# Calculating MBS to BPX (ABRFST)

May 14th, 2007

Please, can anyone help me to know how to calculate MBS regarding SCR and PCR ?

Thanks,

Cleber

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## Replies

paolo bevilacqua Mon, 05/14/2007 - 10:50
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Hi,

from an archived mailing list, Steve Murray:

Here is the function for the relationship of Burst Tolerance BT (time) and Maximum Burst Size MBS (a number of cells).

Most switches allow you to set the MBS and then they calculate the BT based on this function.

Definitions: Ts = 1/SCR, Tr = 1/PCR, ts = BT

MBS = | 1 + ((CDVT + ts) / (Ts - Tr))

where |x| stands for the largest integer less than x. for Tr > 1 and CDVT > Tr -1.

This function is based on using a unit of micro-seconds for each variable. The corresponding function is used to calculate the maximum burst size B (a number of cells) on the PCR policing:

B = | 1 + ((CDVT) / (Tr - 1)) |

For Tr > 1 where |x| stands for the largest integer less than x. CDVT must be greater than Tr - 1.

This function is also based on units of micro-seconds. It breaks down if Tr < 1 microsecond. At that point a smaller unit of time would need to be subtracted from Tr instead of 1 microsecond. This only matters with very large PVCs with a PCR of over 1000000 cells per second.

These values for MBS and CDVT are usually defaulted for the user and the same default applies no matter what the SCR and PCR values are. Some ATM providers set the CDVT and MBS very low and adjust up if problems like you are observing appear.