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Skype for Business codecs - G.711 A-law vs G.711 U-law RRS feed

  • Question

  • Hi all,

    As far as I known SfB uses g.711 codec in mediation server for communication with the PSTN.

    Is it possible to use only one of the u-law or a-law? I see in the traces that both are offered for an outbound call from SfB to PSTN. 

    In the SDP how the preferred order of u-law/a-law is determined? Is it possible to change this ordering?

    Thanks.

     


    • Edited by mpmk Thursday, July 21, 2016 8:39 AM
    Thursday, July 21, 2016 8:13 AM

Answers

  • Hi

    Unfortunately there is no way to prioritise codecs per-say in SfB. They are negotiated based on available bandwidth, so they can be limited on per stream bitrate limits. But as A and U law have the same bitrate, then it is impossible to define. 

    However, the mediation server will convert between u and a to your gateway anyway as long as the gateway side you are accepting only a-law g711.

    thanks


    Note: Please remember to `Mark as Answered` a post that answers your question and/or `Vote as Helpful` posts that have helped you. This will help others find answers to similar problems. For more Skype for Business help visit: http://www.skype4b.uk Please note that answers are based on my experience and opinion only and do not necessarily represent the views of my employer.

    • Marked as answer by mpmk Thursday, July 21, 2016 9:55 AM
    Thursday, July 21, 2016 8:36 AM
  • No there is no way to put PCMA above PCMU.

    We have a way of limiting the available codecs based on stream bitrate by implementing CAC and bandwidth policies, but as PSTN is g711 or RTAudio, this wouldn't help you too much 

    Basically it works like if you apply a limit of 60KBps per stream for audio - then you are getting RTAudio everywhere and not G711 or G722 or SILK etc. but we can't say is use G711 A first, the U then RTAudio then x y and z etc.

    thanks


    Note: Please remember to `Mark as Answered` a post that answers your question and/or `Vote as Helpful` posts that have helped you. This will help others find answers to similar problems. For more Skype for Business help visit: http://www.skype4b.uk Please note that answers are based on my experience and opinion only and do not necessarily represent the views of my employer.

    • Proposed as answer by Ram OjhaMVP Thursday, July 21, 2016 9:10 AM
    • Marked as answer by mpmk Thursday, July 21, 2016 9:55 AM
    Thursday, July 21, 2016 9:06 AM

All replies

  • Hi

    Unfortunately there is no way to prioritise codecs per-say in SfB. They are negotiated based on available bandwidth, so they can be limited on per stream bitrate limits. But as A and U law have the same bitrate, then it is impossible to define. 

    However, the mediation server will convert between u and a to your gateway anyway as long as the gateway side you are accepting only a-law g711.

    thanks


    Note: Please remember to `Mark as Answered` a post that answers your question and/or `Vote as Helpful` posts that have helped you. This will help others find answers to similar problems. For more Skype for Business help visit: http://www.skype4b.uk Please note that answers are based on my experience and opinion only and do not necessarily represent the views of my employer.

    • Marked as answer by mpmk Thursday, July 21, 2016 9:55 AM
    Thursday, July 21, 2016 8:36 AM
  • My SDP looks like this:

    v=0
    o=- 54 1 IN IP4 10.10.xxx.yyy
    s=session
    c=IN IP4 10.10.xxx.yyy
    b=CT:1000
    t=0 0
    m=audio 53354 RTP/AVP 97 101 13 0 8
    c=IN IP4 10.10.xxx.yyy
    a=tcap:1 RTP/SAVP
    a=pcfg:1 t=1
    a=rtcp:53355
    a=label:Audio
    a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:5bkeeFe5yxrPxeV4q2HhMoyDQf+YpADBxHg6tiuB|2^31|1:1
    a=crypto:2 AES_CM_128_HMAC_SHA1_80 inline:VdwLO3inB9hI5YqU9Hzga6PXISW8umcfzr8LwO6w|2^31
    a=sendrecv
    a=rtpmap:97 RED/8000
    a=rtpmap:101 telephone-event/8000
    a=fmtp:101 0-16
    a=rtpmap:13 CN/8000
    a=rtpmap:0 PCMU/8000
    a=rtpmap:8 PCMA/8000
    a=ptime:20

    So you say that there is no way to have PCMA first and PCMU second?

    Aditionally, when you say that there is no way to prioritize codecs in SfB, does this imply that the admin has no control over this, e.g. we cannot manually exclude a codec from being offered?  

    Thursday, July 21, 2016 8:52 AM
  • No there is no way to put PCMA above PCMU.

    We have a way of limiting the available codecs based on stream bitrate by implementing CAC and bandwidth policies, but as PSTN is g711 or RTAudio, this wouldn't help you too much 

    Basically it works like if you apply a limit of 60KBps per stream for audio - then you are getting RTAudio everywhere and not G711 or G722 or SILK etc. but we can't say is use G711 A first, the U then RTAudio then x y and z etc.

    thanks


    Note: Please remember to `Mark as Answered` a post that answers your question and/or `Vote as Helpful` posts that have helped you. This will help others find answers to similar problems. For more Skype for Business help visit: http://www.skype4b.uk Please note that answers are based on my experience and opinion only and do not necessarily represent the views of my employer.

    • Proposed as answer by Ram OjhaMVP Thursday, July 21, 2016 9:10 AM
    • Marked as answer by mpmk Thursday, July 21, 2016 9:55 AM
    Thursday, July 21, 2016 9:06 AM
  • I suppose the natural question is - Is it causing an actual issue? and if so what?

    Note: Please remember to `Mark as Answered` a post that answers your question and/or `Vote as Helpful` posts that have helped you. This will help others find answers to similar problems. For more Skype for Business help visit: http://www.skype4b.uk Please note that answers are based on my experience and opinion only and do not necessarily represent the views of my employer.

    Thursday, July 21, 2016 9:07 AM
  • What's your objective? Why do you want to alter the course of early media negotiation? Are you trying to connect a gateway or device which understands just one form of codec otherwise the communication fails? If yes, which device or gateway it is?

    In my opinion we should let the gateways to select a mutually accepted codec through media negotiation (which happens through standard protocol)... until and unless there is any specific reason (netwrok, security, quality etc) to not follow this advise. 


    RTCPedia - A Unified Communications Journey


    • Edited by Ram OjhaMVP Thursday, July 21, 2016 9:16 AM
    Thursday, July 21, 2016 9:12 AM
  • I need to clarify something with the CAC you mention.

    As far as I know, CAC affects both sides of mediation server i.e. the SfB FE side and the gateway side. The gateway side uses G.711. If limiting to 60 Kbps I guess I won't have communication with the PSTN? 

    Thursday, July 21, 2016 9:50 AM
  • No Mark, I don't have a actual issue. I'm about to connect a non-certified gateway to SfB and I'm doing some preliminary research and I stumbled on this:

    https://social.technet.microsoft.com/Forums/lync/en-US/fc25cef7-6649-462f-b363-d13ce03095e0/what-default-codec-of-lync-2013-g711-alaw-or-g711-ulaw-or-the-other-codec?forum=lyncvoice

    Please, see the last post.

     
    • Edited by mpmk Thursday, July 21, 2016 9:56 AM
    Thursday, July 21, 2016 9:55 AM
  • No worries, CAC will only limit WAN connections really so that is usually client > mediation rather than mediation > gateway, unless of course the gateway is across a WAN from the mediation.

    The 60KBps was just an example but you could enforce RTAudio from client > mediation and then if there is a WAN between mediation and gateway apply a different limit (or no limit)

    thanks


    Note: Please remember to `Mark as Answered` a post that answers your question and/or `Vote as Helpful` posts that have helped you. This will help others find answers to similar problems. For more Skype for Business help visit: http://www.skype4b.uk Please note that answers are based on my experience and opinion only and do not necessarily represent the views of my employer.

    • Proposed as answer by Liinus Thursday, July 21, 2016 11:42 AM
    Thursday, July 21, 2016 10:32 AM