How would a bad UPS affect only the network port

[Tim]

Allegedly, on or about 27 April 2013, Sam Varshavchik sent:
> So, that leaves me wondering how a bad UPS would affect a server's
> network port, and only its network port. 

If it has a noisy power supply, it could generate enough hash to cause
interference with certain things.  I have two or three external hard
drive enclosures that do that.  Whenever they're connected to a
computer, my ADSL2+ modem continually drops out every few seconds.  What
they're connected to doesn't even have to be turned on (e.g. the
computer is plugged into a switched off wall socket), it's just
providing a point where the HDD supply can radiate crap through wiring.
Some switchmode power supplies are just awful.

Bad earthing in the equipment or the building could cause earth loops,
which can create peculiar problems.  Particularly if some equipment is
sensitive to that, but others are not.  It gets even more difficult to
manage when you network between buildings.  And unless you buy the
really expensive network switches or routers, they don't have
transformer isolated inputs on each ethernet port.

> As I understand it, with main power, the UPS is pretty much out of the
> loop, and it kicks in only if mains power is lost.

Well, a truly uninterruptible power supply would work like this:

The mains input charges a battery and provides DC to the inverter.  The
inverter provides AC to its output (going to your PC), either from the
mains supplied DC or the battery.  This way, there's an instant,
unbroken, changeover.  And it can cope with the mains voltage going up
and down, by either the regulator coping with over voltage, or using the
battery when there's undervoltage.

A cheap system might work as you suggest, and suffer glitches at the
times that the mains becomes bad enough that it has to go over to the
battery.

It could well be that you have had a series of bad UPSs, or it could be
that your mains supply is so bad that the UPS spends most of its time
supplying the computer, rather than charging its batteries.  Or you
could be putting too much of a load on it.

-- 
[tim at localhost ~]$ uname -rsvp
Linux 3.8.4-102.fc17.x86_64 #1 SMP Sun Mar 24 13:09:09 UTC 2013 x86_64

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