EXFO FTB-8510B Guia Del Usuario página 1276

Glossary
SONET - Line Overhead (LOH)
Z2: Growth/FEBE (Far-End Block Error)
The Z2 byte is allocated for future growth. This byte is located in the first
and second STS-1s of an STS-3, and the first, second, and fourth through n
of a STS-n signal (12 ≤ n ≤ 48).
E2: Orderwire
The Orderwire provides a 64 Kbps voice channel for communication
between LTEs. This byte is only defined for the first STS-1 frame of a
composite signal.
626
FTB-8100 Series Transport Blazer