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 evidence, vali
16、dity 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 be requ
17、ired 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 2016 All rights reserved. No part of this publication may be reproduc
18、ed, by any means whatsoever, without the prior written permission of ITU. Rec. ITU-T G.8260 (08/2015) iii Table of Contents Page 1 Scope . 1 2 References . 1 3 Definitions 1 3.1 Terms defined in this Recommendation . 1 4 Abbreviations and acronyms 6 5 Conventions 7 6 Description of packet timing con
19、cepts . 7 6.1 The nature of packet timing 7 6.2 Differences between packet-based and physical layer timing systems 8 6.3 Classes of packet clocks . 9 6.4 Two-way timing protocols . 9 6.5 Packet delay sequence measurement 10 6.6 Packet timing signal equipment interface characterization 12 Appendix I
20、Definitions and properties of packet measurement metrics . 13 I.1 Introduction 13 I.2 Definition of the time error sequence . 14 I.3 Packet selection and filtering 16 I.4 PDV metrics estimating the performance of a packet slave clock . 23 I.5 PDV metrics studying floor delay packet population . 38 B
21、ibliography. 45 Rec. ITU-T G.8260 (08/2015) 1 Recommendation ITU-T G.8260 Definitions and terminology for synchronization in packet networks 1 Scope This Recommendation provides the definitions, terminology and abbreviations used in Recommendations on frequency, phase and time synchronization in pac
22、ket networks. It includes mathematical definitions for various synchronization stability and quality metrics for packet networks, and also provides background information on the nature of packet timing systems and the impairments created by packet networks. Ethernet physical layer methods for synchr
23、onization are based on traditional time division multiplexing (TDM) physical layer synchronization and therefore most of the definitions related to these methods are covered by ITU-T G.810. Additional definitions are included in this Recommendation. 2 References The following ITU-T Recommendations a
24、nd other references contain provisions which, through reference in this text, constitute 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 enco
25、uraged to investigate the possibility of applying the most recent edition of the Recommendations 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 do
26、cument, the status of a Recommendation. ITU-T G.810 Recommendation ITU-T G.810 (1996), Definitions and terminology for synchronization networks. ITU-T G.811 Recommendation ITU-T G.811 (1997), Timing characteristics of primary reference clocks. ITU-T G.8261 Recommendation ITU-T G.8261/Y.1361 (2013),
27、Timing and synchronization aspects in packet networks. ITU-T G.8261.1 Recommendation ITU-T G.8261.1/Y.1361.1 (2012), Packet delay variation network limits applicable to packet-based methods (Frequency synchronization). ITU-T G.8263 Recommendation ITU-T G.8263/Y.1363 (2012), Timing characteristics of
28、 packet-based equipment clocks. ITU-T Y.1413 Recommendation ITU-T Y.1413 (2004), TDM-MPLS network interworking User plane interworking. IEEE 1588 IEEE Standard 1588-2008, IEEE standard for a precision clock synchronization protocol for networked measurement and control systems. 3 Definitions 3.1 Ter
29、ms defined in this Recommendation This Recommendation defines the following terms: 3.1.1 adaptive clock recovery: Clock recovery technique that does not require the support of a network-wide synchronization signal to regenerate the timing. In this case, the timing recovery process is based on the (i
30、nter-)arrival time of the packets, e.g., timestamps or circuit emulation service 2 Rec. ITU-T G.8260 (08/2015) (CES) packets. The information carried by the packets could be used to support this operation. Two-way or one-way protocols can be used. 3.1.2 arbitrary reference time clock (ARTC): A refer
31、ence time generator that provides a reference time signal, or simply a reference phase signal, whose frequency has the accuracy of a primary reference clock (PRC) as specified in ITU-T G.811, while the epoch does not necessarily have a relationship with an internationally recognized time standard. 3
32、.1.3 coherent time and frequency: The condition where the timing signal-carrying frequency and the timing signal-carrying time-of-day or phase are traceable back to the same primary source. 3.1.4 floor delay: The notion of “floor delay“ is derived from the notion of minimum possible transit delay of
33、 packets over a network. It may be useful to distinguish the notions of “absolute floor delay“ and “observed floor delay“: absolute floor delay: Absolute minimum possible transit delay of packets of a given size over a network. This may generally be described as the transit delay experienced by a pa
34、cket that has experienced the minimum possible delay through each network element along a specified path. Depending on loading and other considerations, it is possible that in any given finite window of observation interval a packet with delay equal to this absolute minimum may not be observed. Full
35、 knowledge of the packet network, network elements, and routing path must be known in order to perform a theoretical analysis of the minimum transit delay. observed floor delay (OFD): Minimum transit delay of packets of a given size over a network observed over a given observation interval for insta
36、nce, during a packet delay variation (PDV) measurement. NOTE As mentioned above, the observed floor delay during a PDV measurement may differ from the absolute floor delay. 3.1.5 floor delay step: The difference between the observed floor delays of two consecutive, non-overlapping observation interv
37、als, see Figure 1: G . 8 2 6 0 ( 1 2 ) _ F 0 1F l oor de l a y ( ove robs e r va t i oni nt e r va l 2)F l oor de l a y s t e p ( ove robs e r va t i on i nt e r va l s 1 a nd 2)F l oor de l a y ( ove robs e r va t i oni nt e r va l 1)O bs e r va t i on i nt e r va l 1 O bs e r va t i on i nt e r va
38、 l 1Figure 1 Illustration of observed floor delays and floor delay step 3.1.6 packet-based method: Timing distribution method (for frequency or time or phase) where the timing information is associated with packets. The frequency can be recovered using two-way or one-way protocols. Time and phase in
39、formation is recovered with a two-way protocol in order to compensate for the transfer delay from packet master clock to packet slave clock. Rec. ITU-T G.8260 (08/2015) 3 3.1.7 packet-based method with full timing support to the protocol level from the network: Packet-based method (frequency or time
40、 and/or 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., network time protocol (NTP) stratum clocks, precision time protocol (PTP) boundary
41、clock; a mechanism to correct for the delay introduced by the network node or the connected links (e.g., PTP transparent clock). 3.1.8 packet-based method with partial timing support to the protocol level from the network: Packet-based method (frequency or time and/or phase synchronization) where no
42、t all of the network nodes on the path of the synchronization flow implement timing support. 3.1.9 packet-based method with physical frequency support from the network: Packet-based method for time and phase synchronization using frequency support from a traceable network reference clock carried by
43、a physical layer timing trail. NOTE For instance, it can correspond to telecom boundary clocks syntonized by a frequency reference carried at the physical layer. This type of support is expected to provide “phase and/or time holdover“ capacities, enabling phase and/or time local reference to be main
44、tained during periods of failure of the phase and/or time distribution protocol. 3.1.10 packet-based method without timing support from the network: Packet-based method (frequency or time and/or phase synchronization) where the timing packets are transported over a timing transport agnostic network.
45、 3.1.11 packet master clock: A clock that measures the precise times at which the significant instants of a packet timing signal pass the masters timing reference point (e.g., as they enter the network from the packet master clock or as they enter the packet master clock from the network). These mea
46、surements are done relative to the master clocks local time scale. They are forwarded to, and used to control, one or more packet slave clocks. NOTE In the case of a periodic packet timing signal (used for one-way frequency distribution), the event packets enter the network from the packet master cl
47、ock at regular intervals, such that the masters timing information is implied from the nominal frequency of the packets. 3.1.12 packet network timing function (PNT-F): The set of functions within the inter-working function (IWF) that supports the synchronization network clock domain (see Figure B.2
48、of ITU-T G.8261). This includes the function to recover and distribute the timing carried by the synchronization network. The PNT-F clocks may be part of the IWF or may be part of any other network element in the packet network. When the PNT-Fs are part of the IWF, they may support the CES IWF or ch
49、ange the layer over which timing is carried (i.e., from packet to physical layer and vice versa). 3.1.13 packet slave clock: A clock whose timing output is frequency locked or phase aligned or time aligned to one or more reference packet timing signals exchanged with a higher quality clock. 3.1.14 packet timing monitor: A device capable of analysing the packet flow e.g., precision time protocol (PTP) including precise measurement of the sending times and arrival times of timing eve
