如何自定义 Android BLE（蓝牙低功耗）gatt 传输速度？

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/ .