[Asterisk-Users] IAX2/GSM VOIP troubleshooting

[Michael George]

Last week I was able to do some debugging of the problem I'm having with
IAX2/GSM, residential-grade broadband, and VOIP.

To summarize, I am having a great learning experience with * and Zap cards,
SIP and IAX2.  I hit a wall though, when I registered with iaxtel and tried
doing VOIP.

I spend the better part of a workday with the jitterbuffer and all sorts of
settings and finally started to conclude that my systems might just be
inadequate for what I was asking them to do.

However, on Friday I went to our other office to try a DP system out.  I got *
up and running on it and IAX2/GSM was doing VOIP just fine with Digium's
iaxtel number, even with two channels.  I figured that was the problem, not
enough interrupt handling and transcoding capacity in my systems.

But I brought that system back to this office and tried it here and I have the
same choppiness that I have with my other (single processor) systems here.
So, the problem has to be in my net connection.

My ISP is willing to work with me on debugging the issue, and that's good.
But I'm not sure what types of things to look for that we might be able to
remedy.

At the office where it works fine, we have a Comcast cable modem providing
broadband.  It will do up to 5Mb/s down and 256Kb up.  I'm not sure if it's
full or half duplex.  But it sounds gread.

At this office where I normally work, I have a wireless broadband connection
to a POP in a town 5mi from here which connects via wireless to a town 10mi
from there and then it's either phoneline or wireless to the ISP's ISP.  It
seems like it might be a dicey connection, but I've had tremendous luck with
it for over 3 years now.  As a matter of fact, I regularly use a MultiTech
MultiVOIP with a 9.6kb/s voice coder on it and hardly anyone notices that it's
a VOIP connection.  (It *is* a half-duplex connection, if that matters.)
However, with the 13kb/s voice coder of GSM it's so choppy it unusable.

I don't think the problem is in IAX2/GSM.  I don't think the problem is in
bandwidth (I regularly pull 40KB/s down on this connection and I don't think
there's any limiting going up).  But there must be *something* about my
broadband connection here that is causing problems with IAX2/GSM.

I am hoping some of you networking/telephony/VOIP gurus might have some
insight into what I might look into to get this resolved.  Sometimes when I call
Digium's iaxtel number I get the "Received mini frame before first full frame"
error, but often I don't.  I've checked the archives and the solutions there
for that were not helpful.  Is there perhaps fragmenting of the packet?  Maybe
IAX2 need a minimum transmission unit that isn't getting through?  Or maybe
it's sensitive to buffering that a router in transit migth be doing for more
efficient use of the bandwidth?

Next week I might be able to go to my ISP's location with a SIP phone and my
little * testing box to see if eliminating the two radio hops from their
location to me makes a difference.  If not, we have big trouble.  If that does
help, then we need to figure out what in the ISP's equipment is causing the
problems.  

Any helpful hints as to what I could investigate to get this narrowed down?

-- 
-M

There are 10 kinds of people in this world:
	Those who can count in binary and those who cannot.