[Asterisk-Users] Weird IAX trunking/7960/ILBC quality issue

[Chris Bagnall]

Greetings all,

I've been doing some testing with a new client's voicemail system from my
asterisk box at home. I'm using a 7960 (alaw/ulaw) then an IAX2 link between
my server and the client's server.

I've configured IAX settings at both ends as follows: jitterbuffer enabled,
trunking enabled, ILBC/GSM allowed in that order. When listening to the
voice prompts from VoicemailMain at the client's asterisk I hear about one
word in three, very similar to a GSM mobile on the border of its coverage.
Disabling trunking appears to make no difference. Disabling the jitter
buffer appears to solve the problem, but with a connection that's often used
for other things, this is undesirable as a long-term solution.

When I make the same call from a SIP device (GXP2000), the issue disappears
entirely. But the same problem does not occur from the 7960 to a PSTN
gateway (also via IAX2 with exactly the same settings).

To summarize:
7960 -> remote (ILBC + jitterbuffer) = quality problem
SIP -> remote (ILBC + jitterbuffer) = no problem
7960 -> remote (ILBC no jitterbuffer) = no problem

I'm not sure what's to blame here - the 7960 or my IAX settings, or where to
begin looking for solutions.

Any thoughts?

Regards,

Chris
-- 
C.M. Bagnall, Director, Minotaur I.T. Limited
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