From: Nigel.Johnson@barclayscapital.com
Date: Wed May 26 2004 - 08:03:33 GMT-3
Rate limiting an Policing take the 'normal-burst' and 'max-burst' values in
bytes. If we are given these values in Kbps then how do we convert these to
bytes?
The problem is that the bytes value has no time period associated with it so
if we just divide the bps value by 8 then we end up with a value that has
units of Bytes-per-seconds.
Now from the following link we see that CAR uses the following formula to
calculate the normal and extended burst:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos
_c/fqcprt4/qcfpolsh.htm
<http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqo
s_c/fqcprt4/qcfpolsh.htm>
normal burst = configured rate * (1 byte)/(8 bits) * 1.5 seconds
extended burst = 2 * normal burst
I think the key thing to note here is that the 'configure rate' in the
formula is the rate you are looking to rate-limit e.g. If we wanted to
rate-limit to 5Mbps then it gives us 937500 bytes for the normal burst and
twice this, 1875000 bytes, for the max-burst.
However, what I'm interested in is if we are given all the values we want to
achieve and they are all in bps.
E.G. If we want to achieve a rate of 5Mbps with a normal burst of 300kbps
and a max burst of 600kbps, then would the following be correct:
Normal-burst (in bytes) = (300000*1.5)/8 = 56250 bytes
Max-burst (in bytes) = (600000*1.5)/8 = 112500 bytes
The 5Mbps has not been used in the calculation as we are given the burst
values. The more mathematically inclined of you will realise that it is just
knowing what the reference time period is for the burst values. From my
calculations above we take a value in bps (bits divided by seconds),
multiply it by a value in seconds which leaves just bits ( bit/seconds *
seconds, the seconds cancel), then just divide by 8 to get bytes.
However, all this is related to CAR, are the formulas the same for policing?
I hope I've explained this ok.
Any help is much appreciated.
Nigel
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