EXFO FTB-800 Series for FTB-1 - Manual (Page 582)

Glossary
572 FTB-700G/800 Series
SyncE
SyncE
Synchronous Ethernet or SyncE enables a traceable network timing
reference frequency to be distributed node-by-node across a chain of
Ethernet devices (SyncE compliant devices only). The network timing
reference is typically hierarchically traceable to a Primary Reference
Source/Clock (PRS/PRC) in a unidirectional flow. Any node in the chain not
compliant to SyncE automatically terminates the synchronization
propagation but this does not impact the flow of traffic.
The Physical Layer carries the clock frequency and the Data Link layer
carries the Quality Level (QL) associated to the transported frequency via
Synchronization Status Messaging (SSM) embedded in the Ethernet
Synchronization Message Channel (ESMC). The QL is transported via
Information and Event PDUs typically transmitted at a rate of 1 frame per
second but this rate can vary between 1 to 10 messages per second to
respect the slow protocol rules. The Information PDU is used as heart-beat
for the channel while one Event PDU is transmitted asynchronously to the
Information PDU upon change of the QL value. Following this change, the
Information PDU is also adjusted to match the new QL.