A BLE UUID says what kind of data, not which device

The gap

I read "a custom UUID is a unique ID that never clashes" and concluded that each device needs its own UUID. That's wrong, and the word "unique" pushed me there: unique compared to what?

The pieces

• A GATT characteristic is a data element inside a service, for example "battery level" or "temperature". Each characteristic and service is identified by a UUID.
• A custom 128-bit UUID is unique compared to other kinds of data, so your "temperature" never clashes with someone else's.
• Every device running the same firmware exposes the same UUIDs.
• The Bluetooth address identifies one device. On Android, BluetoothDevice is a thin wrapper around that hardware address.

The click

Picture identical apartment blocks built from the same plan. Every block has a flat called "3B" (the UUID: which kind of room), but each block has its own street address (the device address). To deliver to one specific flat 3B, you need the street address first, then "3B".

So a scan filter on a service UUID finds all devices of that kind, and you choose the one you want by its address.

How I checked it

I read the Android reference: a BluetoothGattCharacteristic is "a basic data element used to construct a GATT service" and getUuid() returns the UUID of the characteristic; BluetoothDevice is "a thin wrapper for a Bluetooth hardware address", and getAddress() returns that address.

I didn't test this on two real devices, so this card stays a seedling until I do.

Check yourself

Sources

• Android reference: BluetoothGattCharacteristic
• Android reference: BluetoothDevice