[c-nsp] Frame Circuit Not Cooperating

[John Neiberger]

> The second NIU I had that failed in this way passed the hard loopback
> plug test.  I suspect it's output drivers were damaged to the point that
> they could produce enough signal to make it work on 2 inches of wire
> in a loopback plug, but not enough signal to make it up 1 story to the
> next floor where the router was.  Swapping the NIU fixed the problem
> of course.

The weirdest NIU failure I ever had went like this:

1. Users complain that some file transfers are failing to a remote site
2. Further investigation shows that it is only one particular file
that will not transfer
3. I try testing across the link with various payloads and sizes and I
can't get it to fail, not a single error, yet this one particular file
will fail every single time.

I ended up using a hex editor to split the file into successively
smaller parts. I eventually found an 18-byte pattern that could not
pass through this NIU. Anything less than that particular 18-byte
pattern would succeed with no problem. Qwest wasn't able to replicate
this during testing because their test equipment won't allow test
patterns that big. I convinced them to swap out the NIU and that
solved our problem.

John