EXFO FTB/IQS-85100G Packet Blazer for FTB-500/IQS-600 - Manual (Page 401)

Glossary
Packet Blazer 391
Generic Framing Procedure (GFP)

Payload Information Field
The payload area (also referred to as payload information field)
contains the framed client signal. This variable-length field may include
from 0 to 65,535 – X octets, where X is the size of the payload header
(including the extension header, if present) and the payload FCS field
(if present).
Figure GFP-T vs. GFP-F Frames on page 384 shows the GFP-T and
GFP-F frame structures. As shown, Both GFP-T and GFP-F frame types
share a common core header, payload header, and payload FCS
(optional), and they differ in the way in which the client is mapped into
this payload area.

Payload FCS (pFCS)
This is an optional four-octet-long frame-check sequence. It
contains a CRC-32 check sequence that is designed to validate the
entire content of the payload area. The FCS field presence is
signalled by the PFI bit located in the Type field of the payload
header. The FCS does not correct any errors; it just indicates the
presence of error(s).
In GFP-F, the transmitter encapsulates one entire frame of the
client data into one GFP frame. In this case, the basic frame
structure of a GFP client frame is used, including the required
payload header.
In GFP-T, however, rather than buffering an entire client-data
frame, the individual characters of the client signal are demapped
from the client block codes and then mapped into periodic
fixed-length GFP frames. The transparent GFP client frame uses
the same structure as the frame-mapped GFP, including the
required payload header.