Synchronization Status Message
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.
The SSM is used to transmit the quality level of timing signals on the synchronization timing link. 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.
Bit 5-bit 8 of S1 byte
SDH Synchronization Quality Level
0
The synchronization quality is unknown (the existing synchronous network).
1
Retained
10
G.811 clock signal
11
Retained
100
SSU-ANote 1
101
Retained
110
Retained
111
Retained
1000
SSU-BNote 1
1001
Retained
1010
Retained
1011
Synchronous equipment timing source (SETS) signal
1100
Retained
1101
Retained
1110
Retained
1111
Not used for synchronization
Note 1: The "G.812 Transit Exchange" and "G.812 Local Clock" terms are used in the previous version of ITU-T Recommendations. In the new version of ITU-T G.812, the clock definition is changed to synchronization supply unit (SSU). The SSU is available in A and B types. The SSU-A corresponds to the "G.812 Transit Exchange" and the SSU-B corresponds to the "G.812 Local Clock" that is previously used.
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.
Standard SSM protocol
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.
Extended SSM protocol
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.
In this project, as the transmission network is complicated, and with large distance, there are six main rings, more than 70 sites, if we start the extended protocol, we have to assign the clock ID to NEs and divide the clock synchronization network into small subnets, it is troublesome to maintain and expansion in a complex network. In this project, the standard SSM protocol enabled.
