SDL core BLE sequences and mechanics - CustomSDL/sdl_android_launcher GitHub Wiki

Note, that in BLE communication each device can have either a peripheral or central role.

What’s BLE Central?

A Central is a device, which scans for Bluetooth devices to connect and utilize the information hosted by them. Usually, Central devices are richer in terms of resources such as computing power compared to Peripheral devices. Pre-connection: at startup, a device called Central device, after connected, it called master.

What’s BLE Peripheral?

BLE peripheral could be scanned by the Bluetooth Central device. After a BLE connection has been established, the peripheral device is called a slave.

Role of SDL Android Launcher

SDL Android Launcher is implemented to have a Central role.

Scanning for devices

When the user initiates BLE communication from SDL Android Launcher, it starts scanning all BLE-compatible devices nearby. Once the list of devices is collected, Launcher filters out the list and keeps only those devices that have an advertisement for SDL UUID service (can be specified in the config file in SdlTesterServiceUUID param). After that, Launcher is attempting to connect to the first device in this list.

Establishing BLE connection

To establish BLE communication, you need at least 2 physical devices - one with SDL Android Launcher installed and one with a Mobile application that supports BLE transport. The mobile application should start BLE advertisement if it is going to connect to Launcher over BLE. Once the advertisement is active, the central device will be able to find a peripheral device during the scanning sequence.

NOTE. Peripheral and central devices should NOT BE paired with each other. Both devices should have Bluetooth enabled.

NOTE 2. The central device should also have GPS enabled as BLE implementation requires location permissions according to Android API.

NOTE 3. BLE connection will be closed if the screen of either device is locked (BLE feature), so make sure that the screen of both devices is unlocked during the BLE connection (this can be specified in Developer settings on the Android device).

The process of connection establishing is described by the following sequence diagram - sources image

BLE data flow

If the connection was successfully established, Launcher can initiate corresponding connection channels in SDL core and perform messages exchange in both directions. As BLE is a low-bandwidth transport, there was used a compression feature to reduce size of messages sent over the BLE connection and improve its performance. More details can be found here. The data exchange flow is described by the following sequence diagram - sources image

Finalizing BLE connection

If the BLE connection was not successful, Launcher restarts the scanning sequence described above and tries to connect to the first device in a new list collected after the new scanning attempt. The "scan-connect-disconnect-scan" sequence is repeated until the user requests to stop BLE transport by clicking a separate button on the Main Activity. The following sequence diagram shows what is going on when use clicks this button - sources image