[Asterisk-Users] Problem on Outgoing Calls (FXO - SIP)

[Jim Van Meggelen]

asterisk-users-bounces at lists.digium.com wrote:
> Hello,
> 
> We have a high volume of incoming and outgoing calls that
> come in via our analog POTS lines connected to FXO cards in
> an Adtran TA750.  This is connected to a T100P.

Wouldn't you prefer to connect a PRI directly to the telco? You've got
some fantastic capability in that T100P that is being . . . well . . .
crippled by the analog circuits. How many lines do you have? To have or
not have PRI is generally a cost issue (sometimes facilities as well).
I'd like to understand what the channel bank is doing for you, because
it's kinda like running a Formula 1 car on wooden wheels). OK not that
bad, but you know what I mean?

> We are using Polycom IP 500's.  The problem we are
> experiencing is, on frequent occasions, when someone dials
> out, there is another person who has just dialed in.

It's called "glare" and it is generally a problem on systems where
inbound and outbound channels are selected in the same order, or there
are not enough lines, or the hunting sequence of the incoming lines is
not matched to the PBX line numbering.

If we visualize a five-line system, we might have a scenario like this
(incoming calls start at the top on the left; outgoing on the bottom
right; PBX line numbers in the middle):

--> 001
    002
    003
    004
    005 <--

Line 001 is going to be the busiest for incoming calls; line 005 will be
busiest for outgoing calls.

Now let's say there's two incoming calls on lines 001 and 002, and two
outgoing calls on line 005 and 004.

1-> 001
2-> 002
    003
    004 <-2
    005 <-1

A third incoming call is about to be presented to the system by the CO,
and at the same time somebody in the office wants to make a call out. 

1-> 001
2-> 002
3-> 003 <-3
    004 <-2
    005 <-1

The system could grab line 003 at the exact same time the C.O. begins to
attempt connection to the system. The CO is about to send ringing, but
hasn't yet, so the PBX figures the line is free and grabs it:

The C.O. sees the line go off hook, and, since it was attempting to
connect a call, it assumes someone has answered it. The parties are
connected.

> I have had this problem at my house before (without an *
> system) where I pick up too quickly for it to ring.

Yep, that's glare.

> Are there any methods to mitigate this problem?  I can see
> that Asterisk is recognizing the call, but it still opens a
> zap channel for the outgoing call.

Really? I don't want to be hasty here, but that sounds like a bug. If
the system knows a call is incoming, it should not be connecting it to
someone who'd trying to get an outside line.

Let's leave that for a moment and talk about the glare, which is
generally a split second timing issue, more common on systems where a)
the lines are busy and b) there are not enough lines to handle traffic.
It is pretty much impossible on a digital circuit, as the signalling is
far faster (I don't even know if it is possible on a PRI circuit, as the
endpoints would notice the conflict - I _have_ heard of it on a CAS
circuit, but it's extremely rare).

Another possibility is that the hunting order on your incoming lines is
not in the sequence you think it is. Typically, a hunting pattern for
incoming lines should follow 001, 002, 003 and so on. If for some reason
the second line in the group is, say, line 004, then it could happen
quite often that an incoming call and a request for dial tone overlap.

1-> 001
    002
    003    
2-> 004 <-2
    005 <-1

> Any help would be appreciated.

I would start by having the hunting sequence verified for your incomong
lines; make sure they physically terminate in the same order they hunt.
I've seen this cause glare on numerous occasions.

Next is to look at your traffic. If you have a lot of busy lines, then
you're running your circuits at capacity. Glare is much more likely in
this case.

As for the fact that you're saying the system has detected incoming
ringing and is still giving that circuit to a user requesting a line,
I'd want hear from a few of the Asterisk/Zaptel gurus on that one - that
is not proper behaviour for a PBX. Regardless, the best way to avoid
glare is to reduce the chances that an incoming and outgoing call will
ever have to contend for the same line.

Cheers,

Jim.