EXFO FTB-8500 Series for FTB-200 - Manual (Page 335)

Common Tab
Ethernet and Fibre Channel Application 321
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.

UAS (Unavailable Second): Gives the count of the seconds
corresponding to the periods of time within which SES errors occurred
for at least 10 consecutive seconds. The unavailable period ends at the
beginning of the next available period represented by ten consecutive
seconds within which no SES errors are detected.

ESR (Errored Second Ratio): Gives the ratio of the number of ES in
available time (AS) during a fixed measurement interval.
ESR = ES ÷ AS