Throughput of Nrf52832 with MTU 247

[Linxi]

I am going to find out an ideal throughput of NRF52832. And my environment is NRF52832/S332/MTU 247/connect interval 20ms.

Finally, i got a maximum value about 23Kb/s. I receive data and record timestamp from PC tools(programmed by C# to receive Nordic nus service notification data), when data transmission done,calculate the transmission speed.

Still confused as 23Kb/s is much lower than the target and I think i did everything in right way.

I found that i have only send one packet per connect interval，It's supposed to be more than one.

I use API: ble_nus_string_send to keep sending 244 bytes data until i get error return. Then I wait for BLE_GATTS_EVT_HVN_TX_COMPLETE to send data again. And I found every function call of ble_nus_string_send, i will get corresponding BLE_GATTS_EVT_HVN_TX_COMPLETE.

In my opnion, or the info i have got, for per connect interval, more than one packets can be transmitted(IOS:4, Android:6).

Follows are my questions:

1.Based on my environment, what is the actual transmission speed of Nordic Nus notification?

2.Is there any detail i forgot?  What should i do to improve the speed to more than 40Kb/s.

[Vidar Berg]

Hi,

23 Kb/s sounds a bit low. I can try to repeat the test here with the ble_app_uart sample. What SDK version is this based on? As you may have seen already, the Softdevice specification includes max. throughput numbers of when you have a nRF5 device on both sides of the link: Bluetooth Low Energy data throughput 

[Linxi]

Hi Vidar:

Thanks for your comments. My softdevice veriosn is S332 4.0.2. Perhaps the key is connection bandwidth. In SDK 13.10 document, I find no API to set this. Can you help?

[Vidar Berg]

Hi,

Attached below is a modified version of the ble_app_uart example found in SDK 13.1.0. It's very simple. It sends data as fast as it can to the client when notifications get enabled and use a timer to determine the transfer rate while doing so. Please try and test it on your side and see if it gives you any better results.

 UART log when I tested it with an iPhone 8:

 

Modified ble_app_uart example. It still uses s132, but that shouldn't make a difference.

ble_app_uart_throughput.zip

[Linxi]

Hi, thank you very much.

280 kbps mean 280 * 1000 / 8 / 1.024 = 34.18 KB/s?

My experiment speed is 23KB/s. Sorry for my error writing 23kb/s.

MTU 247,valid user data 244, I expect 4~6 packets per connection interval, means speed can be more than 47KB/s. Is anything wrong with my understanding?

My connection interval is 20ms and throughput is 23KB/s. It seems like only 2 packets sent per connection interval. 1000/20 * 2 * 244bytes / 1024 = 23.8KB/s. Do you have any ideas to improve the bandwidth to let me send more packets per connection interval ?

[Vidar Berg]

Hi,

Linxi said:

280 kbps mean 280 * 1000 / 8 / 1.024 = 34.18 KB/s?

 Yes, correct. Result of the same example with another nRF device on the other side of the link:

Linxi said:

MTU 247,valid user data 244, I expect 4~6 packets per connection interval, means speed can be more than 47KB/s. Is anything wrong with my understanding?

 No, your understanding is correct. The only problem is that the iPhone may choose to receive fewer packets. Here is a thread gives an example of how iPhones may limit the bandwidth: https://devzone.nordicsemi.com/f/nordic-q-a/40597/maximizing-nrf52-ios-ble-throughput-in-both-directions