What is the default codec for translation?

Discussion in '3CX Phone System - General' started by millsey, Jan 3, 2013.

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  1. millsey

    millsey New Member

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    We have our SIP trunk set to PCMA codec, our phones are also set to PCMA codec and we have checked this is all correct.

    The extensions have the option "PBX Delivers audio".

    Most of our calls received are in PCMU, is there a way to change this so when teh PBX generates or translates audio, it uses PCMA?

    All this trnaslation is a waste of CPU time and a reduction in quality. This issue has been identified since I went to G722 (wideband) and people complained that the sound was unnatural, this seems to be because we are stuck with PCMA on the SIP trunk so there is a translation from GCMU to G722 which makes the sound unnatural.

    Thanks

    Millsey.
     
  2. lneblett

    lneblett Well-Known Member

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    To a large degree, the codec will depend on what the SIP provider supports. You don't indicate who your provider is or where you are located.

    In any event you can try from within the 3CX control panel, going to the VOIP Providers tab and then go into the advanced settings and selecting the codecs of interests and their priority.

    In the US, the dominate codec is g711u while in EU, they favor g711a. It may not be possible to "lock" to a single codec if any point in the chain does not support the desired; it may be forced to negotiate.
     
  3. eagle2

    eagle2 Well-Known Member

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    Most probably you have another problem, not related to PCMA to PCMU translation in connection with G.722 codec. I guess some of your phones is not supporting G.722 and there why a low quality codec is used (the sound being not natural). Try capturing traffic with Wireshark and analyze what codecs are offered / negotiated / used. It is not necessary to force 'PBX delivers audio' for phones in one and the same network.

    Regards
     
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  4. leejor

    leejor Well-Known Member

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    In most cases, with most devices/PBX's, (of course there are always exceptions), you choose a list, in order of preference, of the Codecs you (your device) supports. When a call is set-up the other end will go down that list, until one is found that you both support. I'm putting this in very basic terms. You can change the order of the Codecs that 3CX will support , or you can try eliminating some from the list. It all depends on whether or not, the far end supports your first, or only choice. You might want to contact your VOIP provider(s) to get more information on which Codecs they will support. The best option, in most cases, is selecting a Codec that is common to all devices/trunks, if possible.
     
  5. millsey

    millsey New Member

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    Our VOIP provider supports G711a G711u and G729 whatever, we don't want to use that.

    We have Grandstream GXP2100 phones, and we can see on the display which codec it is using during a call. Our VOIP provider has told us that they offer G711a, G729 and G711u in that order. Our Phones are now configured to support G711a then G711u. But all our inbound calls end up as G711u. If there is a translation going on (even from 711a to 711u then I want to remove it so the audio is not repeatedly translated).

    Am I wrong to think that the option per extension "PBX delivers Audio" will only work if the phone has a path to the VOIP provider? In our network that is not the case? Or does that option route all internal-to-internal traffic via the 3CX which is undesirable.
     
  6. leejor

    leejor Well-Known Member

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    "PBX Delivers Audio" will force the voice packets to pass through the PBX if the devices are on the same network. I have not looked into whether that is the case when each end is on a different network (remote extensions). I'm also not sure if audio will automatically pass through 3CX when a call is outgoing, over a trunk (gateway/VoiP provider), even if the same codec is used, as it may just not be supported.

    In an ideal situation, 3CX would be involved in the call set-up, then "drop out", and allow each device to communicate directly, but that is not always possible.

    You might try setting all devices (including the trunk) to use G711u as a first choice, and disable the "PBX delivers Audio" option on the extensions. Check the logs to see if any translations are still happening.
     
  7. millsey

    millsey New Member

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    Thaks Leejor, will try setting all to pcmu and see if things clear up.

    Millsey
     
  8. eagle2

    eagle2 Well-Known Member

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    1. 'PBX delivers audio' is more or less solving NAT traversal issues like packets being lost (effects like one-way audio, etc.). You should turn it off, if your phones belong to one and the same network. There was a nice article on the topic by Kevin. Traffic to provider will always pass through the PBX regardless of this setting for extension.

    2. Changing from PCMU to PCMA should not create any issues. However, if you want to force using only PCMA codec (or also known as G.711a) you must leave it as the only one available in your provider's settings. If your provider disagrees with this the call will fail, which means you can't use PCMA.

    3. Forcing using PCMA codec for extension is configured into the phones themselves. You can also choose to use only PCMA codec. This should solve all your issues.
     
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