Fibre Channel Flow Control Buffer Credit Calculations
Buffer Credit Calculations
• Credit requirement also depends on frame size • Examples based on 10 km link…
1Gb/s
2Gb/s
Payload Size
(bytes) Trans-
mission Time (µs)
2048 19.84 1024 10.20 512 256 128 64 32
5.38 2.97 1.77 1.17 0.87
Credits Required 6
10 19 34 57 87
116 Trans-
mission Time
(µs) 9.92 5.11 2.69 1.49 0.89 0.58 0.43
Credits Required
11 20 37 67
113 173 231
Trans- mission Time
(µs) 4.96 2.52 1.32 0.72 0.41 0.26 0.19
4Gb/s Credits Required
21 40 76
138 231 350 471
The frame size used in an actual customer environment would be based on the I/O characteristics of the customer’s applications. Serialization time is proportional to frame size, so the number of credits required varies with frame size.
For example, with a 10km link, at 2Gb/s, only 11 credits are required if the average payload size is 2048 bytes—say for a data mining application.
However, if the average payload is 256 bytes—say for an OLTP application— 74 credits are required. That’s more than most vendors currently provide for an E_Port.
Fortunately, most applications use larger frames on average, and some WAN links might not need to run at wire speed.