EXFO FTB-800 Series for FTB-1 - Manual (Page 415)

Test Functions
NetBlazer 405
FEAC

Loopback Commands

Current displays the valid message received in the last second of
measurement. A valid message is detected only when receiving 10
consecutive occurrences of a specific Loopback Command
immediately followed by 10 occurrences of a specific Channel
Codeword.

Previous displays the last valid message received excluding the
actual Current message.