HP StorageWorks Fabric OS 6.x administrator guide (5697-7344, March 2008)

364 Administering Extended Fabrics
Table 80 describes Fibre Channel data frames
The term byte used in Table 80 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.
Table 80 Fibre Channel data frames
Start of frame 4 byes 32 bits
Standard frame
header
24 bytes 192 bits
Data (payload) {0 - 2,112 bytes {0 - 16,896} bits
CRC 4 bytes 32 bits
End of frame 4 bytes 32 bits
Total (Nbr
bits/frame)
{36 0 2,148} bytes 288 - 17, 184 bits