A Peripheral advertises, inviting and (perhaps) accepting connections from Central devices. 'Advertising' means transmitting small amounts of data, quite frequently, which other Bluetooth devices can receive and act upon if they think the advertising device is of interest.
A Central device scans, looking for advertising packets and based on their content, may decide to connect to a device it thinks is suitable.
A Broadcaster is like a peripheral in that it advertises but it does not accept connections. It's sole purpose is to advertise.
An Observer scans and processes advertising packets but never tries to connect to another device.
The BBC micro:bit is a Peripheral.
A smartphone is typically a Central but some newer devices can also act as Peripherals with the right application software running.
A Bluetooth beacon (iBeacon, AltBeacon, EddyStone and so on) is a Broadcaster.
A beacon application on a smartphone which alerts you to special offers broadcast by beacons for example, is an Observer.
A Peripheral cannot create connections, only accept them. Therefore a Peripheral cannot connect to another Peripheral. A Peripheral cannot scan for advertising packets from other devices, only transmit them. Therefore two Peripherals cannot interact using advertising data only.
The outcome of this is that one micro:bit cannot talk to another micro:bit *directly* using Bluetooth. You'd need a Central mode device acting as a hub and relaying messages between micro:bits. My Heart Rate Monitor video shows a smartphone acting as a Central hub which is connected to two Peripherals, a BBC micro:bit and a heart rate monitor.
Technically, a device like the BBC micro:bit could have the ability to be either (or both) a Peripheral and a Central but there's just not enough memory available on the micro:bit to squeeze the larger Bluetooth stack that would be required to support both GAP roles. Maybe as and when there's a 32K micro:bit (complete conjecture / wishful thinking!) this will happen.
That's it. An introduction to Bluetooth GAP roles and their relevance to BBC micro:bit. Amaze your friends at your next dinner party ;-)
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