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.
If the lightstream does traffic shape properly this should not be an issue. If you do have traffic shaping on the lighstream
you could set the PCR=SCR. You will lose a small amount of burst but will be able to sustain at SCR. The bursts do not
really buy you a large amount of extra traffic anyway (unless CDVT and MBS are set high) they are really just there to
allow for variations in accuracy of policing between devices.
Steve Murray
MCI Worldcom
steve.murray@wcom.com