Bluetooth HFP

Two roles defined - Audio Gateway and HFP.

Mobile phone - Audio Gateway - Gate way for audio input/output.
HF Device - Acts as remote device for audio input/output.

1) First service level connection is established and
2) then audio connection is established for active call

Service level connection happens with exchange of AT format commands and responses between AG and HF device

AT commnds that exhanged are

AT+BRSF - Bluetooth Retrieve Supported Features.
AT+BAC - Bluetooth available codes
AT+CIND - Call indicators supported
AT+CHUP - Call hang up/Or reject incoming call by HF
AT+CMER - Event reporting activation/deactivation.
RING - Ring alert from AG -> HFdevice
ATA - Call Answer by HF
+CIEV: (call = 1) - Call answered by AG, indicate to HF
+CIEV: (callsetup = 0) Call iNterrupted/rejected in AG
AT+BCS - Codec Selection( eSCO + mSBC, or SCO_CVSD)
 +CIEV unsolicited result code with the corresponding indicator value whenever a change in service, call, or call setup status occurs.

Service Level connection is assumed successful once either of AT+CMER, OR AT+CHLD are exchanged.