Fibre Channel Flow Control Buffer Credit Calculations
Buffer Credit Calculations
Credits =
(Round Trip Time + Processing Time) Serialization Time
As a rule of thumb, at 2Gb/s and with a 2KB payload we require a credit for each km plus one
2Gb/s 10km ISL
Frames
Initiator E_Port
Credits
Target E_Port
Buffer credit calculations depend on link rate and total round-trip latency: • At 2Gb/s, a FC frame takes 9.92 microseconds (μs) to serialize—that is, to transmit every bit of that frame.
• The speed of light in fiber is about 5μs/km. With a link distance of 10km, each bit of that frame will take about 50μs to reach its destination.
• Given a frame serialization time of about 10μs, and a total round-trip time of 100μs, the total latency is about 110μs.
• Using the formula above, we can calculate that 11 buffer-to-buffer credits are required to make full use of the bandwidth of the link.