ATM Switch Interoperability

Martin, Christian CMartin at
Wed Jan 20 21:14:43 UTC 1999


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


> 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 at
> >>> "Martin, Christian" <CMartin at> 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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