How can I increase the throughput of my BLE application?

Question

I have an android client that functions as a central and have an app on my MAC (peripheral) that this central connects to and sends data.

At this point, I need to wait almost 100ms after I call writeCharacteristic(..) to receive the onCharacteristicWrite(..) callback. I am sending strings. If I send smaller strings, the throughput is great (understandably). When the string contains about 200 characters and I send 20 byte chunks, it takes almost a second before the entire string is seen at the peripheral. When I set the write type to NO_RESPONSE before writing the characteristic, I see no data on the peripheral.

After I connect, I have done the following to improve throughput:

• Stopped discovery after services are discovered because it is an expensive operation
• I set the write type to default first - When I do this, I see data on the peripheral. But, there is a significant delay. When I set the writeType to NO_RESPONSE, I see no data on the peripheral. I have no logic in onCharacteristicWrite(..) either. Sometimes, I see the data getting truncated on the peripheral.
• I have set the desired connection latency to low on my mac app. Is there a way to set a value (as 7.5ms perhaps?).
• When I set the write type to default and send a string of 200 characters - I split the string into 20 byte chunks. I now have 10 chunks to send. If I set characteristic value and call writeCharacteristic(..) in loop, I see no data. When I add a ~100ms delay after writeCharacteristic(..) before it executes the next iteration of the loop, I see data on the peripheral.

I see a huge increase in throughput between an iOS central - iOS peripheral. I don't see why Android central - iOS peripheral shouldn't work he same way. From my understanding, Android and iOS use the same chip.

Any reason the performance is so poor? Is there anything else I can do to improve throughput?

Answer 1

Please have a look at the MTU size. My experience:

Using a iOS central, the central automatically starts the MTU size negotiation with some large value. I think it is larger than 200 bytes.

On most Android devices I tested this does not start automatically but you have to start the MTU size negotiation by your app (central). If you do not do that, Android cuts your data into 20 byte pieces. This has big influence on your throughput.