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.
These lines specify G711 mu and a-law, which is telecom’s classic 64kbps pulse code modulation (PCM) codec using different companding laws. 0 and 8 are the static payload types for PCMU and PCMA respectively. Technically these lines do not to be present since this information can be inferred by the codec list in the media line – PCMU or PCMA.