EXFO FTB-800 Series for FTB-1 - Manual (Page 444)

Test Functions
434 FTB-700G/800 Series
OH - SONET/SDH

J2 Trace. Available when the Trace format is set to 1 Byte (Refer to
Traces (SONET/SDH) on page 239).
SONET: VT Path Trace: The J2 Trace byte provides a 16 or 64 bytes fixed
string allowing the receiving VT PTE to verify its continued connection
to the intended transmitting VT PTE.
SDH: Path Trace: The J2 byte is used to repetitively transmit a
Lower-Order Access Path Identifier so that a path receiving terminal
can verify its continued connection to the intended transmitter.

Z6/N2
Z6 (SONET): VT Tandem Connection Monitoring or VT Path Growth.
The Z6 byte is allocated for future growth.
N2 (SDH): (Network operator byte) Tandem Connection Monitoring for
the VC2, VC-12, and VC-11 level.