[asterisk-users] Negotiates g729 but RTP contains g711

David Herselman dhe at syrex.co
Mon Sep 21 23:51:17 CDT 2020


We have a scenario where inbound calls from an upstream provider (chan_sip) sent downstream (chan_iax2) negotiates only g729 yet RTP media contains g711. Both the upstream and downstream trunks are limited to only offering g729 whilst the initial invite from our upstream provider offers both g711 and g729. Asterisk presumably simply forwards the media from iax2 trunk encapsulation to sip encapsulation. Most calls surprisingly work, presumably by the caller's system identifying the incoming media as g711, whilst very few callers don't hear the IVR prompt. The downstream is unfortunately not within our control but can't be anything other than Asterisk, considering it's using iax2 in trunk mode.

We are running Asterisk 16.13.0, not sure what version the downstream is using.

caller -> upstream -> us -> downstream (IVR)

Herewith the SIP portion of the call, between upstream and us:
[cid:image002.jpg at 01D690AC.C02E4200]

Wireshark unfortunately still cannot dissect iax2 trunk captures though, so I didn't know how to conclusively identify where this problem originates. I do however have a concern that the media we are receiving's packet size indicates that it is most likely G729.

Herewith the IAX2 trunk portion of the call, between us and downstream:
[cid:image004.jpg at 01D690AC.C02E4200]

David Herselman
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