HP StorageWorks Fabric OS 6.2 administrator guide (5697-0016, May 2009)

424 Administering extended fabrics
The following table describes Fibre Channel data frames.
NOTE: The term byte used in Table 81 equals 8 bits. The maximum Fibre Channel frame is 2148 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. Only
E_Ports can be configured for extended distances. Changes made by this command are persistent across
switch reboots and power cycles.
Enter the portCfgLongDistance command to select one of the following four options for buffer credit
allocation:
NOTE: Long distance modes L0.5, L1, and L2 are not supported on Fabric OS 6.x.
• Level 0 static mode (L0)–L0 is the normal (default) mode for a port. Each user port reserves eight buffer
credits and competes with other ports for additional buffer credits. No buffer credits are reserved for
extended distance ISLs.
• Level E static mode (LE)–LE reserves a static number of buffer credits, which 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 Gb/s. This yields the following values for 10 km:
• 5 credits per port at 1 Gb/s
• 10 credits per port at 2 Gb/s
• 20 credits per port at 4 Gb/s
• 40 credits per port at 8 Gb/s
• 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. If the
measured distance is more than desired distance, the desired distance is used in the calculation;
otherwise, the measured distance is used. This is a mechanism for controlling the number of reserved
buffer credits ensure buffer availability for other ports in the same group.
• Static long-distance mode (LS)–LS calculates a fixed number of buffer credits based on a desired
distance value.
NOTE: For the LD and LS distance levels, see ”Determining how many ports can be used for long
distance” on page 425 to get an approximation of the calculated number of buffer credits.
Table 81 Fibre Channel data frames
Fibre Channel Frame fields Field size
Start of frame 4 bytes 32 bits
Standard frame header 24 bytes 192 bits
Data (payload) 0–2112 bytes 0–16,896 bits
CRC 4 bytes 32 bits
End of frame 4 bytes 32 bits
Total (Number bits/frame) 36–2148 bytes 288–17,184 bits