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.
The ISAC (Internet Speech Audio Codec) is a wideband speech codec for high quality conferences. The 16000 indicates that ISAC is going to be used at 16kbps. Because this is first, 16kbps will be considered before ISAC at 32kbps as specified in the next line.