Reference Guide

TABLE 12 Fibre Channel gigabit values (continued)
Gigabit value Line rate
16 Gbps 17
32 Gbps 28.05
Buffer credit allocation based on full-size frames
Assuming that the frame is a full-size frame, one buffer credit allows a device to send one payload up to 2,112 bytes (2,148 with headers).
Assuming that each payload is 2,112, you need one credit per 1 km of link length at 2 Gbps (smaller payloads require additional buffer
credits to maintain link utilization). Refer to Allocating buffer credits based on average-size frames on page 131 for additional
information.
Fibre Channel data frames
The final frame size must be a multiple of 4 bytes. If the data (payload) needs to be segmented, it will be padded with 1 to 3 "fill-bytes" to
achieve an overall 4-byte frame alignment. The standard frame header size is 24 bytes. If applications require extensive control
information, up to 64 additional bytes (for a total of an 88-byte header) can be included. Because the total frame size cannot exceed the
maximum of 2,148 bytes, the additional header bytes will subtract from the data segment by as much as 64 bytes (per frame). This is
why the maximum data (payload) size is 2,112 (because [2,112 - 64] = 2,048, which is 2 kb of data). The final frame, after it is constructed,
is passed through the 8-byte-to-10-byte conversion process.
Table 13 describes Fibre Channel data frames.
TABLE 13 Fibre Channel data frames
Fibre Channel frame fields Field size Final frame size
Start of frame 4 bytes 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 (number bits/frame) 36-2,148 bytes 288-7,184 bits
Allocating buffer credits based on full-sized frames
You can allocate buffer credits based on distance using the portCfgLongDistance command. The long-distance link modes allow you to
select the dynamic mode (LD) or the static mode (LS) to calculate the buffer credits.
For LD, the estimated distance in kilometers is the smaller of the distance measured during port initialization versus the desired_distance
parameter, which is required when a port is configured as an LD or an LS mode link. A best practice is to use LS over LD. The
assumption that Fibre Channel payloads are consistently 2,112 bytes is not realistic in practice. To gain the proper number of buffer
credits with the LS mode, there must be enough buffer credits available in the pool, because Fabric OS will check before accepting a
value.
NOTE
The desired_distance parameter of the portCfgLongDistance command’s is the upper limit of the link distance and is used to
calculate buffer availability for other ports in the same port group. When the measured distance exceeds the value of
desired_distance , this value is used to allocate the buffers. In this case, the port operates in degraded mode instead of being
disabled as a result of insufficient buffer availability. In LS mode, the actual link distance is not measured; instead, the
desired_distance value is used to allocate the buffers required for the port.
Buffer-to-Buffer Credits and Credit Recovery
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