1、 International Telecommunication Union ITU-T G.8260TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (02/2012) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS Packet over Transport aspects Quality and availability targets Definitions and terminology for synchronization in packet
2、 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-TRANSMISSION SYSTEMS G.200G.299 INDIVIDUAL CHARACTERIST
3、ICS 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 AND LINE TELEPHONY G.450G.499 TRANSMISSIO
4、N 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.1000G.1999 TRANSMISSION MEDIA CHARACTERISTICS
5、 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.8600G.8699 ACCESS NETWORKS G.9000G.9999 Fo
6、r further details, please refer to the list of ITU-T Recommendations. Rec. ITU-T G.8260 (02/2012) i Recommendation ITU-T G.8260 Definitions and terminology for synchronization in packet networks Summary Recommendation ITU-T G.8260 provides the definitions, terminology and abbreviations used in ITU-T
7、 Recommendations on timing and synchronization in packet networks. 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 Keywords Frequency, packet delay variation, phase and time, synchronization definitions. ii Rec. ITU-T G.8260 (02/2012)
8、 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 responsible for stu
9、dying 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 the ITU-T study gro
10、ups 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 collaborative basis with
11、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 certain mandatory provisi
12、ons (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 requirements. The use of suc
13、h 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. ITU takes no positio
14、n 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 property, protected by
15、 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 2012 All rights reserved. No part of this
16、publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. Rec. ITU-T G.8260 (02/2012) 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 5 5 Conventions 6 6 Descript
17、ion of packet timing concepts . 6 6.1 The nature of packet timing 6 6.2 Differences between packet-based and physical-layer timing systems 7 6.3 Classes of packet clocks . 8 6.4 Two-way timing protocols . 8 6.5 PDV measurement 9 6.6 Packet timing signal equipment interface characterization 10 Append
18、ix I Definitions and properties of packet measurement metrics . 11 I.1 Introduction 11 I.2 Definition of the time error sequence . 13 I.3 Packet selection 13 I.4 PDV metrics estimating the performance of a packet slave clock . 18 I.5 PDV metrics studying floor delay packet population . 30 I.6 Summar
19、y of metric classifications . 33 Bibliography. 34 Rec. ITU-T G.8260 (02/2012) 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, pha
20、se and time synchronization in packet 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
21、 physical layer methods for synchronization 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
22、 following ITU-T Recommendations and 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
23、 Recommendation are therefore encouraged 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 doe
24、s not give it, as a stand-alone document, 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 Recommend
25、ation ITU-T G.8261/Y.1361 (2008), 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
26、 (2012), Timing characteristics of 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 Cont
27、rol Systems. 3 Definitions 3.1 Terms defined in this Recommendation This Recommendation defines the following terms: 2 Rec. ITU-T G.8260 (02/2012) 3.1.1 adaptive clock recovery: Clock recovery technique that does not require the support of a network-wide synchronization signal to regenerate the timi
28、ng. In this case, the timing recovery process is based on the (inter-)arrival time of the packets, e.g., timestamps or circuit emulation service (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 re
29、ference time clock (ARTC): A reference time generator that provides a reference time signal, or simply a reference phase signal, whose frequency has the accuracy of a PRC as specified in ITU-T G.811, while the epoch does not necessarily have a relationship with an internationally recognized time sta
30、ndard. 3.1.3 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 equivalent to the notion of minimum possible transit
31、 delay of 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 experience
32、d by a packet 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 obser
33、ved. Full 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: Minimum transit delay of packets of a given size over a network observed over a given observation interval (for i
34、nstance, 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 i
35、ntervals, see Figure 1: G.8260(12)_F01Floor delay (overobservationinterval 2)Floor delay step (overobservation intervals 1 and 2)Floor delay (overobservationinterval 1)Observation interval 1 Observation interval 1Figure 1 Illustration of observed floor delays and floor delay step Rec. ITU-T G.8260 (
36、02/2012) 3 3.1.6 packet-based method: Timing distribution method (for frequency and/or time and/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 information is recovered with a two-way protocol in ord
37、er to compensate for the transfer delay from packet master clock to packet slave clock. 3.1.7 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 a phy
38、sical 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/time holdover“ capacities, enabling to maintain phase/time local reference during periods of
39、 failure of the phase/time distribution protocol. 3.1.8 packet-based method with timing support from the network: Packet-based method (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 fun
40、ctional support: termination and regeneration of the timing (e.g., NTP stratum clocks, PTP boundary clock); a mechanism to measure 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.
41、3.1.9 packet-based method without timing support from the network: Packet-based method (frequency or time-phase synchronization) where the timing packets are transported over a timing transport agnostic network. 3.1.10 packet master clock: A clock that measures the precise times at which the signifi
42、cant 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 measurements are done relative to the master clocks local time-scale. They are forwarded to
43、, 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 clock at regular intervals, such that the masters timing information is implied from the n
44、ominal frequency of the packets. 3.1.11 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 of ITU-T G.8261). This includes the function to recover and distribute the timing carrie
45、d 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 and/or change the layer over which timing is carried (i.e., from packet to physical layer and
46、 vice versa). 3.1.12 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. 3.1.13 packet timing signal: A signal, consisting of a series of event packets or frames,
47、that is used to convey timing information from a packet master clock to a packet slave clock. Event packets in a packet timing signal may travel from a packet master clock to a packet slave clock or vice versa, but the flow of timing information is always in the direction from master to slave. The s
48、ignificant instants of the packet timing signal are measured relative to the masters local time-scale as they pass the masters timing reference point, and these measurements are communicated to the packet slave clock. 4 Rec. ITU-T G.8260 (02/2012) The significant instants of the packet timing signal
49、 are also measured relative to the slaves local time-scale as they pass the slaves timing reference point. NOTE 1 The significant instants of the signal are the set of times that a defined location in each event packet or frame passes a given reference point in the network (e.g., the interface between the packet master clock and the network). Conventionally the defined location is the end of the start-of-frame delimiter, but it may be defined differently in any given packet timing protocol provided the d
