HP StorageWorks 8/80 HP StorageWorks Fabric OS 6.1.x administrator guide (5697 - Page 348

Table 76 describes Fibre Channel data frames Table 76 Fibre Channel data frames Start of frame 4 bytes Standard frame 24 bytes header Data (payload) {0 - 2,112 bytes CRC 4 bytes End of frame 4 bytes Total (Nbr bits/frame) {36 0 2,148} bytes 32 bits 192 bits {0 - 16,896} bits 32 bits 32 bits 288 - 17, 184 bits The term byte used in Table 76 means 8 bits. The maximum fibre channel frame is 2,148 bytes. You can allocate buffer credit using the portCfgLongDistance command, which allows you to allocate sufficient numbers of full-size frame buffers on a particular port or to support a long distance link up to 500 km. The port can only be used as an E_Port. Changes made by this command are persistent across switch reboots and power cycles. Use the portCfgLongDistance --distance_level command to select one of the following four options for buffer credit allocation. • Level 0 static mode (L0) - L0 is the normal mode for a port. An extended ISL port competes with other ports for buffers. No buffer credits are reserved for extended ISLs. • Level E static mode (LE) - LE reserves a static number of buffer credits that supports distances up to 10 km. The number reserved depends on the port speed. The baseline for the calculation is one credit per km at 2 Gbps. This yields the following values for 10 km: • 5 credits per port at 1 Gbps. • 10 credits per port at 2 Gbps. • 20 credits per port at 4 Gbps. • 40 credits per port at 8 Gbps. • Dynamic Mode (LD) - LD calculates buffer credits based on the distance measured during port initialization. An upper limit is placed on the calculation by providing a desired distance value. When the measured distance is more than desired distance, the desired distance is used in the calculation. This is a mechanism for controlling the number of reserved buffer credits ensuring buffer availability for other ports in the same group. • Static long distance mode (LS) - LS calculates a static number of buffer credits based on a desired distance value. For both LD and LS distance levels, you can use the following formula to get an approximation of the calculated number of buffer credits. 348 Administering Extended Fabrics