[TriLUG] Managed & Unmanaged switches

Thu Mar 3 01:02:04 EST 2005

A typical PoE switch costs around ~$2100 and can power up 24 phones.
A decent switch ($800) and 24 Power bricks ($800) cost ~$1600

So a PoE switch costs about $500 more than just buying a switch and
separate power supplies for each phone - but some folks say the
convenience is well worth the cost. 

We have a few customers with Cisco 3550 PoE switches and they love them.
It makes moving phones a snap. Plus they don't need UPS's for the
phones. They just put a beefy UPS on the Cisco 3550 switch and the
phones stay up even during power outages.

QoS is a good thing on a network that runs Voice over it.

Even on a fully switched network you will want some QoS on your switches
(if you are mixing Voice and Data on the same network). Local data
movement across your network from client-to-client or client-to-server
can slow the network and delay packets. It is ridiculous to say that it
doesn't happen. We have all seen it... and it can have a very bad effect
on Voice.

Voice traffic is not bandwidth intensive, but latency intensive. Any
delays in delivering Voice packets will cause breaks or gaps (called
"jitter") in the communication. To prevent jitter during heavy network
use you either need "infinite" bandwidth or to:
 - reserve some of your internal bandwidth for Voice, and
 - give Voice traffic higher priority on your network, or
 - segregate your Voice traffic from your Data traffic

Don't be fooled by the "Big Sky" point of view. If you have ever seen
your network responses slow down due to load then you need QoS on your
network in order to run Voice across it properly.

Also, I've never seen anyone ever recommend running a connection used
for Voice applications at Half-Duplex. You always need your connections
to run at Full Duplex. 

You can always tell if there is a Half-Duplex connection between you and
the party you are talking with. It's like talking over an old pair of
walkie talkies. You find yourself wanting to shout "Over" once you've
finished talking. Move the connection to Full-Duplex and the
conversation is more like face-to-face speech.  

Momma always said "Full-Duplex is best for Voice". And that's all I have
to say about that.

Jon Carnes
FeatureTel

On Wed, 2005-03-02 at 22:02, Chris Bullock wrote:
> Depending on the type of phone you are getting you may need to get PoE
> (Power over Ethernet) switches.  Only reason you may need manages switches
> is that some IP systems make you set your speed to 100mb/HD at the switch
> to guarentee no crosstalk.
> Chris
> --- Jon Carnes <jonc at nc.rr.com> wrote:
> 
> > No flawed reasoning here. Your setup works in the real world as well as
> > in theory... but if you are going to segregate your traffic anyway you
> > really don't need any managed switches.
> > 
> > Buy a nice router (or build a nice firewall) that has two NIC's in it - 
> > one that connects to your switch with VoIP phones and the other that
> > connects to the switch with everything else.
> > 
> > Then you can prioritize at the port level on the router/firewall and
> > simply give any traffic coming in/out via the Voice NIC high priority
> > and any any traffic coming in/out the Common NIC low priority.
> > 
> > Heck, a Linksys WRT54GS will do this and it only costs ~$90
> > 
> > Jon Carnes
> > FeatureTel
> > 
> > BTW: Depending on how you implement VoIP (Hosting the server yourself or
> > using a hosted provider) you may want to look at doing some bandwidth
> > shaping on your Router/Firewall.
> > 
> > On Wed, 2005-03-02 at 09:15, Brian Henning wrote:
> > > This is probably going to be right up Jon Carnes' alley:
> > > 
> > > Somewhere in the distant future, my current employer may be moving to 
> > > VoIP telephony.  I remember hearing that it's best to have managed 
> > > switching hardware supporting a VoIP infrastructure, as it allows a
> > way 
> > > to guarantee that the phones always have the bandwidth they need.  My 
> > > question is thus:
> > > 
> > > Would it work to have one managed switch to serve the VoIP phones,
> > which 
> > > would also feed an unmanaged switch to handle other nodes?  Such as 
> > > described by the following beautiful diagram:
> > > 
> > > }}}}
> > >   }}}}}}}}}
> > > Internet }}--[firewall]---[managed sw]----[unmanaged sw]
> > >   }}}}}}}}}                   |    |              |
> > > }}}}                         /    \              |
> > >                              /      \         [Rest of the computers]
> > >                   [VoIP phones]     |
> > >                             [Some computers]
> > > 
> > > (Where of course "computer" means any node that isn't a VoIP phone)
> > > 
> > > It seems to me that the above arrangement would allow the managed
> > switch 
> > > to, er, manage the total allocation of bandwidth between outside and
> > the 
> > > phones, and all the traffic passing through the unmanaged switch could
> > 
> > > be clamped by the managed switch on its way to the outside if 
> > > necessary...  Right?  And that would allow us to continue getting
> > value 
> > > out of our current hardware..
> > > 
> > > Or am I completely flawed in my reasoning?
> > > 
> > > Thanks as always!
> > > 
> > > Cheers,
> > > ~Brian
> > > 
> > 
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> 
> 
> 
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