HFP describes how a gateway device can be used to place and receive calls for a hand-free device.

• Audio Gateway (AG) -This is the device that is the gateway of the audio, both for input and output, typically a mobile phone.
  
• 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. Hands-Free Control entity responsible for Hands-Free unit specific control signalling; this signalling is AT command based.

Summary:

• HFP allows sending AT commands between HF and HF Audio gateway (RFCOMM)
• Bi-directional audio can be transmitted over eSCO/SCO connection

• Audio quality: full-duplex, mono, 8kHz, 8bits -> 64kbps