Odd One-way Audio Problem

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Asterisk Users 2 Comments

Hi all,

I have a user who is reporting one-way audio, but only when a call is made to or from particular PSTN (cell) numbers.

Their phones are behind a NAT router and my server is on the open Internet.

Calls within their office sound fine. Calls to/from most numbers sound fine.

When they took their phones home, those same phone numbers still had problems.

So, I don’t think it’s their network. I’ve taken pcaps of both legs of example calls. On the provider-side, I see 2-way audio. On the client-side, I only hear one side.

Most of the time, though, their phones work correctly.

Any ideas where to look to fix this?

Thanks in advance.

2 thoughts on - Odd One-way Audio Problem

  • I’m assuming you’re using a PSTN trunking provider to connect to those numbers
    (ie: you don’t have your own on-site gateway device).

    Do you use only a single trunking provider, through which some calls show this problem, but most don’t, or do you use several trunking providers, and the call numbers showing this problem always go via the same one?

    I think that basically rules out the common NAT reasons for one-way audio.

    …almost certainly also behind NAT…

    But presumably other numbers didn’t? (Important to check!)

    From what you’ve said, I think you’re right.

    Please explain that in a bit more detail.

    You have an Asterisk server on the Internet, presumably with one IP address
    (or maybe two, but one IP4 and one IP6).

    Where are you capturing “the provider side” and “the client side”?

    Can you show us the tshark / tcpdump / whatever commands you are actually using to perform these captures, and make clear which machine/s you’re running those commands on?

    Only two things spring to mind so far:

    1. Transcoding?

    2. IPv4 on one side and IPv6 on the other (although I’m hard pushed to see how this could create one-way audio rather than no audio)?

    I think the key thing I would look for in the pcaps is for any re-invites – is one side telling the other “oh, you can get my audio from here” and that’s not an accessible address?

    However, why this would be specific to particular _numbers_ rather than particular SIP connections puzzles me too.

    Antony.

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    Please apologise my errors, since I have a very small device.

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  • My comments below:

    I only have the one inbound provider, and all numbers go through that provider.

    I use voipmonitor to create sniffer traces of calls on my server. I get 2 pcap files for each call:

    1. Traffic from the phone to my server.
    2. Traffic from my server to the trunk provider.

    In the cases I’m concerned about, one of these legs (only) will exhibit the one-way audio.

    There is no transcoding. I ONLY allow ulaw for voice.

    My server doesn’t have IPv6 configured at all.

    I will take a more thorough look at the pcaps.

    Thank you for your time, and for validating/confirming my conclusions.

    Any other ideas would be most welcome!