Serial Interface Unit (SIU) phase errors are logged when the SIU is unable to properly receive an incoming cell. In particular, the SIU timed out while looking for the frame alignment pattern (FAP) in the serial data and gave up until the next frame cycle.
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An SIU always sends and receives, and sends Null BFrames when it has no other data available. When an SIU phase error occurs, either a Null BFrame or a Data Frame might have dropped, so traffic might be disrupted.
They are errors on the point-to-point data path, so they fit into the general category of debug.