GENERIC FRAMING PROTOCOL

The Generic Framing Protocol (GFP), defined in ITU-T G.7041, is a mechanism for mapping constant and variable bit rate data into the synchronous SDH/SONET envelopes. GFP support many types of protocols including those used in local area network (LAN) and storage area network (SAN). In any case GFP adds a very low overhead to increase the efficiency of the optical layer.

Currently, two modes of client signal adaptation are defined for GFP:

• Frame-Mapped GFP (GFP-F), a layer 2 encapsulation PDU-oriented adaptation mode. It is optimized for data packet protocols (e.g. Ethernet, PPP, DVB) that are encapsulated onto variable size frames.
• Transparent GFP (GFP-T), a layer 1 encapsulation or block-code oriented adaptation mode. It is optimized for protocols using 8B/10B physical layer (e.g. Fiber Channel, ESCON, 1000BASE-T) that are encapsulated onto constant size frames.

GFP could be seen as a method to deploy metropolitan networks, and simultaneously to support mainframes and server storage protocols.

  Data packet aggregation using GFP. Packets are in queues waiting to be mapped onto a TDM channel. At the far-end packets are drop again to a queue and delivered. GFP frame multiplexing and sub multiplexing. The figure shows the encapsulation mechanism and the transport of the GFP frames into VC containers embedded in the STM frames

Figure       GFP frame formats and protocols

Framed-mapped GFP

In Frame-mapped GFP (GFP-F) one complete client packet is entirely mapped into one GFP frame. Idle packets are not transmitted resulting in more efficient transport. However, specific mechanisms are required to transport each type of protocol .

Figure   GFP mapping clients format

GFP-F can be used for Ethernet, PPP/IP and HDLC-like protocols where efficiency and flexibility are important. To perform the encapsulation process it is necessary to receive the complete client packet, but this procedure increases the latency, making GFP-F inappropriate for time-sensitive protocols.

Transparent GFP,GFP-T

Transparent GFP (GFP-T) is a protocol-independent encapsulation method in which all client code words are decoded and mapped into fixed-length GFP frames The frames are transmitted immediately without waiting for the entire client data packet to be received. Therefore it is also a Layer 1 transport mechanism because all the client characters are moved to the far-end independently it does not matter if they are information, headers, control or any kind of overhead.