oad Type Identifier Identification PFI: Payload FCS Indicator FCS: Frame Check Sequence Fig. 11.18 Generic framing procedure frame structure. Generic Framing Procedure end of the GFP frame is. The next core header will occur immediately after the end of this current frame, SO if another valid 32-bit pattern is found in that location, the receiver can be sure that it has acquired framing for the GFP stream. The CRC is adequate to provide a simple, single error correction capability for the PLI, which increases the robustness of the GFP framing. In contrast, an error in an HDLC start/end flag has no error correction, which makes HDLC framing more vulnerable to transmission errors. This low-complexity frame delineation ensures that GFP scales to high bitrates such as 40 Gbit/s. In addition, because GFP supports variable-length PDUs, it does not need complex SAR functions or frame padding to fill unused payload space, resulting in a deterministic transport bandwidth overhead. In contrast, compare HDLC, in which to prevent erroneous packet delineation, to special escape characters that have to be inserted into the datastream if the client data contains the start/end flag; this leads to an unpredictable transport band- width because it is influenced by the content of the client data. The GFP payload area consists of payload header information, a payload field that contains the client data, and an optional Frame Check Sequence (FCS) CRC-32 over the Payload field. There are two types of payload headers. The payload type header is used in all GFP frames except idle frames and consists of a 2-byte Payload Type field (protected by a CRC-16). If additional payload information is required, then a 60-byte extension is avail- able. In these circumstances, the Payload Type field consists of four subfields: Payload Type Identifier (PTI) consists of three bits and distinguishes between two currently defined frame types: client data frames (PTI = 0) and client management frames (PTI = 4). Payload FC Indicator (PFI) indicates whethe