North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical RE: RE: ATM Switch Interoperability
Steve, In your experience, then, would you agree that higher SCR rates should have higher MBSs, so that the Burst Tolerance linearly increases with SCR? I have a problem with an MBS of 100 for SCR 1000 up to 1000000. CDVT gives us some bucket depth, but I don't think we're clumping. The LS1010 doesn't shape VBRnrt, just polices. At the far end is a Fore ASX1000. So, we have three different vendors in the mix. Our ATM provider sets MBS to 100 no matter the contract. This seems too restrictive. Chris > > 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 > [email protected] > > > >>> "Martin, Christian" <[email protected]> 01/15 9:19 PM >>> > >We have it matched to the policing policy of the Network > Provider. By > >increasing the CDVT, we gain some bucket depth, but we are > only MUXing 5 > >VCCs over one VP. There shouldn't be too much clumping, espescially > >considering the load on the switches(low). One thing is interesting, > >though: The far end (customers) are all running over DS3 using HEC > >delineation. > > > >A curiosity: Given the parameters below for the contract, > particularly > >the MBS and CDVT, does it appear that we are being cheated? From my > >understanding of the UNI 3.1 spec, the MBS should be adjustable > >according to SCR/PCR, given the fact that CDVT is defined as the same > >for both SCR and PCR buckets. > > > >One obstacle that myself and my colleagues have not been able to > >overcome is the use (aside from bucket calculation) of the Burst > >Tolerance. It is obviously proportional to the MBS, but again, it > >appears as if a contract specifying a PCR/SCR of > 100000/50000 cells per > >sec, should have a higher MBS than a 10000/5000 cell/sec contract. > > > >Any insights? > > > >Chris > > > > - > >
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