Reading more in the rfc7587, you will find out that, in SDP, you will always see the codec being using “OPUS/48000/2″, no matter the real sampling rates.
无论采样率为多少，codec采样的参数都是”OPUS/48000/2″
实际的采样率协商可以使用以下附加参数，见第三行：
m=audio 54312 RTP/AVP 101
a=rtpmap:101 opus/48000/2
a=fmtp:101 maxplaybackrate=8000; sprop-maxcapturerate=8000; maxaveragebitrate=12000
3.1.1. Recommended Bitrate
For a frame size of 20 ms, these are the bitrate “sweet spots” for Opus in various configurations:
o 8-12 kbit/s for NB speech,
o 16-20 kbit/s for WB speech,
o 28-40 kbit/s for FB speech,
o 48-64 kbit/s for FB mono music, and
o 64-128 kbit/s for FB stereo music.