EXFO FTB-800 Series for FTB-1 - Manual (Page 283)

Test Results
NetBlazer 273
Alarms/Errors

FEC-UNCORR (FEC - Uncorrectable)
RX: Statistics on the detected codewords (CW) having uncorrectable
errors.
TX: FEC-UNCORR-CW (Forward Error Correction - Uncorrectable -
Codeword) generates 16 symbol (bytes) containing 8 bits in error each,
in each codeword.

FEC-STRESS (Forward Error Correction - Stress)
TX: Generates correctable errors composed of a random number of
symbol errors (less or equal to 8) containing a random number of bits
distributed all over the OTU frame.

MFAS (Multiframe Alignment Signal)
RX: The MFAS bits are in error.