I have a question regarding the SDP part of the Invite request.

suppose user agent  "A" wants to talk to user agent "B", he sends a INVITE
request to B.  the SDP part of the invite request is as follows,


v= 0
o= A 1464112736 1464112736 IN IP4 151.209.33.190
s=Calling
c=IN IP4 pgs-477
t=0 0
m=audio 2074 RTP/AVP 0 3 5
a=rtpmap:0 PCMU/8000
a=rtpmap:3 GSM/4000
a=rtpmap:5 ADPCM/4000

If user agent accepts the request and agrees to talk to user agent A, he
sends a OK response with SDP as followed.

v= 0
o= B1464112736 1464112736 IN IP4 151.209.33.194
s=Answering the call
c=IN IP4 pgs-451
t=0 0
m=audio 2074 RTP/AVP  3 5
a=rtpmap:3 GSM/4000
a=rtpmap:5 ADPCM/4000

now user agent A sends an ACK and starts communication. My question is ,
How user agent A is going to know the type of encoding done in the voice
packet recieved from "B"? that is how he is going to  come to know whether
it is GSM or ADPCM?





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