EXFO FTB-8510B Guia Del Usuario página 1307

The figure below explains how (in GFP-F) the transmitter encapsulates
one entire frame of the client data.
SONET/SDH Application
Type HEC (tHEC) Field: A two-octet field that contains a CRC-16
sequence to protect the integrity of the type field. The tHEC
sequence is computed over the core header bytes using standard
CRC-16. As with the cHEC, CRC-16 enables both single-bit error
correction and multibit error detection.
Channel Identifier (CID): A one-byte field that is only available
when the EXI field is configured to Linear. The CID byte is used to
indicate one of 256 communication channels at a GFP termination
point.
Spare: A one-byte field that is only available when the EXI field is
configured to Linear. This field is reserved for future use.
Extension HEC (eHEC): A two-byte field that contains a CRC-16
check sequence that protects the integrity of the contents of the
extension. CRC-16 enables both single-bit correction and multibit
error detection.
GFP-F vs. GFP-T Frame Structure
Next-Gen - Generic Framing Procedure (GFP)
Glossary
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