Synchronization-SSM protocol

Reference source selection falls into two cases, namely, under SSM control and without SSM control. Under SSM control, first the reference source with higher SSM level is preferred. If with the same SSM level, the reference source with higher priority is preferred. If available reference source with the highest level is missing, the CLK board will auto switch to the reference source with secondary highest level. When the original reference source recovers, the CLK board can auto switch back to it. Without SSM control, first the reference source with higher priority is preferred. If available reference source with the highest level is missing, the CLK board will auto switch to the reference source with second highest level. When the original reference source recovers, the CLK board can auto switch back to it. To select the clock source correctly, devices on the network also transmits the Synchronization Status Message (SSM) with the clock information. Synchronization Status Message (SSM) should be sent together with reference clock to enable the downstream devices to use this message to select the appropriate clock source. Two synchronization trails each traceable to a different source shall be available at each network node. In case of active source or link failure the device shall switch to backup trail immediately. On the SDH network, the quality level of the clock is transmitted through out-band overhead byte in the SDH frame. The SSM is used to transmit the quality level of timing signals on the synchronization timing link. A node on the Synchronous Digital Hierarchy (SDH) network can obtain the clock information of the upstream device by parsing the SSM message. The node performs operations such as tracing, switching, or holding the local clock source according to the SSM message, and then forwards the SSM message to the downstream device. The SSM message contains a 4-bit code, which can express 16 types of signals to indicate different quality levels. Table 3-1 lists the quality levels in the SSM message. So, after setting the clock source and specifying the clock priority level for the NEs, the clock synchronization network should enable the standard SSM protocol or extended SSM protocol to prevent the NEs from tracing an incorrect clock source. The standard SSM protocol is a mechanism for network synchronization management. It uses bit 5~8 of the SSM byte to exchange clock source quality information between nodes to ensure that the device automatically selects the clock source with the highest quality and highest priority to prevent clock interlocking. The extended SSM protocol is a concept that Huawei adds the clock ID based on the standard SSM protocol. It uses the 1~4 bit of the SSM byte as the unique ID of the clock source and transmits the ID with the SSM. After receiving the SSM byte, the node checks the clock ID in bit 1~4 to determine whether the clock is sent by the local station. If yes, it is considered that the source is unavailable, thereby avoiding the generation of a timing loop. The extended SSM protocol is used for the interconnection between Huawei transmission equipment.