RFCOMM/HFP协议
RFCOMM
Set Asynchronous Balanced Mode (SABM) command 向对方请求建立异步平衡模式（ABM）的建立
Unnumbered Acknowledgement (UA) response 对SABM或者DISC的响应帧，表示确认收到
Disconnected Mode (DM) response，disconnect mode下做出的响应
Disconnect (DISC) command
Unnumbered information with header check (UIH) command and response
RFComm 帧格式
HFP
Audio Gateway (AG) – This is the device that is the gateway of the audio, both for
input and output. Typical devices acting as Audio Gateways are cellular phones.
Hands-Free unit (HF) – This is the device acting as the Audio Gateway‘s remote audio
input and output mechanism. It also provides some remote control means.
These terms are used in the rest of this document to designate these roles
AT+CMER enables the Signal strength Indication‖ function in the AG.
AT+BIA allows the HF to deactivate/reactivate individual indicators.
HF shall send AT+COPS=3,0 command to set name format to long alphanumeric