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ATM nrt-VBR MBS calculations

Hi all!

Is there anybody has knowledge and/or experience in MBS (or BT) calculations?

I've tried to calculate MBS and BT at given PCR and SCR

for Cisco Router equipped with PA-A3 and BPX8620 ATM Switch.

There are two formulas I've found:


Acquired from ATM Forum (ATM TM 4.0):


(in Cisco BPX Configuration Guide the formula looks like this but a little more complex).


Acquired from ATM PA-A3 Configuration Guide (MBS Configuration Guidelines section)


MBS=BT*(PCR-SCR)/424, or

MBS=BT*(PCR-SCR) (if PSR and SCR measured in cps)

From graphics, pictures and explanations in documentation I understand that MBS (definition (2)) has meaning as rectangle square bounded by BT value and (PCR-SCR) value. At this point I think about MBS as of number of maximum bursted cells allowed for BT period. Is it clearly? Of course!

As I understand the (1) definition, the ATM Forum explains its formula through 1/PCR and 1/SCR values,

and defines MBS as function (cumulative value of differences) between the real cell arrival time (at PCR) and TAT (Theoretical Arrival Time) - at SCR. (I also think of MBS here as of sum of accumulated 'penalties').

When I've tried to convert one graphic (and formula) into another

I've got distinguished meanings and values of MBS.

Please look at given example:

( - "How PCR Bursts Reduce Latency" Section

PCR = 800 Kbps = 1887 cps

SCR = 64 Kbps = 151 cps

MBS = 32 cells

and BT calculated by formula (2) is equal to 0.018sec:

BT = MBS/(PCR-SCR)=32/(1887-151)=0.018

I clearly understand that this time is enough to send 1500-byte packet (32 cells) at PCR rate

because time to send 1500-bytes packet at 800kbps is equal 0.015 sec.

And meaning of MBS here is to give enough capacity to LeakyBucket_II to send whole 1500-bytes packet (32cells)

at PCR in 0.015 (or 0.018) seconds.

Let's try to use the formula (1) from ATM Forum:

MBS=[1+BT/(1/SCR-1/PCR)]=[1+0.018/(1/151-1/1887)]=3 (NOT 32 !!!)

Let's try to calculate BT from given MBS=32:

BT=(MBS-1)*(1/SCR-1/PCR)=(32-1)*(1/151-1/1887)=0.189 sec (NOT 0.018 !!!)

Which formula is right?

I should not configure the Cisco ATM Router with MBS value of 32

and the BPX ATM Switch with MBS value of 3 (with same PCR and SCR parameters) at the same time!


Konstantin Okounev

RIM Telecom


Re: ATM nrt-VBR MBS calculations

What is your ATM router model? I think for each router model, there are separate BTS calculation documents. So, if you specify the exact router model, it will be further easy to filter through and provide you with a more relevant or exact information.

New Member

Re: ATM nrt-VBR MBS calculations

7204VXR with PA-A3-OC3SMI

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