Two-fiber Unidirectional Path Switched Ring 2F-UPSR

THE GENERIC FRAMING PROCEDURE GFP 41 G GFP client-dependent aspects GFP client-independent aspects SONETSDH G.709 Ethernet IP over PPP SAN F P Figure 2.23 The GFP stack. administration, and management OAM . The resulting GFP frames are transmitted over SONETSDH, or over the new ITU-T optical transport G.709 otherwise known as the digital wrapper . G.709 is described in detail in Section 9.3.

2.7.1 The GFP Frame Structure

Two different types of GFP frames have been defined: GFP client frames and GFP con- trol frames . The GFP client frames can be either client data frames or client management frames . GFP client data frames are used to transport client data, and GFP client manage- ment frames are used to transport information with the management of the client signal or the GFP connection. The GFP frame structure, as shown in Figure 2.24, consists of the GFP core header and the payload area. The GFP core header consists of the following fields: • Payload length indicator PLI : A 2-byte field used to indicate the size of the payload area in bytes. Values between 0 and 3 are reserved for internal usage. • Core head error control core HEC or cHEC : A 2-byte field used to protect the PLI field. It carries the frame check sequence FCS obtained using the standard CRC-16, which enables single-bit error correction and multiple-bit error detection. The GFP payload area consists of the payload header, the payload, and an optional payload frame check sequence FCS. See Figure 2.24. The payload header has a variable Payload area Core header Payload length Payload length Core HEC Core HEC Payload header Payload Payload FCS Figure 2.24 The GFP frame structure. 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