In need of GSM codec specifications in specific format

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?

Solution

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.