BlueTooth Random Info:

ACL (Asynchronous Connectionless Link) This is a control data link.
SCO (Synchronous Connection Oriented Link) This is a voice data link.   While Bluetooth is the technology used to connect devices from different manufacturers without wires, the devices still have to talk to each other while providing what ever service they are designed for. Headsets are audio devices for speech, talk and listen; but when a headset is not doing its job it is still communicating with the other device referred to as the audio gateway (AG). This would be a cell phone or USB dongle for instance.   Once the headset has completed its pairing to the AG it goes into a stand by mode of sorts. As long as the headset and AG are within range they are connected by the ACL. The headset and AG are checking in with each other to verify range, check for control signals like incoming calls, redial or voice control requests.   If the AG receives a call and the headset ACL is good the AG will signal the headset. The headset will beep to let the user know a call has arrived. Pressing the call control on the headset lets the AG know it is okay to open the SCO and voice data will be exchanged between the headset and AG.