EXFO FTB-800 Series for FTB-1 - Manual (Page 436)

Test Functions
426 FTB-700G/800 Series
OH - SONET/SDH

M0 or M1/Z2 (SONET)
M0: REI-L: The M1 byte is used for line Remote Error Indication (REI-L)

For STS-1e and OC-1: The M0 byte located in the STS-1 indicates
BIP violations.

For OC-192: The M0 bytes located in the STS-1 #4 indicates BIP
violations when combined with the M1 byte (see M1 byte below for
more information).
M1: REI-L. The M1 byte is used for line Remote Error Indication (REI-L).

For STS-3e and OC-3: The M1 byte located in the STS-1 #3 indicates
BIP violations.
M0, bits
234 5678
Indicates
000 0000 0 BIP violation
000 0001 1 BIP violation
: :
000 1000 8 BIP violations
000 1001 to
1111 1111
0 BIP violation
M1, bits 234 5678 Indicates
000 0000 0 BIP violation
000 0001 1 BIP violation
: :
001 1000 24 BIP violations
001 1001 to
1111 1111
0 BIP violation