[asterisk-users] Quiet 24 port POE gig switch

[D Tucny]

2009/2/2 Steve Underwood <steveu at coppice.org>

> Bernd Felsche wrote:
> > Ian Cowley <ianc at moffat.co.uk> wrote:
> >
> >
> >> Beware PoE switches that can't handle Class 3 (15W) on all ports.
> >> Most have fans because 24 (or 48) x 15W is hot!
> >>
> >
> > That's the power supplied .. which'd be at the far end of the wire.
> >
> > The efficiency of the PSU plays a big part in the heat dissipation.
> > The push to compact dimensions doesn't help ... a 400W or
> > thereabouts PSU with 24 independent outputs in 1U height? I suppose
> > if the switch were quite deep it could be workable and quiet.
> >
> > The problem isn't simply of being "fanless". But being quiet.
> > Preferably below 32 dBA at 1 metres for most offices.
> >
> > You can do that by using fans other than the tiny, whiney, 40mm fans
> > that vibrate at 6000 to 18,000 Hz. A couple of 80 or 120 mm muffin
> > fans at the back or front, pushing air in (hence the deep
> > dimensions), but the top and bottom would need recesses to allow
> > sufficient airflow when the positions above and below are filled.
> >
> So, size does matter after all. :-)
>
> 24 x 15W => 360W. Its not that big a supply really, and spread across a
> 1U case its not that dense a supply. A 360W desktop PC supply can be
> pretty quiet, so its sad none of the 1U chassis supplies are. Probably
> if they used a large impeller fan they could get the noise down. I guess
> they assume these things will be in cupboards or data centres where
> nobody cares. This is a poor assumption.
>
>
The problem is squeezing fans in that can push enough air to keep it cool...
For a 1U device, you have only 4.445cm to work with, with a 4mm fan, that
would be 2.2mm of space for casing etc above and below, reasonably tight
already... A quiet 80mm fan as you may find in a PC PSU that puts out
somewhere between 15-20dBA of noise will typically move between 20 and 30
cfm of air... A quiet 120mm fan at the same noise levels would typically
move between 30 and 50 cfm of air and a quiet 40mm at those levels would
move about 5 cfm of air... Obviously, they aren't using quiet 40mm fans...
To get the airflow of the quiet 80mm fans, a 40mm fan has to go very fast
and you're looking at noise levels of approx 40-60dBA, not exactly quiet,
but, that's not all, even if the fan was silent, forcing the air through the
small cramped chassis of a 1U device is going to be noisy...

The assumption made when they make these devices is that the vast majority
of people will put this kit somewhere out of the way in a likely temperature
controlled, reasonably sound insulated environment, with the rest of their
hardware that lives hidden from people... These people will likely prefer
that kit uses the space as efficiently as possible, so, squeezing as much
functionality into as few rack units as possible is important... They have
typically made a good assumption in this I would say... Admittedly, people
who are planning an office for their first time may more commonly neglect IT
hardware, it's requirements (and those of those people around it), from an
IT standpoint, it's a significant pain to deal with, but, in most cases I've
seen, it's something that's considered very carefully if planning an office
in the future...

I suspect the lack of larger quieter units in the market is reflective of
the much lower demand for these, somewhat specialised devices... On the
otherhand, soundproofed rack cabinets that have integrated cooling and look
nice/plain enough that they don't scare people in an office should be
generic enough that there would, I suspect, be sufficient demand from those
that didn't consider IT requirements when fitting out an office to justify
making them...

d
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