同步以太网的SSM-USB迷|专注于互联网分享

2024年3月9日发(作者：妫晗日)

同步以太网的同步状态信息（参见G.8164）

11 SSM for synchronous Ethernet

11.1 Packet-level SSM

For existing SDH-based SSM, the SSM message is carried in fixed locations within the SDH frame. In the case of

Ethernet, there is no equivalent of a fixed frame. Overhead for various functions, e.g., pause, OAM, etc., is carried

via protocols running over the PHY layer. As such, SSM must be carried over a protocol.

11、同步以太网的同步状态信息（SSM，译者注）

11.1分组级的SSM

对于现存的、基于SDH的SSM，SSM是在SDH帧内固定位置传送的。在以太网情况下，不存在等效的固定帧。各种功能的开销（例如中断、OAM等）是借助运行在物理（PHY，译者注）层上的协议传送的。因此，SSM必须在协议上传送。

Logically, the SDH SSM overhead can be viewed as a dedicated unidirectional communication channel between

entities that process SSM messages. Figure 11-1 shows a simplified example of two network elements connected

to one another. Each is also connected to an SSU. Selectors are provided within each network element to provide

the source selection for the system clock. Selectors are under the control of a block called "sync control". This

block would also be responsible for controlling timing protection. Not shown in the figure is an interface to the

management system.

在逻辑上，SDH 的SSM可以看作为一个专用的、处理SSM实体之间的单向通信通路。图11-1给出了一个两个网元互连的简化例子。每个网元也连接到一个同步供给单元（SSU，译者注）。为了提供系统时钟的来源选择，在每个网元内都提供了选择器。选择器受到一个称为“同步控制”功能（“功能”二字为译者所加）块的控制。本功能（“功能”二字为译者所加）块也负责控制定时保护。没有在图中给出与管理系统的接口。

The sync block may be implemented as software running on a network element and may take as input the quality

level SSM on the various inputs (e.g., the external inputs or the line inputs). The sync control block may also be

responsible for generating an SSM message on the appropriate outputs to indicate certain conditions (for example,

insertion of DNU on some ports –see [ITU-T G.781]).

The SSM represents an indication of the quality level of the transmitting clock, and hence represents a

unidirectional channel between the sync control block in the transmit NE and that of the receive NE.

同步功能（“功能”二字为译者所加）块可以通过运行在网元中的软件来实现，并且可以提取输入到各种输入（例如外部输入或线路输入）处的质量等级SSM。为了表明临界条件，同步功能（“功能”二字为译者所加）块也可以负责在相应输出端口上产生SSM（例如在某些端口上产生同步不可用DNU——参见G.781）。

SSM代表了发送时钟质量等级的指示。所以，代表了发送网元同步控制功能（“功能”二字为译者所加）块与接收网元同步控制功能（“功能”二字为译者所加）块之间的一个单向通路。

11.2 Sync selection based on SSM

SSM messages represent the quality level of the system clocks located in the various network elements. Quality

level refers to the holdover performance of a clock. The two clocks defined for synchronous Ethernet equipment

in [ITU-T G.8262] have different characteristics, and slightly different holdover performance.

NOTE – For the purposes of SSM selection, the ITU-T G.8262 EEC option 1 clock is treated as a ITU-T G.813

option 1, while the EEC option 2 is treated as an ITU-T G.812 type IV clock (i.e., QL-SEC and QL-ST3,

respectively). The SSM messages are provided in Table 11-1.

Synchronization selection is detailed in Annex A.

11.2基于SSM的同步选择

SSM代表了位于各种网元内部的系统时钟质量等级。质量等级与时钟的延续性能有关。在ITU-T

G.8262中为同步以太网设备定义的两种时钟具有不同的特性，而且具有些许不同的延续性能。

注——为了SSM选择的目的，ITU-T G.8262 EEC选项1时钟（EEC1，译者注）被看作为ITU-T G.813选项1，而EEC选项2被看作为ITU-T G.812 IV型时钟，即分别为QL-SEC和QL-ST3。在表11-1中给出了SSM。

表11-1 同步以太网的SSM

时钟

EEC1

EEC2

同步选择的细节参见附录A。

11.3 SSM for synchronous Ethernet: Format and protocol

As noted above, clock quality level indication is carried via a protocol running over the synchronous Ethernet link.

信息

QL-EEC1

QL-EEC2

SSM编码

0xB

0xA

Synchronization status messages for Ethernet implement the SSM channel using an IEEE 802.3 organizational

specific slow protocol (OSSP). Network level SSM is defined in [ITU-T G.781]. Message processing times

contained within [ITU-T G.781] are based on network reconfiguration objectives, which are defined based on the

performance characteristics of the system clocks (SEC for the case of SDH, EEC for the case of synchronous

Ethernet). In order to meet the performance requirements for reference switching in [ITU-T G.781], two types of

protocol message types are defined. In general terms, a background or "heart-beat" message is used to provide a

continuous indication of the clock quality level. A message period of one second meets the message rate

requirements of IEEE 802.3 slow protocols. To minimize the effects of wander that may occur during holdover, an

event type message with a new SSM quality level is generated immediately, subject to the clock processing

requirements in [ITU-T G.781]. To protect against possible failure, the lack of the messages is considered to be a

failure condition. The protocol behaviour is such that the SSM value is set to DNU if no SSM messages are

received after a five second period. Details are contained within the following subclauses.

11.3同步以太网的SSM：格式和协议

正如上面所表明的，时钟质量等级指示通过运行在同步以太网链路上的协议传送。以太网的SSM利用IEEE 802.3规范专用慢协议（OSSP）实现SSM通路。网络等级SSM在ITU-T G.781中定义。在ITU-T

G.781内获得的处理时间信息基于网络重构指标，它的定义是基于系统时钟的性能特性（SDH情况下为SEC，同步以太网情况下为EEC）。为了满足ITU-T G.781中参考倒换的性能要求，定义了两种类型的协议信息类型。通常，背景或“心律节拍”信息用于提供一个持续的时钟质量等级指示。每秒的信息周期满足IEEE 802.3慢协议的信息速率要求。为了使可能发生在延续期间的漂移缺陷达到最小，立即产生一个具有新SSM质量等级的事件类型信息，以满足于ITU-T G.781中的时钟处理要求。为了防止可能的失效，信息缺失被认为是一种失效条件。如果在5秒周期后SSM没有被接收到，协议就将SSM值设定为不可用DNU。详细的情况可以在下面的子条中获得。

11.3.1 ESMC format

The Ethernet SSM is an ITU-T defined Ethernet slow protocol. The IEEE has provided ITU-T with an

organizationally unique identifier (OUI) and a slow protocol subtype. These are used to distinguish the Ethernet

SSM PDU. The values assigned by the IEEE are indicated in Table 11-2.

The SSM quality level is carried in a type length value (TLV) field, which is contained within the ESMC PDU.

Two types of ESMC PDU frames are defined and are distinguished by the C flag. These are the ESMC

information PDU and the ESMC event PDU.

11.3.1 ESMC格式

以太网SSM是一个ITU-T定义的以太网慢协议。IEEE已经将一个规范一致识别符（OUI）和一个慢协议子类型提供给ITU-T。这些被用于以太网SSM协议数据单元（PDU）。IEEE安排的这些值示于表11-2中。

表11-2 IEEE安排的OUI和慢协议子类型

规范专用识别符

慢协议子类型

00-19-A-7

0x0A

SSM质量等级在类型长度值（TLV）域中传送，它被放置于ESMC PDU中。定义了两种类型ESMC PDU帧，并且通过C标识别。这些就是ESMC信息PDU和ESMC事件PDU。

11.3.1.1 ESMC PDU format

The ESMC PDU format is shown in Table 11-3. The QL TLV is shown in Table 11-4.

Table 11-3 indicates the bit and byte locations for the PDU format. The order of transmission is as follows: Octet

number 1 in the table is transmitted first. For each octet, the first bit transmitted is the least significant bit (LSB).

ESMC PDUs have the following fields in the order specified above:

a) Destination address (DA): This is the IEEE-defined slow protocol multicast address. The format is defined in

Annex 43B of [IEEE 802.3].

b) Source address (SA): The source address is the MAC address associated with the port through which the

ESMC PDU is transmitted.

c) Slow protocol Ethertype: ESMC PDUs must be type encoded and carry the slow protocol type field value.

d) Slow protocol subtype: Assigned by the IEEE and fixed with a value of 0x0A.

e) ITU OUI: Organizational unique identifier assigned by the IEEE registration authority.

f) The ITU subtype is assigned by ITU-T. The value of 00-01 applies to all usage defined in this

Recommendation.

g) Version: The four-bit field indicates the version of ITU-T OSSP frame format. This field shall contain the

value 0x1 to claim compliance with version 1 of this protocol.

h) Event flag: This bit distinguishes the critical, time-sensitive behaviour of the ESMC event PDU from the

ESMC Information PDU. A value of 1 indicates an event PDU, a value of 0 indicates an information PDU.

NOTE 1 – The behaviour of the event PDU is similar to the critical event defined for Ethernet OAM in clause

57 of [IEEE 802.3]. Event messages need to meet processing times defined in [ITU-T G.781].

i) Reserved for future standardization (27 bits). These fields are set to all zero at the transmitter and are ignored

by the receiver.

j) Data and padding: This field contains data and necessary padding to achieve the minimum frame size of 64

bytes. The PDU must be an integral number of bytes (octets). Padding characters are not defined and are ignored

by receivers.

NOTE 2 – The recommended maximum size for the ESMC PDU is 128 bytes as per Annex 43B of [IEEE

802.3]. However, PDU sizes greater than 128 bytes may be permitted.

k) FCS: Four-byte frame check sequence as defined in clause 4 of [IEEE 802.3].

11.3.1.1 ESMC PDU格式

ESMC PDU格式参见表11-3。QL TLV参见表11-4。

表11-3表明PDU格式所使用的比特和字节位置。发送顺序如下：首先发送表中8比特数组1（字节1，译者注）。对于每个字节，发送的第1比特是最小有效位（LSB）。

表11-3 ESMC PDU格式

字节编号

1-6

7-12

13-14

16-18

19-20

22-24

25-1514

最后4字节

在上面规范的顺序中，ESMC PDU具有如下域：

a）目的地址（DA）：这是IEEE定义的慢协议多播协议。格式在IEEE 802.3附录43B中定义。

b）源地址（SA）：源地址是与通过发送ESMC PDU端口相关的 MAC地址。

c）慢协议以太网类型：ESMC PDU必须是编码类型，并且传输慢协议类型域值。

d）慢协议子类型：IEEE 规范的，并且具有固定值0x0A 。

e）ITU OUI：由IEEE注册机构规范的组织一致识别符。

f) ITU子类型由ITU-T规范。00-01值应用于本协议定义的所有用途。

g) 版本：4比特域表明ITU-T OSSP帧格式的版本。本域应该保持值0x1与本协议版本1相兼容。

h) 事件标记：该比特将ESMC事件PDU准则、时间灵敏性能与ESMC信息PDU区别。值1表示一个事件PDU，值0表示一个信息PDU。

注1——事件PDU的性能类似于在IEEE 802.3中57条为以太网OAM定义的准则事件。事件信息需要满足在ITU-T G.781中定义的处理事件。

i）为将来标准预留（27个比特）。这些域在发送机被全部置零，而在接收机中被忽略。

j）数据和填充：为了实现64字节的最小帧尺寸，该域包含数据包含了数据和必要的填充。PDU必须是整数个字节（8比特）。填充特性没有定义，并且在中予以忽略。

注2——按照IEEE 802.3附录中43B，建议的ESMC PDU 最大尺寸是128字节。然而，可能允许PDU尺寸大于128字节。

尺寸（长度，译者注）

6字节

2字节

1字节

3字节

2字节

4比特

1比特

3比特

3字节

36-1490字节

4字节

源地址

域

目的地址=01-80-C2-00-00-02

慢协议以太网类型=88-90

慢协议子类型=0x0A

ITU-OUI=00-19-A7

ITU-T子类型

版本

事件标记

预留

数据和填充（参见点j）

帧校验序列

k）4字节帧校验序列（FCS，译者注）：FCS在IEEE 802.3第4条定义。

11.3.1.2 QL TLV format

The format of the QL data is indicated in Table 11-4 below and is the format used for both information and event

messages. The length field encompasses the entire TLV, including the type and length fields. This follows the type

length value convention described in clause 57.5.2.1 of [IEEE 802.3].

To allow for potential hardware implementations, the SSM TLV is always sent as the first TLV in the data/padding

field. This means that the QL indication always remains fixed in the PDU. Any padding must occur after the SSM

TLV.

11.3.1.2 QL TLV格式

QL数据的格式在表11-4中给出，并且是应用于数据和事件信息的格式。长度域封装了整个TLV，包括类型和长度域。这符合在IEEE 802.3中57.5.2.1条所描述的类型和长度值集合。

为了允许可能的硬件实现，SSM TLV总是作为数据/填充域中第1个TLV被传送。这意味着，QL指示总是固定保留在PDU中。任何的填充都必须在SSM TLV之后发生。

表11-4 QL TLV格式

8比特

16比特

4比特

类型：0x01

长度：00-04

0x0（未使用）

SSM编码

11.3.2 Protocol behaviour

The ESMC PDU contains the QL TLV for synchronous Ethernet. Synchronization source selection using SSM is

defined in [ITU-T G.781]. [ITU-T G.781] is applicable to both SDH and synchronous Ethernet. The protocols for

carrying SSM differ for both SDH and synchronous Ethernet. Pre-processing in the appropriate atomic functions

provides a uniform interface to the synchronization processing algorithm. The protocol described within this

clause adheres to the requirements for slow protocols given in Annex 43B of [IEEE 802.3].

The SSM code contained in the QL TLV represents the free-run accuracy of the clock that is currently the clock

source of the synchronization trail. Specific bounds on the processing times of messages are defined in [ITU-T

G.781].

NOTE – The processing times are specified for ideal conditions. There may be cases where these processing times

may not be met.

When a network element is operating in the QL-enabled mode, the protocol generation and reception must meet

the criteria below in clauses 11.3.2.1 and 11.3.2.2, respectively. The synchronization selection function in Annex A

of [ITU-T G.781] has been modelled using SDL descriptors. The critical aspects of the algorithm are based on the

input quality level of each of p inputs (i.e., QL[p] in [ITU-T G.781]). The output QL, QL_out, is the SSM code

that is to be transmitted on output ports of the NE. [ITU-T G.781] describes cases where DNU is applied instead

of the active quality level. Note that Annex A of [ITU-T G.781] is normative, it describes the NE behaviour. It

does not necessarily mandate a specific implementation.

Within this Recommendation, QL states are used to describe the protocol behaviour. This is the QL state that

would be present at the input to the synchronization selection algorithm at the SD_CI interface.

Generation is based on the QL_out state, while reception is based on the QL[p] state. It is assumed that these

states are maintained within appropriate atomic functions for synchronous Ethernet or SDH (e.g., the ETY/SD or

MS/SD functions, respectively).

11.3.2协议性能

ESMC PDU 包含同步以太网的QL TLV。使用SSM的同步源选择在ITU-T G.781中定义。ITU-T G.781适用于SDH和同步以太网。对于SDH和同步以太网，传输SSM的协议是由差异的。在相应原子功能中，预处理为同步处理机理提供了一个唯一的接口。在本条中描述的协议，遵循在IEEE 802.3中附录43B给出的慢协议要求。

包含在QL TLV中的SSM编码代表了当前同步踪迹时钟源的时钟自由震荡的准确度。在处理时间信息上特别绑定在ITU-T G.781中定义。

注——处理时间规范了详细的条件。可能会引起这些处理时间不能满足的地方。

当网元工作于QL使能方式时，协议产生和协议接收必须分别满足下述11.3.2.1和11.3.2.2条的准则。ITU-T G.781附录A中同步选择已经使用SDL描述符模拟过。机理的准则概念基于每个p输入的输入质量等级（即ITU-T G.781中的QL[p]）。输出QL、QL-out是在网元（NE，译者注）输出端口上发送的SSM编码。值得注意，ITU-T G.781附录A是基准的，它描述了NE的性能。它不需要指定一个特定的实现。

在本协议内，QL状态用于描述协议性能。这是在SD_CI接口、出现于输入到同步选择机制的QL状态。

产生是基于QL_out状态，而接收则基于QL[p]状态。假设：这些状态被保存在同步以太网或SDH的相应原子功能（即分别为ETY/SD或MS/SD功能）中。

11.3.2.1 QL generation

An ESMC information PDU, containing the current QL used by the system clock selection algorithm, is generated

once per second.

In the case of a change in the QL level, an ESMC event PDU (i.e., the event flag is set) containing the new QL

TLV is generated upon detection of the QL change, subject to message transition and processing times given in

[ITU-T G.781].

In no case can more than 10 ESMC PDUs (information and/or event) be generated in any one-second period as

per Annex 43B of [IEEE 802.3].

NOTE – The ESMC PDU is only one slow protocol PDU that may be present in a system. [IEEE 802.3] places

limits on the total number of slow protocols, each subject to a maximum transmission of 10 frames per second.

Implementations that implement multiple slow protocols should consider the time-sensitive nature of the event

message type.

11.3.2.1 QL产生

一个包含系统时钟选择机制使用的、当前QL的ESMC信息PDU在每秒钟产生一次。

在QL等级中变化的情况，一个包含新QL TLV的ESMC事件PDU（即事件标记被设置）是根据QL变化的检测而产生的，遵循在ITU-T G.781给出的信息跃迁和处理时间。

按照IEEE 802.3附录43B，在任何1秒周期内产生不大于10个ESMC PDU（信息和/或事件）。

注——ESMC PDU仅仅是一个可能出现在系统中的慢协议PDU。IEEE 802.3将限制放置于慢协议总数上，每个都遵循每秒10帧的最大传输。多重慢协议的实现应该考虑事件信息类型的时间敏感特性。

11.3.2.2 QL reception

The QL state, QL_out is utilized by synchronization selection algorithm described in [ITU-T G.781] (see Annex A

of [ITU-T G.781]). For synchronous Ethernet, the slow protocol used for the transmission of the SSM code relies

on the use of a "heart-beat" timer. ESMC information PDUs are sent periodically at a rate of one PDU per second.

Lack of reception of an ESMC information PDU within a five-second period results in the QL being set to DNU.

The default (initial) value for the QL is DNU and must only change when a valid QL TLV is received.

Upon reception of an event TLV, the QL state is changed to the new QL value, and the information timer is reset.

If the NE is operating in QL-enabled mode and no QL TLV is received within a five-second period, the QL state is

set to DNU. The synchronization reference is now subject to a wait-to-restore period as defined in [ITU-T G.781].

11.3.2.2 QL接收

通过ITU-T G.781（参见ITU-T G.781附录A）中描述的同步选择机制，使用QL状态、QL_out。对于同步以太网，用于SSM编码传输的慢协议依靠“节拍”定时器的使用。以每秒1个PDU的速率，周期性的发送ESMC信息PDU。在5秒内一个ESMC信息PDU接收的丢失引起QL被设置成DNU。

QL的缺省（固有）值是DNU，并且仅当接收到一个有效的QL时必须改变。

根据一个事件TLV的接收，QL状态被变成新QL值，而且信息定时器复位。

如果NE工作中QL使能方式和在5秒周期内没有接收到QL TLV，QL状态就被设置为DNU。同步参考遵循ITU-T G.781中定义的等待-恢复周期。

11.4 ESMC PDU extensions

Future extensions to the ESMC PDU are for further study. However, it is expected that future extensions will be

specified in terms of a TLV format. The TLV format is indicated in Table 11-5. The size of the TLV is calculated

as the total number of octets in the TLV structure. The length includes the type and length fields. Padding of data

is required to ensure that the TLV contains an integral number of octets.

NOTE – Two bytes are used to represent the length field. The suggested maximum size of a slow protocol PDU is

128 bytes as per Annex 43B of [IEEE 802.3]. However, slow protocol PDU lengths greater than 128 bytes are

permissible.

Details of usage are for further study.

11.4 ESMC PDU扩展

对ESMC PDU的未来扩展需要进一步研究。然而，预期未来扩展将会按照TLV格式规范。TLV格式在表11-5中给出。TLV的尺寸根据结构中字节总数计算得到。长度包括类型和长度域。为了保证TLV包含整数个字节，要求进行数据填充。

注——2字节用于表示长度域。按照IEEE 802.3附录43B，建议的、慢协议PDU的最大尺寸为128字节。然而，慢协议PDU长度大于128字节是被允许的。

表11-5 TLV结构

1字节

2字节

3字节

类型

长度（字节）

数据加填充

11.5 Interworking with existing synchronization networks

See clause A.5 of [ITU-T G.8261].

11.5 现存同步网络的互操作

参见ITU-T G.8261的A.5条。