OOB key: The 16 byte SMP TK value is shared between the host PC and the BTH LE device. Only used to represent unique peripherals. Must be as unique as the Bluetooth Mac Address. DeviceID: uniquely identify a supporting device and should be 8 bytes in length to avoid device conflicts. The OEM must define the appropriate threshold. Set as a minimum signal strength before pairing takes place. RSSI Threshold: Defines a distance threshold in dB for when peripherals are “out of range of the host PC”. After the UEFI variables are defined, they must be locked to be read-only.įor each device supporting automatic pairing, the host PC should program the following OOB information into its NVRAM: The feature allows this functionality to persist on system recovery and clean installs. This feature is dependent on storing information to aid the pairing process in UEFI. Store for Device ID (hashed from Bluetooth Address) & TK value. Microsoft-defined Bluetooth HCI commands for optimized battery life. They must also guarantee security and that they will all be in the same box when shipped to customers. OEMs must determine on their own how to manufacture these devices. Users must leave the range of the host PC, to be able to pair their device to other Bluetooth enabled PCs. When a bundled device is unpaired within the range defined by the RSSI value, the Bluetooth device automatically re-pairs to Windows. All data pre-provisioned must be stored in UEFI to preserve this functionality on clean installs and system restores. The distance from the host PC in which the peripheral may pair is determined by another pre-provisioned minimum RSSI value, which is represented in dB, so ranges may vary. 
This is through Out-Of-Band Pairing which uses a separate secret OOB key that was also pre-provisioned. If the device is in range and its advertisement matches a pattern that is pre-provisioned in the host PC during manufacturing, then the device is paired. That advertisement is then picked up by the host PC. When an LE peripheral supporting this feature is supplied power for the first time, it sends out proprietary Microsoft-defined data in an undirected connectable advertisement. However, if the peripherals are unpaired and within range of the host PC, they will automatically re-pair and connect. These peripherals can still be used as regular Bluetooth devices, and maintain full functionality when out of range of the host PC. Users do not need to keep LE devices paired and connected after both the host PC and bundled devices are supplied power for the first time. Windows 10 supports LE Bluetooth peripherals automatically pairing to a single PC, after all devices have been provisioned and enabled during the manufacturing process.
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