EXFO FTB-800 Series for FTB-1 - Manual (Page 261)

Test Results
NetBlazer 251
Alarms/Errors
DS1
Alarms

AIS (Alarm Indication Signal)
RX: An unframed all-ones signal is received.

OOF (Out-OF-Frame)
RX: Four consecutive frame bit errors are detected.

RAI (Yellow) (Remote Alarm Indication)
RX:
SF framing: Bit 2 in each timeslot contains “0”.
ESF framing: Eight “ones” followed by eight “zeros” pattern is
received continuously in the data link (FDL).
Errors

Framing Bit
RX: An incorrect value appeared in a bit position reserved for framing.

CRC-6 (Cyclical Redundancy Check) is only available with ESF
framing.
RX: One or more bit errors have been detected in a block of data
through cyclical redundancy check.