Generic Framing Procedure

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Generic Framing Procedure (GFP) is a multiplexing technique defined by ITU-T G.7041. This allows mapping of variable length, higher-layer client signals over a circuit switched transport network like OTN, SDH/SONET or PDH; the client signals can be protocol data unit (PDU) oriented (like IP/PPP or Ethernet Media Access Control) or can be block-code oriented (like Fibre Channel).

There are two modes of GFP: Generic Framing Procedure - Framed (GFP-F) and Generic Framing Procedure - Transparent (GFP-T):

  • GFP-F maps each client frame into a single GFP frame. GFP-F is used where the client signal is framed or packetized by the client protocol.
  • GFP-T, on the other hand, allows mapping of multiple 8B/10B block-coded client data streams 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 High-Level Data Link Control (HDLC), 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 currently 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, and is better known as an idle frame.

Frame format[edit]

A GFP frame consists of:

  • a core headers
  • a payload header
  • an optional extension header
  • a GFP payload
  • an optional payload frame check sequence (FCS).

Modes[edit]

  • Framed GFP (GFP-F) is optimized for bandwidth efficiency at the expense of latency. It encapsulates complete Ethernet (or other types of) frames with a GFP header.
  • Transparent GFP (GFP-T) is used for low latency transport of block-coded client signals such as Gigabit Ethernet, Fibre Channel, ESCON, FiCON, and Digital Video Broadcast (DVB). In this mode, small groups of 8B/10B symbols are transmitted rather than waiting for a complete frame of data.

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