4. Profiles: The More The Merrier, Continued

Once the GAP bridges the connection between the headset and the cell phone, information on both devices in the Service Discovery Application Profile details what services are offered by the distant end and how they can be used. If both devices are linked and the available services are known, they can be selected and used.

The headset profile is thus in effect a service that enables the mobile phone to use the headset. It in turn is based on the Serial Port Profiles (SPPs). As might be guessed from the name, SPPs define rules governing how a serial cable connection between two Bluetooth devices is emulated. Thus, the actual connection between the headset and the cell phone, also called the Audio Gateway (AG), is made by emulating a serial cable connection between the devices. In principle, communication between the headset and the mobile phone is based on transmitting and receiving AT commands, such as those used to control a modem.

In addition to the conventional Headset Profile (HSP), Logitech's Mobile Bluetooth Headset also has a Hands-Free Profile (HFP). It was originally developed as a standard for brand-independent communication between a mobile phone and a stationary hands-free setup mounted in a car. In principle, it can also be used to control/operate a fixed car phone with Bluetooth functions or a cell phone with an appropriate headset.

The difference between a headset that conforms to the Headset Profile and one that conforms to the Hands-Free Profile is in the number of functions the headset "knows" for operating/controlling the cell phone or car phone by voice commands. The HFP, moreover, better fulfills the requirements of car-bound telephony, because it also has integrated mechanisms for noise suppression and echo compensation.