I have been playing with this for a while.
Here's what I've done. I looked at the Serial Interface on one of my routers and wrote down the values for the total bytes input and the total bytes output.
15 minutes later I went back and wrote down the valuse as they were then.
I subtracted the starting values for each from the ending values for each, and added the resulting values together to get a total bytes transferred amount. This value was approximately 19 MB.
However when I looked at the interface in NTA for the same 15 minute time period the value that was showing was 43.6 MB.
I'm a little unsure of the reason for the apparent discrepency. Am I looking at apples and oranges here, or is there something I am not understanding?
Check to make sure you have 64 bit counters enabled in your NPM monitoring config for that node. Having a 32-bit counter overrun will screw up your calculations completely.