About compression in BLE transport - CustomSDL/sdl_android_launcher GitHub Wiki

To increase the speed of data transfer via the BLE protocol, we apply preliminary compression of data that exceeds the current BLE bandwidth (MTU). Each SDL frame that can't be sent in a single BLE message, will be compressed, sent, and then decompressed by the receiver.

On the transmitting side, which is provided as a BluetoothLongWriter, it is determined whether the data should be compressed. To indicate that the data is compressed, we provide this information in an additional byte located after the information about the number of frames.

Thus, a separate data packet looks like this

  frames_count 4 bytes    compression 1 byte     data 
|______________________|______________________|__________ _ _ _ 

Accordingly, on the receiving side (BluetoothLongReader), information about whether the data has been compressed is read from the packet header. And if they were compressed, then the message will be decompressed.

Tools for compressing and decompressing data are provided in the CompressionUtil class in the byte [] compress (byte [] data) and decompress (byte [] data) methods, respectively. For these operations, the library implemented in the android SDK ZLIB is used, which made it possible for us not to import additional libraries into the project.

The same implementation is provided in the SPT application for tests of sending and receiving data.

There are some differences to be noted when using compression:

  			    |   without compression   |  with compression 
--------------------------------------------------------------------------
 RAI request / response     |           7 sec         |  1 sec
--------------------------------------------------------------------------                        
PutFile 5MB (filled with 0s)|          ~4 min         |  ~4 sec
--------------------------------------------------------------------------
RPC StartService            |           3 sec         |  2 sec