EXFO FTB-8510B Guia Del Usuario página 1279

Glossary
SONET - Path Overhead (POH)
G1: Status
The G1 byte provides a method to communicate the far-end path status
back to the path originating equipment.
F2: User Channel
The User Channel provides a 64 Kbps channel for communication between
two PTEs. This byte is only defined for the first STS-1 frame of a composite
signal.
H4: Multiframe Indicator
The H4 byte provides a multiframe phase indication of a VT payload to
identify phases of a SF as well as to convey the control packet information
in VCAT.
Z3 and Z4: Growth
The Z3 and Z4 bytes are allocated for future growth.
N1: Tandem Connection
The N1 byte (formerly referred to as the Z5 byte) is allocated for Tandem
Connection Maintenance and the Path Data Channel.
629
SONET/SDH Application