[asterisk-users] network design philosophy and practice

[Jerry Jones]

On Oct 29, 2008, at 12:30 PM, David Gibbons wrote:

> 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.


Allow me to clarify.

Yes I do advocate seperate cable runs for phones and computers.

Do not care if they both use a single switch as long as they are VLANd  
on seperate paths, either port based or tag based.

And before everyone starts up again - :) - let me say that YES, I do  
install single cable fully integrated systems - when I manage the  
network. If I remember the OP was looking for real world examples and  
guidance. In the real world, just last week I picked up a new  
customer, drove 6 hours to a branch office of theirs that kept  
complaining about voice performance, and threw out the hub I found  
they had installed when they moved into their brand new building. Had  
a nice new switch - which I was told about - for their pc's. But all  
phones were on a hub - which I had not been told about. The new switch  
had been sent down to plug the phones into, but yeah.


So in the real world I really like the KISS principle. Of course if  
there are qualified data folk ALWAYS makeing sure network is setup  
properly then feel free to disregard.