Is there a "rule-of-thumb" for setting the ATM VBR-nrt burst-window size

First off, the correct way in ATM is to match the SCR, PCR, and MBS on both ends.  A mismatch should get thrown away with the policing.

Is your telco running vbr-nrt or vbr-nrt with tagging instead of policing?  Most people don't understand ATM vbr correctly.  VBR has three major parameters, SCR (sustained Cell Rate), PCR (Peak Cell Rate), and MBS (Maximum Burst Size).

The part people don't understand is that PCR is not Burst Rate in the sense of what people think of as bursting.  The best way to think of vbr-nrt is to imagine that you are going to get SCR all the time.  However, every once in a while, your allowed to increase your data rate (making sure you don't exceed PCR) for up to the MBS.  Many people understand that; however, they don't usually understand the implications.

Here's an example much like yours.  Let's assume the MBS is 256 cells.  You have a SCR or 20Mb/s and a PCR or 40Mb/s.  To make things nice, we'll assume that a DS3 is actually 40Mb/s even after the overhead.  You can think of a cell as a time unit since every cell is exactly the same length of time.  If you look at the possible cells that could fit on a DS3, you have a SCR that will allow every other cell to be used by this PVC.  Because of your PCR and MBS, your allowed to use all the cells in a row for up to 256 extra cells.  That means the amount of time you can burst can be calculated as:

256 cells
x 2    because only half of your cells are above SCR
X 53   53 bytes/cell
x 8    8 bits/byte
= 217,088 bits
now take:
(217,088 bits)(40,000,000 bits/sec) = 0.0054272 seconds

Now what happens when you burst to 40Mb/s for the 5ms that your allowed?

The rules say that you must drop to the 20Mb/s rate and never burst again until you've dropped below the 20Mbs for the number of cells that your going to burst for.

Think of MBS as a bucket full of "free passes for cells that are above SCR and not above PCR".  When you put a cell into the pipe that is over SCR, you have to take a pass out of the bucket.  On an MBS of 256, if you use 60, you still have 196 left.  If you use all 256, you have none left and are not allowed above the 20Mb/s (every other cell).  If you go for say 3 cells without sending a packet, you just earned back one free pass because there was a cell that you were allowed to have under the 20Mb/s SCR that you didn't use.  As a result, you get back one of your burst passes.  The bucket is only 256 cells though so if you run at 0 forever, you still only get 256 cells of burst when you want them.

Where does all this take us, well for any given length of time, the maximum throughput on vbr-nrt is SCR (in Mb/s) + MBS * 53 * 8 / (length of time).  Since the + MBS part of that is so small, it's approximately SCR in addition, it gets closer and closer to SCR as the length of time goes up.  At 5 ms, it might be 40Mb/s at 50ms, it's going to be around 22Mb/s, at 1s, it's going to be just slightly above 20Mb/s.

So, if your looking at performance tools, you probably won't see anything above 20Mb/s unless your monitoring your network to death which is a bad thing.

The next thing everyone says is, why then have the 40Mb/s PCR when I can't use it.  Well, the issue is that you can and do use it without realizing it.  Most of your traffic is highly bursty (web browsing, interactive telnet's, ssh's).  Those types of traffic do really well because a given html page may only be a few KB and a few keystrokes on an ssh session is only a few bytes.  The shortened response time by getting to use those extra cells every so often is very benefitial.  And if you watch the traffic on a link, when people aren't browsing or hitting keystrokes, you have a chance to get those MBS "passes" back into the bucket.

So that's the detailed explanation.  The short version is that PCR was never meant for large transfers like FTP.  Yes it will benefit a bit, but your never going to get closer to PCR than SCR when talking about FTP.  If you need 40Mb/s for FTP then you need to buy 40Mbs SCR or you need UBR traffic.