Two-fiber Bidirectional Line Switched Ring 2F-BLSR

42 SONETSDH AND THE GENERIC FRAME PROCEDURE GFP Payload header Payload Payload FCS Payload type Payload type Type HEC Type HEC 0-60 Bytes of extension header PTI UPI PFI EXI Figure 2.25 The GFP payload structure. length between 4 bytes and 64 bytes see Figure 2.25. The following fields have been defined: • Payload type : A mandatory 2-byte field that indicates the content and format of the payload. The following subfields have been defined within the payload type: ◦ Payload type identifier PTI: A 3-bit subfield that identifies the type of client frame i.e., client data frame and client management frame. ◦ Payload FCS indicator PFI: A 1-bit subfield that indicates the presence or absence of the optional payload FCS. ◦ Extension header identifier EXI: A 4-bit subfield that identifies the type of exten- sion header. ◦ User payload identifier UPI: An 8-bit field that identifies the type of payload. Defined UPI values for client data frames include: Frame-mapped Ethernet Frame-mapped PPP including IP and MPLS Transparent-mapped Fiber Channel Transparent-mapped FICON Transparent-mapped ESCON Transparent-mapped Gigabit Ethernet GbE • Type head error control type HEC or tHEC : A 2-byte field that protects the payload header. It carries the FCS obtained using standard CRC-16. As with the core HEC, it enables both single-error correction and multiple-error detection. • Extension headers : A flexible mechanism for header extension is supported in order to facilitate adaptation of GFP to diverse transport mechanisms. The payload contains a GFP frame. It is a variable-length area ranging from 0 bytes to 65,535 bytes, minus the size of the payload header and if present the size of the payload FCS. Finally, the GFP payload FCS consists of an optional 4-byte FCS generated using CRC-32.

2.7.2 GFP Client-independent Functions

GFP supports the following basic procedures, which are common to all payloads: frame delineation, frame multiplexing, header and payload scrambling, and client management. Frame delineation checks the GFP frames to make sure they are extracted correctly from the bit stream that SONETSDH delivers to the GFP client-independent layer. It is