[asterisk-users] Simultaneous Inbound and Outbound calls on analog lines...

[Jay R. Ashworth]

On Thu, Feb 28, 2008 at 08:47:37AM -0600, James Texter III wrote:
> On Feb 27, 2008, at 2:55 PM, Tim Nelson wrote:
> > Hello! I've run into a problem where a user is making an outbound  
> > call at the same time that an inbound call is being made on the same  
> > analog line. It appears that as the zap channel is opened for the  
> > outbound call, it is simply answering the inbound call. Obviously,  
> > both parties involved in the calling get a bit confused. Previously,  
> > it happened only on an occasional basis. However, as this  
> > installation gets more and more use, we are finding it happens more  
> > often. How can this situation be prevented? Shouldn't zaptel see an  
> > incoming call and simply choose another trunk? We are running  
> > Asterisk 1.2.12.1 and Zaptel 1.2.22.1. Any ideas?!?
>
> In the telephony world, this is called glare, it's most prevalent on  
> Analog (though you can have the same thing happen with robbed-bit  
> T1).  There really isn't much you can do to prevent it, only minimize  
> it.  You need to have your inbound and outbound starting at opposite  
> ends.  If your incoming calls are coming top down, then you need to  
> use G<your group number> in your Dial app so that outbound calls go  
> bottom up, or vice versa.

And, to expand a bit, this comes from the fact that on analog (and I
guess RBT-1, though I hadn't realized it happened there), there is not
a 3-way handshake to open the channel; each end can open it
unilaterally.  This leads to a race condition, and thus, 'glare'.

The *reliable* way to fix this is to go to PRI, if you can a) get it,
b) afford it, c) terminate it, and d) profit!!   

Oh, sorry; that's Slashdot.  Never mind.  :-)

Cheers,
-- jra
-- 
Jay R. Ashworth                   Baylink                      jra at baylink.com
Designer                     The Things I Think                       RFC 2100
Ashworth & Associates     http://baylink.pitas.com                     '87 e24
St Petersburg FL USA      http://photo.imageinc.us             +1 727 647 1274

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