[asterisk-users] network design philosophy and practice

[David Gibbons]

Fair enough, I guess I was concentrating on this line in Jerry's message :)
> The only reason I can think of not to is to eliminate the cost of the second cable.

I believe you're mistaken about the QOS though.
> QoS is not required on lightly loaded links and will do nothing for you on over loaded ones.

QOS will absolutely allow voice traffic to pass with priority over heavily loaded links -- this is in fact the reason that it would be implemented. Obviously giving priority to the voice traffic on these heavily loaded links serves to mitigate both latency and jitter.

> The concern is almost never one of taking bandwidth away from the desktop, but one of the desktop taking bandwidth
> (especially by introducing latency) away from the phone.

Agreed -- but with VLAN tagging and QOS, the issue of how much bandwidth the desktop uses and/or needs becomes moot since the phone is given priority.

Dave

David Gibbons wrote:
> Two separate networks? Did I miss something? I feel like I'm taking crazy pills! Two separate physical networks means twice the hassle, twice the maintenance, twice the cost, twice the headache. Not to mention the fact that the whole idea of VOIP is to simplify IT and focus on converging data and voice networks.
>
> This is what VLANs and QOS do best. I dare say it's what they were designed foe. I can't think of any reason that I would ever recommend two ports per desk to support telephony -- ever. It's ludicrous to think that two ports will be better than one if we're setting up our VLANs and QOS properly. A phone takes very, very little bandwidth away from the desktop and a decent one will support tagging its frames for the alternate voice VLAN.
>
> --snip--
> In almost all cases it is much better to have two seperate networks.
> This may be impractical in some smaller installs, but in any office
> setting we always do this. The only reason I can think of not to is to
> eliminate the cost of the second cable.
> --snip--
>


That's two _logically_ separate networks. The key point is that the
"last yard" cable to the phone is not shared with the computer.
The issue is not a lack of bandwidth but that the phone has to try and
get its little packets inserted between the massive packets of a
database lookup or file transfer in a timely manner (latency and jitter).

You might get away with a single logical network on a smaller site or a
larger one with very light traffic.

QoS is not required on lightly loaded links and will do nothing for you
on over loaded ones. I only use it on WAN links where bandwidth is more
expensive.

regards,

Drew

--
Drew Gibson

Systems Administrator
OANDA Corporation
www.oanda.com


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