同步以太网的SSM

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

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.