[TriLUG] Slightly OT: network switch question

[Matt Flyer via TriLUG]

I have an application that uses two wireless modems, running Linux as
their OS, as a wireless Ethernet bridge.  The devices have been up for
a little over 5 years without issue until about six months ago when the
link stopped working.  At the time, removing and reinserting the
network cable between the main building network switch and one of the
modems got things working again.

Last Saturday they stopped communicating again.  I did a reset of both
of the wireless modems.  In one case, I pulled the POE / Ethernet cable
out of the modem performing a power reset on it.  Similarly on the
other modem, I performed a power reset by pulling the AC plug on the
POE injector as the modem is physically inaccessible without a lift /
bucket truck.  This had no effect and did not solve the problem.

I was reviewing some documentation on the previous failure and noticed
that it got 'fixed' when someone pulled the Ethernet cable between the
building switch and the POE injector.  This would have interrupted the
communications, but not power reset the wireless modem.  It would have
also broken the physical connection as far as the building switch was
concerned.

My question is, what happens at the switch when you pull the network
cable and break the circuit?  More importantly, what happens that could
have caused things to 'reset'? I am starting to think that this is a
switch problem and not an issue with the wireless modem, which is what
I had initially assumed.  

My knowledge of Ethernet doesn't go down to the physical level, so I
thought I would throw this question out in the hopes that someone has
an idea.