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Editorial Team
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Asked: 2026-05-29T20:57:03+00:00

What is the default encoded frame size of gsm codec? What is the default

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  • What is the default encoded frame size of gsm codec?
  • What is the default decoded frame size?
  • What are the media attreibutes?
  • What is the codec id?

For example same questions for G729’s answers are 20, 160, “fmtp:18 annexb=no” and 18 respectively. I am working on a softphone and I want to add the gsm codec to it.

Any help?

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1 Answer

  1. Editorial Team

    This could be what you’re looking for, from RFC 3551 (RTP Profile for Audio and Video Conferences with Minimal Control) in section 6:

               PT   encoding    media type  clock rate   channels
                    name                    (Hz)
               ___________________________________________________
               <snip>
               3    GSM         A            8,000       1
               4    G723        A            8,000       1
               <snip>

               Table 4: Payload types (PT) for audio encodings

    so your SDP body will look, with the boring bits cut away, like this:

      m=audio 49170 RTP/AVP 4
  a=rtpmap:4 GSM/8000

    for a GSM-only connection.

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