[asterisk-users] Max amount of concurrent calls on and iax trunk

[Gordon Henderson]

On Thu, 7 Aug 2008, Mattias Andersson wrote:

> I agree bandwidth is the limit, however the reason to use IAX is it is
> saving bandwidth.
> I am runging 2 Trixbox CE with IAX over a 2 Mbit line.
> I have never had any isues with the IAX trunk.
> I wish that I could get son good IAX phones to the office, tan would we skip
> on Trixbox and run the phones directly over a VPN Chanel. SIP over VPN are
> giving more hassle then IAX sound wise.

So As I Understand It... The bandwidth saving on using IAX is using it in 
a trunk mode - using it on separate phones will use almost as much as SIP 
phones will.

In my (rough calculations!) world, a SIP call uses 80Kb/sec b/w (each way, 
but we'll quietly ignore thant for now) A single IAX call uses almost the 
same, (78 according to a quick check on iftop) but in an IAX trunk, each 
additional call is only adding on about 65Kb/sec - whereas in a SIP world, 
each additional call is adding on another 80Kb/sec... Or you can have 
approx. 15 to 20% more IAX calls than SIP calls over the same Internet 
path, if they're inside an IAX trunk.

Lets see if that works:

On a 2Mb line:

   SIP is: 2048/80          = 25 concurrent SIP calls.
   IAX is: ((2048-80)/65)+1 = 31 concurrent IAX calls.

(And 31 calls is 31*50 = 1550 packets a seconds each way, so make sure the 
router can handle that!)

Personally, I tend to suggest to my customers that if they have more than 
4 or 5 extensions in one location that they're better off with a "micro" 
PBX to trunk the calls out - especially if they're making a lots of calls, 
and a few internal calls (as on NAT it'll bounce the call out & in again 
)-: This is based on typical UK ADSL line speeds though (up to 420Kb/sec , 
outgoing speeds on normal lines, 830Kbps on 'business' lines) Although 
typically what happens is that they want to keep their existing 
branch-office POTS lines and integrate them into the "system", so they get 
a mini PBX rather than a micro one!

However I did see some more IAX phones recently when looking:

Atcom -
   http://www.voipon.co.uk/voip-ip-telephones-atcom-iax-ip-telephone-c-1_61.html

(thats a UK site, obvously)

It also irritates me when I see the likes of Snom (and Doro) phones which 
are Linux based not supporting IAX ...

Gordon