Inconsistent latency with ble_app_uart sending via ble

[Shri1395]

Hi,

We are trying to optimize the speed at which we send data over BLE UART with the nrf52832. Currently, we are using the ble_nus_data_send() function in a while loop with no delay to send as fast as possible. On the nrf toolbox app and the Adafruit BLE app, however we are getting a few sets of 20 bytes with very inconsistent latency (burst sends and hangs and bursts again at random intervals). How do we have the nrf52 send our 20 byte data consistently with low latency. Here is our latency output from our testing software.

Connected to device!

data received: 20 bytes

LATENCY: 8079

data received: 20 bytes

LATENCY: 241

data received: 20 bytes

LATENCY: 658

data received: 20 bytes

LATENCY: 0

data received: 20 bytes

LATENCY: 1

data received: 20 bytes

LATENCY: 5468

data received: 20 bytes

LATENCY: 50

data received: 20 bytes

LATENCY: 481

data received: 20 bytes

LATENCY: 490

data received: 20 bytes

LATENCY: 2

data received: 20 bytes

LATENCY: 528

data received: 20 bytes

LATENCY: 480

data received: 20 bytes

LATENCY: 1

data received: 20 bytes

LATENCY: 0

data received: 20 bytes

LATENCY: 238

data received: 20 bytes

LATENCY: 0

data received: 20 bytes

LATENCY: 1

data received: 20 bytes

LATENCY: 1

data received: 20 bytes

LATENCY: 4

data received: 20 bytes

LATENCY: 475

data received: 20 bytes

LATENCY: 1

data received: 20 bytes

LATENCY: 1

data received: 20 bytes

LATENCY: 0

data received: 20 bytes

LATENCY: 238

data received: 20 bytes

LATENCY: 1

data received: 20 bytes

LATENCY: 0

data received: 20 bytes

LATENCY: 1

data received: 20 bytes

LATENCY: 0

data received: 20 bytes

LATENCY: 176

data received: 20 bytes

LATENCY: 1

data received: 20 bytes

LATENCY: 61

data received: 20 bytes

LATENCY: 241

data received: 20 bytes

LATENCY: 181

data received: 20 bytes

LATENCY: 477

data received: 20 bytes

LATENCY: 840

data received: 20 bytes

LATENCY: 0

data received: 20 bytes

LATENCY: 1

data received: 20 bytes

LATENCY: 1

data received: 20 bytes

LATENCY: 0

data received: 20 bytes

LATENCY: 1

data received: 20 bytes

LATENCY: 5936

data received: 20 bytes

LATENCY: 241

data received: 20 bytes

LATENCY: 59

data received: 20 bytes

LATENCY: 241

data received: 20 bytes

LATENCY: 0

data received: 20 bytes

LATENCY: 179

data received: 20 bytes

LATENCY: 1

data received: 20 bytes

LATENCY: 0

data received: 20 bytes

LATENCY: 1263

data received: 20 bytes

LATENCY: 1

data received: 20 bytes

LATENCY: 0

data received: 20 bytes

LATENCY: 1

data received: 20 bytes

LATENCY: 235

data received: 20 bytes

LATENCY: 1

data received: 20 bytes

LATENCY: 1

data received: 20 bytes

LATENCY: 0

data received: 20 bytes

LATENCY: 0

data received: 20 bytes

LATENCY: 238

data received: 20 bytes

LATENCY: 3

Connection status: Disconnected from device.

When we try the same setup in SDK 12, we are able to get consistent latency. Using Adafruit's bootloader and library we are getting 5 ms consistent latency on SoftDevice 0.5.0 and 60 ms consistent latency on SoftDevice 5.0. All of these involve sending a 20 byte packet over the BLE UART.

Our current setup is with SDK 15 and SoftDevice 6.0 and we would like to optimize our sending to be as fast as it was with Adafruit's library. How do we acheive this consistent low latency via BLE? I've read that data rates can be as high as 128 kBps.

[Shri1395]

Hi,

We tried using this function, and it did actually make the data send rates a bit more faster. To be more precise, the SDK 11 was 4 times more faster than SDK 15, but after this test, we found out that SD15 is now almost as fast as the SDK 11. That means the speed did increase.

We set the transmission queue to 20.

My question is: We usually get a lot less speed in the beginning and then it fastens. Is there a way to constantly get the same speed, in order to perform better than SDK 11. 

PS: Quick information:

SDK 11, 20K bytes in 5.6 seconds.

SDK 15 before setting the hvn_tx_queue_size to 20 , 20k bytes in 21 seconds.

SDK 15 after the TX_COMPLETE function with hvn_tx_queue_size set to 20, 20k Bytes in 5.8 sec. 

[bjorn-spockeli]

Could you elaborate on what you mean by "We usually get a lot less speed in the beginning and then it fastens." 

If you want to maximize the throughput then I suggest that you increase the ATT MTU Size, i.e. increase the number of payload bytes in the packet. This is done by increasing NRF_SDH_BLE_GATT_MAX_MTU_SIZE in sdk_config.h

The S132 SoftDevice specification lists the throughput for different ATT MTU, connection interval and method configurations, it can be found under Bluetooth low energy data throughput.

[BTAdam]

Hi,

Can you please elaborate on how you were using Adafruit's library? We're seeing very similar inconsistent latency issues with using the Adafruit's nRF52 feather board and Adafruit's Arduino libraries. We're attempting to send 5 bytes at a 4ms interval and we'll see them being sent at intervals like 4ms, 4ms, 21ms, 4ms, 25ms, 4ms, etc. Was wondering how you were able to achieve consistent intervals with Adafruit's bootload and library.

Thank you,

Adam