EXFO FTB/IQS-85100G Packet Blazer for FTB-500/IQS-600 - Manual (Page 397)

Glossary
Packet Blazer 387
Generic Framing Procedure (GFP)

Core Header
The GFP core header consists of a two-octet length field, specifying the
length of the GFP frame's payload area in octets, and a two-octet field
containing a CRC-16 error-check code.

Payload Length Indicator (PLI): The PLI is a two-byte field
indicating the size in bytes of the GFP payload area. It indicates the
beginning of the next GFP frame in the incoming bit-stream as an
offset from the last byte in the current GFP core header. PLI values
in the range of 0 to 3 are reserved for GFP internal use and are
referred to as GFP control frames. All other frames are referred to
as GFP client frames.

Core HEC (cHEC): The cHEC is a two-byte field containing a cyclic
redundancy check (CRC-16) sequence that protects the integrity of
the core header. The cHEC sequence is computed over the core
header bytes using standard CRC-16. The CRC-16 enables both
single-bit error correction and multibit error detection.

Payload Header
The payload header is a variable-length area, 4 to 64 octets long,
intended to support data-link management procedures specific to the
transported client signal. The payload header contains two mandatory
fields, the Type field and Type Header Error Correction (tHEC) field.
The payload header also supports an additional variable number of
subfields referred to, as a group, as the extension header.

Payload Type Identifier (PTI): A three-bit subfield that identifies
the type of GFP client frame. The following table lists the currently
defined user frames.
PTI Description
000 Client Data Frame
100 Client Management Frame
Others Reserved