In need of GSM codec specifications in specific format

Question

• 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?

Answer 1

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.

Answer 2

Actually SDP should better look like this: v=0 o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com s= c=IN IP4 host.atlanta.example.com t=0 0 m=audio 49170 RTP/AVP 3 a=rtpmap:3 GSM/8000/1 And encoded length is 33 in bytes and decoded lenght is 320 bytes which equals to 160 PCM frames.