Bitrate control errors when framerate at 15 fps

[ChengNing]

Hi, everyone

I meat a serious issue，

I use webrtc.framwork（m52）in my app，and i thought 15fps is enough for me，then set framrate at 15fps according to following method

https://groups.google.com/forum/#!searchin/discuss-webrtc/WebRTC$20doesn$27t$20deal$20with$20maxFrameRate$20constraint$20correctly$20on$20iOS/discuss-webrtc/CRPefY-n9so/PVbEhTzxOAAJ

the issue is，

the bitrate is expected at 1200 kbps,but often reduce to about 500 kbps for some time(about 1 minute) then resume 1200kbps, some time later reduced to about 500 kbps again, two devices in the same network.