aspects. Currently, two modes of client signal adaptation are defined: Framed (GFP-F) and Trans- parent (GFP-T). GFP-F is a layer-2 encapsulation mode that is optimized for data packet protocols (e.g., Ethernet, IP/PPP, DVB) of variable-size frames. GFP-T is a layer-1 encapsula- tion or block coded-oriented adaptation mode, optimized for protocols that use 8B/10B line coding (e.g., Fibre Channel, ESCON, 1000Base-T) and that are of constant-size frames. Chapter 11 Next Generation Synchronous Digital Hierarchy Ethernet IP/PPP Fibre FICON ESCON Other Client Channel Signals Frame GFP - Client Specific Aspects Transparent Mapped (payload dependent) Mapped GFP - Common Aspects (payload independent) Fig. 11.17 Functional block SDH/SONET Path Other CBR Path OTN Path diagram of GFP. GFP Frame Structure A GFP frame consists of a core header and a payload area, as shown in Figure 11.18. The core header supports frame delineation procedures and essential data link operations func- tions independent of the higher-layer PDUs. Within the header, a 2-byte field indicates the size of the GFP payload in bytes (4-65,535 bytes), and a 2-byte field (cHEC) contains a CRC sequence that protects the integrity of the core header. Rather than relying on a special character for frame delimiting (as is the case for HDLC), the GFP receiver begins the framing process by looking for a 2-byte field that is followed by a correct CRC for that field. When such a 32-bit pattern is found, it is very likely to be the core header at the begin- ning of a GFP frame because the probability of such a pattern randomly occurring in the client data is 2-32. The GFP receiver then uses the PLI information to determine where the Payload Length Indicator Payload Type GFP Core Header 0-60 bytes Payload Header Optional Extension Client Header Spare Payload Payload Fixed or Variable Length Packet FCS (optional) cHEC: Core HEC (header CRC) EXI: Extension Header Identifier tHEC: Type HEC UPI: User Payload Identifier eHEC: Extension HEC CID: Channel/Customer PTI: Payl