Now we come to the rtpmap lines for the formats included in the “m” lines above to map the payload types and the codecs. We also have a number of the fmtp lines to describe characteristics of the offered codecs. The order in which the formats are written in the media line is important as it defines the priority in which the browsers would like to use them.
a=rtpmap:13 CN/8000
The dynamic RTP payload types above (except payload type 13 which is static) indicate that Comfort Noise – CN – is going to be used for codecs of rates 48000, 32000, 16000 and 8000kbits/s.