[asterisk-dev] Intermittent busy tone detection on loopback setup

Manish Sapariya manish at gslab.com
Tue Aug 7 09:51:50 CDT 2007

Hi List,

I am using asterisk to test another asterisk/voip software, by
generating user agent on asterisk which can be used to place
calls to unit under test. I have also tests in which I place
calls over PSTN lines and receive them back on PSTN line and
verify the results.

Everything is fine, except some times asterisk sees busy tone
on callee side when the caller side plays the audio file, for
pstn call. To understand the problem I ran simple test by using
a loopback cable from fxs to fxo, repeatedly. What I found is that
the received file indeed was so distorted and had some busy tone
like sounds that asterisk identified it as busy tone and hanged
up the line.

There are following questions that I am trying to answer:
- Why the busy like distortion is generated on the PSTN line?
- When I recorded the wav files to see the actual quality of
the voice I found them not very good. My assumption was that
the loopback setup should give the best result possible. Did
I misunderstand anything?
- Will PSTN hardware, I have digium cards, cause such intermittent
noise generation when use repeatedly to place calls.

Any ideas?

Thanks and Regards,

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