Sunday, January 21, 2007

Calculating BB credits for a link ??

The brocade FC101 is a great resource to start learning SAN design. The other day I was just revising my SAN design course and thought of just adding some more explanations to the FC101 material via this blog.

LATENCY OF LIGHT WAVE = 5 nsec/m or 5 Usec/km

So for 10 km = 10 x 5 = 50 Usec one way or 50 x 2 = 100 Usec round trip.

For a 20 Kbyte frame propagation delay @ 100 MB/sec is T = D/S

(2 x 10^3) / (100 x 10^6) = [2 x 10^(-5)] = [20 x 10^(-6)] = 20 Usec

So to keep a 10 km "pipe" full, the switch would need atleast

100Usec/20Usec = 5 E_Port credits.

Proportionally, a 2 KB frame at 50 k needs (5 E_Port credits for 10 km x 5) = 5 x 5 = 25

At high speeds more buffers are needed, at 200 MB/sec, it would be 40 USec

For 10 km at 200 MB/sec, we would need 10 E_Port buffers.

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