[Asterisk-Users] Digium Quad Span Cards

[Matt Roth]

Initially, I believed that the limitation was the PCI bus, but I was 
mistaken.  There is a lot of confusion surrounding this issue, and it 
would be great if someone stepped forward with a concrete answer.  That 
said, here's what I've learned about the issue through my research.

We started off asking a Digium representative about putting 4 quad-span 
cards in a single machine and got the following response:
- I'd use two machines, with two quad cards each.  And then, I'd need to 
be using only the G.711 ulaw protocol.  Then, I'd still use a  mid-range 
dual Xeon CPU machine in the 2.4GHz+ spectrum.

David Mandelstam of the Asterisk Biz List provided me with this explanation:
- Zaptel drivers produce at least one interrupt per millisecond per 
board, which is minimal if the interrupt handlers are short. But on a 
heavily loaded machine doing lots of echo cancellation, each interrupt 
can approach 1 millisecond in length. So if there are interrupts coming 
from several cards, you can see how you could get into trouble.

So it looks like processor interrupts are the culprit.

Possible solutions to this problem include (please feel free to add to 
this list):
- An Asterisk slave server pool ( 
http://home.comcast.net/~mroth01/LargeAsteriskSetup.gif )
- A TDM-VoIP gateway (Cisco, Quintum, AudioCodes, Lucent)
- Using Sangoma cards (As per David Mandelstam, Sangoma cards use 
proprietary drivers and there are operational setups using 4 quads per 
machine)

I'm not confident that the Asterisk software scales well under certain 
conditions, such as using Monitor to digitally record 16 spans of voice 
channels, so solving the card issue may not be the last step in a large 
installation.  If anyone has any insight on this, please post it to the 
list.

Hope that was helpful,

Matthew Roth
http://www.voip-info.org/tiki-index.php?page=Running%20Asterisk%20on%20Debian

Matthew Boehm wrote:

>>From what I understand (and this could be completely wrong), the Digium
>cards use a bunch of processor interrupts and too many cards will use up all
>the interrupts. (again, that could be completely wrong).
>
>What kind of calls are they? G711->PRI? Not much CPU needed there. G729 ->
>PRI? Yes, you would need something along the lines of a dual Xenon 3.6Ghz do
>do that. Its all in the transcoding. If just passthru, not much cpu is
>needed.
>
>-Matthew
>  
>