EXFO FTB-8500 Series for FTB-500 - Manual (Page 377)

Common Tab
Ethernet and Fibre Channel Application 363
Performance Monitoring (PM)

SES (Severely Errored Second)
For G.821: Gives the number of seconds within which a bit error ratio
is  10
-3
, or during which Loss Of Signal (LOS), Link down, or Pattern
Loss is detected.
For G.826: Gives the number of seconds within which EB count is 
SES threshold (30%), or at least one NE defect occurred.

BBE (Background Block Error) (G.826 only): Gives the count of Errored
Block not occurring as part of a SES. A Block corresponds to a
complete Ethernet/Fibre Channel frame.

AS (Available Second): Gives the count of the seconds corresponding
to the periods of time within which no SES errors occurred for at least
10 consecutive seconds.