[Asterisk-Users] Type of T1 for T100P card

[Peter Svensson]

On Thu, 28 Oct 2004, Steve Underwood wrote:

> The original poster is asking about 2-way telephony. All  the normal 
> forms of telephony on T1 can support 2-way operation, and Asterisk 
> supports them. However, ISDN and SS7 are more robust than the robbed bit 
> signalled forms, like wink start. 2-way just means the same T1 can 
> handle a mixture of incoming and outgoing calls on the same T1. With 
> high call volumes on robbed bit signalled T1s the likelyhood of incoming 
> and outgoing calls clashing (glare) can sometimes be unacceptable. ISDN 
> should be rock solid under these conditions.

Isdn can be totally glare free, but not in the Asterisk implementation.  
Asterisk treats the isdn B channels as normal channels and the D channel
as a signalling channel. It allocates a B channel from what it beleives to
be a free channel and sends a SETUP message indicating that channel to the
network end. There is a risk that the same channel was siezed by the
network which will den disconnect the outgoing call and let the incoming
call through. So, there is no glare problem as long as the net and the cpe 
end hunt in opposite order. The collision will only happen when the last 
channel is contended.

However, if the network and the cpe end are not set to opposite hunting 
order this sort of clash can occur even when not all B channels are used. 
Since asterisk does neither retry when the channel selection is rejected 
by the net nor allow the net end to dictate the channel (an option allowed 
by isdn to prevent glare-like rejection of outgoing calls) it is important 
to have the opposite ends hunt in opposite order. 

Just a point in case anyone experiences something resembling glare on an 
isdn link. 

> Your post is also refering to telephony modes for T1s. RBS gives you all 
> 24 channels, but it doesn't give you 24 *clear* channels. Some bits have 
> been robbed. Most commonly ISDN gives only 23 voice channels. However, 
> ISDN with NFAS and SS7 can give you 24 clear voice channels with Asterisk.

The controlling D channel for isdn NFAS has to be delivered somewhere.  
With the current nfas implementation in asterisk this has to be over an
e1/t1 (can it even be delivered in any other way?). This gives you
24*n-1-b B channels where n is the number of T1s and b is the number of
backup D channels. I think of nfas more in terms of resiliency then 
more efficient use of the channels.

Peter