IEEE 802.1AS：时间同步技术在局域网中的应用

802.1as协议

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"802.1AS协议是2011年发布的一项标准，由IEEE LAN/MAN Standards Committee（局域网/城域网标准委员会）赞助，旨在为时间敏感应用提供桥接局域网络中的定时和同步。该标准详细规定了如何在局域网和城域网中实现精确的时间同步，对于需要严格时间一致性的应用，如音频、视频流媒体、工业自动化和物联网(IoT)等，具有重要意义。" 802.1AS协议是IEEE 802.1时间敏感网络(TSN)系列标准的一部分，主要关注时间同步。在2011版中，它引入了一种机制，允许网络设备在分布式环境中保持纳秒级的精度，这对于实时通信和时间关键型应用至关重要。802.1AS解决了传统网络中时间同步的挑战，比如基于IP的网络无法保证时钟同步的准确性，因为它们依赖于相对不可靠的网络传输。该协议的核心功能包括： 1. **主时钟选举**：网络中的设备通过协议自动选举一个全局同步源，称为Grandmaster，它提供精确的时间参考。所有其他设备都会与Grandmaster同步，确保整个网络的时钟一致性。 2. **时间戳传递**：802.1AS使用时间戳来跟踪数据包的传输时间，每个节点在接收和转发数据包时都会附加时间戳，这样可以精确地计算出数据包在网络中的传输延迟。 3. **GPTP（通用精确时间协议）**：802.1AS引入了GPTP，它是PTP（Precision Time Protocol）的增强版本，用于在以太网网络中提供高精度时间同步。 4. **多路径和优先级**：协议支持多路径传输和优先级控制，确保时间敏感的数据包能在网络拥塞时优先传输，以满足严格的延迟要求。 5. **同步和校准**：802.1AS定义了周期性的同步和校准过程，以补偿网络中的时钟漂移和传输延迟，从而保持长期的精度。 6. **网络层次化**：支持网络的多层次结构，允许不同的子网或区域拥有自己的Grandmaster，同时在整个网络中保持统一的时间框架。 802.1AS的应用场景广泛，不仅限于媒体流和娱乐产业，还涵盖工业自动化、电力系统、智能交通系统和航空航天等领域。通过提供高精度的时间同步，802.1AS为网络中的各种时间敏感应用提供了坚实的基础，确保了可靠性和效率。

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Figure E.4—Propagation delay and residence time over a CSN Backbone................................................ 262

Figure E.5—CSN node-to-node path delay measurement........................................................................... 263

Figure E.6—IEEE 802.1AS Sync Message Propagation over the CSN backbone ..................................... 265

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List of tables

Table 6-1—Primitive data types .................................................................................................................... 15

Table 8-1—Illustration of formation of clockIdentity from EUI-48 ............................................................. 31

Table 8-2—Default values for priority1, for the respective media................................................................33

Table 8-3—timeSource enumeration............................................................................................................. 36

Table 10-1—Port role definitions .................................................................................................................. 68

Table 10-2—Destination address for Announce and Signaling messages .................................................... 86

Table 10-3—Ethertype for Announce and Signaling messages .................................................................... 86

Table 10-4—PTP message header................................................................................................................. 88

Table 10-5—Values for messageType field .................................................................................................. 88

Table 10-6—Values of flag bits..................................................................................................................... 89

Table 10-7—Announce message fields ......................................................................................................... 90

Table 10-8—Path trace TLV ......................................................................................................................... 91

Table 10-9—Signaling message fields .......................................................................................................... 92

Table 10-10—Message interval request TLV ............................................................................................... 93

Table 10-11—Interpretation of special values of linkDelayInterval ............................................................. 94

Table 10-12—Interpretation of special values of timeSyncInterval.............................................................. 94

Table 10-14—Definitions of bits of flags field of message interval request TLV ........................................ 95

Table 10-13—Interpretation of special values of announceInterval.............................................................. 95

Table 11-1—Destination address for Sync, Follow_Up, Pdelay_Req, Pdelay_Resp,

and Pdelay_Resp_Follow_Up messages ..............................................................................................123

Table 11-2—Ethertype for Sync, Follow_Up, Pdelay_Req, Pdelay_Resp, and

Pdelay_Resp_Follow_Up messages ..................................................................................................... 123

Table 11-4—Values of flag bits................................................................................................................... 125

Table 11-3—Values for messageType field ................................................................................................ 125

Table 11-6—References for sequenceId value exceptions.......................................................................... 126

Table 11-7—Value of control field ............................................................................................................. 126

Table 11-5—Value of correction field......................................................................................................... 126

Table 11-8—Sync message fields................................................................................................................ 127

Table 11-9—Follow_Up message fields ..................................................................................................... 127

Table 11-10—Follow_Up information TLV ............................................................................................... 128

Table 11-11—Pdelay_Req message fields .................................................................................................. 129

Table 11-12—Pdelay_Resp message fields................................................................................................. 130

Table 11-13—Pdelay_Resp_Follow_Up message fields ............................................................................ 130

Table 12-1—Parameters of MLME-TIMINGMSMT.request..................................................................... 139

Table 12-2—Parameters of MLME-TIMINGMSMST.confirm ................................................................ 140

Table 12-3—Parameters of MLME-TIMINGMSMT.indication ................................................................ 143

Table 12-4—Values of the Type field in the VendorSpecific information element.................................... 144

Table 13-1—TIMESYNC message fields ................................................................................................... 150

Table 14-1—Default Parameter Data Set Table .......................................................................................... 162

Table 14-2—Current Parameter Data Set Table.......................................................................................... 163

Table 14-3—Parent Parameter Data Set Table............................................................................................ 165

Table 14-4—Time Properties Parameter Data Set Table ............................................................................ 166

Table 14-5—portRole enumeration ............................................................................................................. 167

Table 14-6—Port Parameter Data Set Table ............................................................................................... 170

Table 14-7—Port Parameter Statistics Table.....................................................................................

.......... 173

Table 14-8—Acceptable Mas

er Table Parameter Data Set Table.............................................................. 174

Table 15-1—IEEE8021-AS MIB structure and object cross reference....................................................... 177

Table B.1—Wander generation TDEV requirement for LocalClock entity................................................ 246

Table B.2—ADEV limit corresponding to wander generation requirement of Table B.1.......................... 247

Table B.3—PTPDEV limit corresponding to wander generation requirement of Table B.1 ...................... 248

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IEEE Standard for

Local and metropolitan area networks—

Timing and Synchronization for

Time-Sensitive Applications in

Bridged Local Area Networks

IMPORTANT NOTICE: This standard is not intended to ensure safety, security, health, or

environmental protection. Implementers of the standard are responsible for determining appropriate

safety, security, environmental, and health practices or regulatory requirements.

This IEEE document is made available for use subject to important notices and legal disclaimers. These

notices and disclaimers appear in all publications containing this document and may be found under the

heading “Important Notice” or “Important Notices and Disclaimers Concerning IEEE Documents.”

They can also be obtained on request from IEEE or viewed at http://standards.ieee.org/IPR/

disclaimers.html.

1. Overview

1.1 Scope

This standard specifies the protocol and procedures used to ensure that the synchronization requirements are

met for time-sensitive applications, such as audio and video, across bridged and virtual bridged local area

networks consisting of local area network (LAN) media where the transmission delays are fixed and

symmetrical; for example, IEEE 802.3

full-duplex links. This includes the maintenance of synchronized

time during normal operation and following addition, removal, or failure of network components and

network reconfiguration. It specifies the use of IEEE 1588

specifications where applicable in the context

of IEEE Std 802.1D

-2004 and IEEE Std 802.1Q

-2005.

Synchronization to an externally provided

timing signal (e.g., a recognized timing standard such as UTC or TAI) is not part of this standard but is not

precluded.

1.2 Purpose

This standard enables stations attached to bridged LANs to meet the respective jitter, wander, and time

synchronization requirements for time-sensitive applications. This includes applications that involve

Information on references can be found in Clause 2.

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