EXFO FTB-8500 Series for FTB-500 - Manual (Page 378)

Common Tab
364 FTB-8500 Series and FTB-8120NGE/8130NGE
Performance Monitoring (PM)

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

SESR (Severely Errored Second Ratio): Gives the ratio of the number of
SES in available time (AS) during a fixed measurement interval.
SESR = SES ÷ AS

BBER (Background Block Error Ratio) (G.826 only): Gives the ratio of
BBE in available time (AS) to total frames (fps - Frames per second) in
available time during a fixed measurement interval. The count of total
frames excludes all frames during SESs.
BBER = BBE ÷ ((AS - SES) * fps)