Generic Framing Procedure (GFP) is defined by ITU-T G.7041. This allows mapping of variable length, higher-layer client signals over a transport network like SDH/SONET. The client signals can be protocol data unit (PDU) oriented (like IP/PPP or Ethernet Media access control [MAC]) or can be block-code oriented (like fiber channel).
There are two modes of GFP, viz., GFP-F and GFP-T. GFP-F maps each client frame into a single GFP frame. GFP-T, on the other hand, allows mapping of multiple 8B/10B client data frames into an efficient 64B/65B block code for transport within a GFP frame.
GFP utilizes a length/HEC-based frame delineation mechanism that is more robust than that used by HDLC (High-level Data Link Control), which is single octet flag based.
There are two types of GFP frames: a GFP client frame and a GFP control frame.
A GFP client frame can be further classified as either a client data frame or a client management frame. The former is used to transport client data, while the latter is used to transport point-to-point management information like loss of signal, etc. Client management frames can be differentiated from the client data frames based on the payload type indicator. The GFP control frame consists only of a core header field with no payload area. This frame is used to compensate for the gaps between the client signal where the transport medium has a higher capacity than the client signal.