[Asterisk-Dev] PRI/Zap

[Peter Svensson]

On Wed, 4 May 2005, Tais M. Hansen wrote:

> I've configured 4 spans into one group. When dialing via PRI I send the call 
> to this group.
> 
> Sometimes it happens that my provider rejects the call with a cause 44. That 
> apparently means that the requested channel is unavailable. If I retry the 
> call, Asterisk will chose the same channel again, giving me the same error.
> 
> Is there any way I can make Asterisk/ZAP retry a different channel, if that 
> error occurs?
> 
> Also, my provider told me that we were always using "explicit" when choosing 
> channels, instead of "preferred", which they recommend. I haven't looked into 
> that yet, so I can only guess what it means. I have a feeling it's closely 
> related.

The problem is that Asterisk treats a PRI link like a lot of independant 
channels which happen to share a signalling channel. Asterisk will first 
select a B-channel for a call and then use that channel in the setup 
message.

The net end is free to reject a specific B channel. Most telecom cpe
equipment leave the B-channel selection to the net end, or is at least
capable of retrying using a different channel. Unfortunatly this is not
possible to implement with the current zap channel implementation in
Asterisk.

Peter