proximity-based fob security device with bluetooth: how it works

Question

i have been reading about proximity security devices through bluetooth, but i am wondering how it works in reality. As far as i have been reading, there are two techniques:

1. constant scanning from a master. The central device scans the target device every x minutes. When the target is into range, it gives back MAC bluetooth address and RSSI. The negative side is that being constantly into discoverable mode drains the battery, so does the master device that has to be constantly scanning for devices.

2. Paired and connected. As some product details: "Your laptop locks itself down when you step a certain distance away, and opens again once you're in range". It means that after paring and connected processes, the master can have information when the slave device is out of range, or get inside range again. In bluetooth 4, is going to be even as a profile.

Regarding to the second option, which feature is this? how it works? i mean, maybe you can code a sequence number counter that counts everytime the slave sends a packet through RFCOMM every x milisecs (android or ios), but what i dont understand is how a slave can connect to master again in a transparent way, when normally two bluetooth devices disconnect when they are out of range each other (so you cant send information again from the same bluetooth socket RFCOMM connection).

How proximity devices works in reality? how can you code it?

Answer 1

Bluetooth 4.0 (BT Low Energy) is a whole other animal than traditional Bluetooth. It is designed from the ground up to be good at things like the Proximity profile you describe. Basically a computer and a fob create a connection to periodically communicate at a slow interval, say once every 5 seconds. When you go out of range the connection drops due to a timeout and the PC knows the fob is gone. The PC side isn't power constrained, so it can constantly or periodically scan for the fob when connections are dropped. The fob can advertise its presence using infrequent advertisement packets, which costs only a small amount of power.

Note that there is no RFCOMM in BT LE. Android and other BT stacks currently lack BT LE support because it is too new. Only a few devices exist which actually use BT LE, so getting much real info is hard right now. In a few years this will be much bigger. The new iPhone 4S and new macbooks have the 4.0 BT chips, so you can expect more usage soon.