[Asterisk-Dev] IAX phone quality issues (proposed solution)

Steven Critchfield critch at basesys.com
Wed Jun 25 11:04:07 MST 2003

At my office we recently relocated our PSTN connection to consolidate
all our critical servers at our colocation facility. Since I have had
more than 6 months of good reliable IAX calls to my home over the cable
modem we choose to backhaul our office calls via IAX from our colo to
our office via our dedicated data T1.

With the default settings for IAX, we had problems with the call
beginning to have long delays, and drops of a half second or longer
where audio didn't get passed to the phone at all. This bothered me
because the phone service to my home was so much better than the office,
but to me the reverse should have been true.

During a talk one day about it in my office, we started to guess why
would there be a problem with calls on the 4ms ping loop as opposed to
the 50ms ping loop time. It occurred to us that the jitter buffer might
be causing the echo canceler to go nutty and drop the audio out for a
while.  We have disabled the jitter buffer on our office link, and now
we experience perfect audio quality on our lines.

I'll admit to not knowing enough about the jitter buffer and echo
canceling. I may be way off on the cause of the problem above. I do know
that by turning off the jitter buffer we have consistently good quality
calls to the point that our customers are surprised when we tell them
what we are doing.
Steven Critchfield  <critch at basesys.com>

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