How can I customize Android BLE (Bluetooth Low Energy) gatt transfer speed?

Question

I'm building an application that transfers images using BLE technology. The images need not be transferred right away, hence upon reading some documents, I concluded 1-3 Mbit/s is a reasonable choice.

(From Wikipedia)

Distance/Range <100 m (<330 ft)
Over the air data rate  125 kbit/s – 1 Mbit/s – 2 Mbit/s
Application throughput  0.27-1.37 Mbit/s [38]

However, actual testing with BLE showed the transmission rate is very slow. (almost 100 byte/s) I was testing with,

• two latest update Android phones: Samsung galaxy 8, Samsung galaxy note 10.
• debugging with IntelliJ

To enhance the rate of transfer, I changed the buffer size from default to 500 byte using below code.

 gatt.requestMtu(CommonConstants.FILE_BUFFER_SIZE);
 @Override
 public void onMtuChanged(BluetoothGatt gatt, int mtu, int status) {
    Log.e("ERROR", "stats=>" + String.valueOf(status));
    Log.e("INFO", "CONNECTED");
    Log.i("ERROR", "Connected to GATT server.");
    Log.i("ERROR", "Attempting to start service discovery:" + gatt.discoverServices());
 }

However, changing Maximum Transfer Unit did not change the rate of data transfer.

• What stuffs I am missing out with BLE regarding data transfer speed?
• Shouldn't increased MTU result in increased speed in data transfer?

Answer 1

I faced the same problem when I hard coded the payload length to 20 bytes and it took too long to transfer 1MB of file. So, I read about MTU

MTU (Maximum Transfer Unit) : MTU specifies the maximum number of bytes that can be sent in a single write operation.

Just requesting for MTU will not speed-up the transfer time. requestMtu() will simply return the maximum number of bytes that can send to other device while writing on device.

To speed-up process you need to update the payload which you are sending to write on connected device.

Let's assume your current bytes were sent as 100 bytes /sec:

Total Bytes: 5000 bytes

Payload length: 100 bytes

1 write operation: 100 bytes / sec

Total write operations will be: 5000/100 = 50 sec

Now let's say after requesting MTU value from device, it comes out to be 250 bytes, so again:

Total Bytes: 5000 bytes

Payload length: 250 bytes

1 write operation: 250 bytes / sec

Total write operations will be: 5000/250 = 20 sec // see the time is reduced now.

So, how you can achieve this:

1: Connect to the device

2: Request for MTU value (with maximum 512 and it will return you the supported MTU value of device. In most cases it is 251, but just for the information 3 bytes are used for internal purposes, so the maximum size is MTU-3 )

3: Use this MTU value to create the payload

4: Do the write operation

Answer 2

A large MTU with a short connection interval usually gives the best performance. If both devices support LE Data Length Extension and 2 Mbit/s PHY then you will increase the performance to the max.

See https://www.novelbits.io/bluetooth-5-speed-maximum-throughput/ .