well.. I'm not sure where to put this thread..
but anyone can give me some idea what is it?
where can I find some resources about 95th percentile rule..? tq..

The 95th percentile method is a way of measuring bandwidth usage. In all cases I know of, it is computed by measuring the bytes transfered over 5 minute timeslices, ordering the values, and taking the 95 percentile value as the "bandwidth used".

To take a simple example, if every month was 100 minutes long ( ;) ) and your server used 10, 50, 20, 60, 30, 70, 40, 80, 50, 90, 70, 20, 30, 90, 50, 30, 40, 90, 150, 30 kbps in the 20 5-minute intervals, the 95th percentile method would order those: 10, 20, 20 ... 90, 90, 150 and pick out the 19th value out of the 20, ie 90 kbps.

In general the 95th percentile method gives higher values than the "average bandwidth" method. For this reason many people dislike it. Personally, I like the 95th percentile method since it (theoretically at least) equates closely to the actual cost of supplying bandwidth -- a very "bursty" packet flow is more expensive to carry than a "non-bursty" flow carrying the same number of bytes.

This is just another short description:

Average: billed as the average of your traffic (spikes and all).

95 Percentile: similar, yet you are billed at the highest speed you needed that month, minus the top 5% for the whole month.

You've seen both the simple and more complex answers already. I have both and a graph that helps illustrate it, along with benefits, on my site.