1、 International Telecommunication Union ITU-T G.8265/Y.1365TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (10/2010) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS Packet over Transport aspects Quality and availability targets SERIES Y: GLOBAL INFORMATION INFRASTRUCTURE, INTER
2、NET PROTOCOL ASPECTS AND NEXT-GENERATION NETWORKS Internet protocol aspects Transport Architecture and requirements for packet-based frequency delivery Recommendation ITU-T G.8265/Y.1365 ITU-T G-SERIES RECOMMENDATIONS TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS INTERNATIONAL TELEPHO
3、NE 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 INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON RADIO
4、-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 NETWORKS G.800G.899 DIGITAL SECTIONS AND DIGITAL
5、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.8999 Ethernet over Transport aspects G.8000G.8099
6、MPLS over Transport aspects G.8100G.8199 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.8265/Y.1365 (10/2010) i Recommendation ITU-T G.8265/Y.1365 Architect
7、ure and requirements for packet-based frequency delivery Summary Recommendation ITU-T G.8265/Y.1365 describes the architecture and requirements for packet-based frequency distribution in telecom networks. Examples of packet-based frequency distribution include the network time protocol (NTP) and IEE
8、E-1588-2008, briefly described here. Details necessary to utilize IEEE-1588-2008 in a manner consistent with the architecture are defined in other Recommendations. History Edition Recommendation Approval Study Group 1.0 ITU-T G.8265/Y.1365 2010-10-07 15 ii Rec. ITU-T G.8265/Y.1365 (10/2010) FOREWORD
9、 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 studying tec
10、hnical, 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 groups which
11、, 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 ISO and I
12、EC. 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 provisions (to e
13、nsure, 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 such words d
14、oes 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 position concern
15、ing 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 not received notice of intellectual property, protected by pate
16、nts, 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 2011 All rights reserved. No part of this publi
17、cation may be reproduced, by any means whatsoever, without the prior written permission of ITU. Rec. ITU-T G.8265/Y.1365 (10/2010) iii Table of Contents Page 1 Scope 1 2 References. 1 3 Definitions 1 3.1 Terms defined elsewhere 1 4 Abbreviations and acronyms 2 5 Conventions 2 6 General introduction
18、to packet-based frequency distribution . 2 6.1 Requirements for packet timing . 3 7 Architecture of packet-based frequency distribution 3 7.1 Packet-based frequency distribution . 3 7.2 Timing protection . 4 7.3 Packet network partitioning 7 7.4 Mixed technologies 8 8 Packet-based protocols for freq
19、uency distribution . 8 8.1 Packet-based protocols . 8 8.2 PTP IEEE 1588 general description 8 8.3 NTP General description . 9 9 Security aspects 9 Appendix I Bibliography 11 Rec. ITU-T G.8265/Y.1365 (10/2010) 1 Recommendation ITU-T G.8265/Y.1365 Architecture and requirements for packet-based frequen
20、cy delivery 1 Scope This Recommendation describes the general architecture of frequency distribution using packet-based methods. This version of the Recommendation focuses on the delivery of frequency using methods such as NTP or the precision time protocol (PTP) IEEE 1588. The requirements and arch
21、itecture form a base for the specification of other functionality needed to achieve packet-based frequency distribution in a carrier environment. The architecture described covers the case where protocol interaction is at the end points of the network only, between a packet master clock and a packet
22、 slave clock. Details of requirements for other architectures involving devices that participate between the packet master and packet slave clocks are for further study. 2 References The following ITU-T Recommendations and other references contain provisions which, through reference in this text, co
23、nstitute 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 encouraged to investigate the possibility of applying the most recent edition of the
24、 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 document, the status of a Recommendation. ITU-T G.8260 Recommendation ITU-T G.8260
25、 (2010), Definitions and terminology for synchronization in packet networks. ITU-T G.8261 Recommendation ITU-T G.8261/Y.1361 (2008), Timing and synchronization aspects in packet networks. ITU-T G.8264 Recommendation ITU-T G.8264 (2008), Distribution of timing information through packet networks, plu
26、s Amendment 1 (2010). IEEE 1588 IEEE STD 1588-2008, Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems. IETF RFC 5905 IETF RFC 5905 (2010), Network Time Protocol Version 4: Protocol And Algorithms Specification. 3 Definitions 3.1 Terms defined elsew
27、here This Recommendation uses the following terms defined elsewhere: 3.1.1 packet master clock ITU-T G.8260. 3.1.2 packet slave clock ITU-T G.8260. 3.1.3 packet timing signal ITU-T G8260. 2 Rec. ITU-T G.8265/Y.1365 (10/2010) 4 Abbreviations and acronyms This Recommendation uses the following abbrevi
28、ations and acronyms: CDMA Code Division Multiple Access DSL Digital Subscriber Line EEC Ethernet Equipment Clock GM Grand Master GNSS Global Navigation Satellite System LSP Label Switched Path LTE Long Term Evolution MINPOLL Minimum Poll interval NTP Network Time Protocol PDV Packet Delay Variation
29、PON Passive Optical Network PRC Primary Reference Clock PTP Precision Time Protocol QL Quality Level RTP Real Time Protocol SDH Synchronous Digital Hierarchy SEC SDH Equipment Clock SSM Synchronization Status Message TDM Time Division Multiplexing VLAN Virtual Local Area Network WIMAX Worldwide Inte
30、roperability for Microwave Access 5 Conventions Within this Recommendation, the term PTP refers to the PTP version 2 protocol defined in IEEE 1588. NTP refers to network time protocol as defined in IETF RFC 5905. 6 General introduction to packet-based frequency distribution The modern telecom networ
31、k has relied on accurate distribution of frequency in order to optimize transmission and TDM cross-connection. In contrast, packet networks and packet services are highly buffered by their nature and, as a result, do not require accurate timing for their operation. The migration towards converged pa
32、cket networks on the surface leads to the belief that frequency distribution will not be required as packet network technology becomes more prevalent in the network. While this may be true for certain services (Internet is one example), the underlying transport mechanisms that deliver these timing a
33、gnostic services may require stringent timing requirements that must be provided in the new converged network paradigm. For example, in some cases, support of circuit emulation services over a packet-based infrastructure requires the presence of a stable Rec. ITU-T G.8265/Y.1365 (10/2010) 3 frequenc
34、y reference to enable the service. Likewise, in wireless access technologies (e.g., GSM, LTE, WIMAX, CDMA, etc.) the air interface requirements have stringent synchronization requirements that need to be met, even though the end-user service (e.g., mobile Internet) may seemingly not require timing.
35、In order to enable timing distribution in packet-based networks, ITU-T has developed specification for synchronous Ethernet ITU-T G.8261, b-ITU-T G.8262, ITU-T G.8264 for the physical layer frequency distribution, which is similar to what was provided by SDH. This Recommendation describes the use of
36、 packet-based mechanisms that are intended to be used to transport frequency over a packet network in the absence of physical layer timing. 6.1 Requirements for packet timing Packet-based mechanisms for frequency distribution must meet the following requirements: 1) Mechanisms must be specified to a
37、llow interoperability between master and slave devices (clocks). 2) Mechanisms must permit consistent operation over managed wide area telecom networks. 3) Packet-based mechanisms must allow interoperation with existing SDH and synchronous Ethernet-based frequency synchronization networks. 4) Packet
38、-based mechanisms must allow the synchronization network to be designed and configured in a fixed arrangement. 5) Protection schemes used by packet-based systems must be based on standard telecom operational practice and allow slave clocks the ability to take timing from multiple geographically sepa
39、rate master clocks. 6) Source (clock) selection should be consistent with existing practices for physical layer synchronization and permit source selection based on received QL and priority. 7) Packet-based mechanisms must permit the operation of existing, standards-based, security techniques to hel
40、p ensure the integrity of the synchronization. 7 Architecture of packet-based frequency distribution In contrast to physical layer synchronization, where the significant edges of a data signal define the timing content of the signal, packet-based methods rely on the transmission of dedicated “event
41、packets“. These “event packets“ form the significant instants of a packet timing signal. The timing of these significant instances is precisely measured relative to a master time source, and this timing information is encoded in the form of a time-stamp which is a machine-readable representation of
42、a specific instance of time1. The time-stamp is generated at a packet master function and is carried over a packet network to a packet slave clock. As time is the integral of frequency, the time-stamps can therefore be used to derive frequency. 7.1 Packet-based frequency distribution The three main
43、components are the packet master clock, the packet slave clock and the packet network. A packet timing signal generated by the packet master clock is transported over the packet network so that the packet slave clock can generate a clock frequency traceable to the input timing _ 1In some cases frequ
44、ency may be derived from the arrival rate of incoming packets that do not contain a time-stamp, but that rather are generated at precise intervals. As this Recommendation deals with the use of time-based protocols, methods to derive frequency from the arrival rate of packets are outside the scope of
45、 this Recommendation. 4 Rec. ITU-T G.8265/Y.1365 (10/2010) signal available at the packet master clock. The packet master clock is presented with a timing signal traceable to a PRC. The clock produced at the packet slave clock represents the clock traceable to the PRC plus some degradation () due to
46、 the packet network. The general architectural topology is shown in Figure 1. The synchronization flow is from master to slave. In cases where the reference to the master is provided over a synchronization distribution network, additional degradation of the frequency signal may be present at the inp
47、ut to the master and therefore also at the output of the slave. F +out 1iPacket networkNetworkFa) The reference may be from a PRC directly, from a GNSS or via a synchronization networkPacket timing signalsPacketslave clockPacketmaster clockReferencea)F +out 3F +out 2Packetslave clockPacketslave cloc
48、kG.8265/Y.1365(10)_F01Figure 1 General packet network timing architecture 7.2 Timing protection 7.2.1 Packet master protection In traditional synchronization networks, timing availability is enhanced by the use of timing protection whereby the timing to a slave clock (e.g., SEC, or EEC) may be provi
49、ded over one or more alternative network paths. In the case of the packet-based timing architecture, slave clocks may have visibility to two or more master clocks, as shown in Figure 2. In contrast to physical layer timing, where the selection of the clock is performed at the slave clock, selection of a secondary master clock may involve some communication and negotiation between the master and the slave, and the secondary master and slave. Rec. ITU-T G.8265/Y.1365 (10/2010) 5 G.8265/Y.1365(10)_
