Small throughput in nrf52-ble-app-uart-c-multilink

[Michael01101]

Hello,

currently I am working on a multilink project. I have a nRF52840DK as central and 4 evaluation boards (BMD-300-EVAL, contains nRF52832) as peripherals. 

Code Central: nRF_ble_app_uart_c_multilink (https://github.com/NordicPlayground/nrf52-ble-app-uart-c-multilink)

Code Peripheral: ble_app_uart

Each peripheral generates 8kBytes per second. So each peripheral needs to send data at 64kBit/s to the central. This should be possible in theory. But in reality I am unable to get only one peripheral to work. At the moment I trigger the ble_nus_data_send() function of ble_app_uart with a timer every 30ms and a payload of 240 Bytes to get this 64kBit/s. But when 3 to 10 pakets are queued I get a NRF_ERROR_RESOURCES.

I already use every option which increases throughput:

<info> app: Phy Updated. TX: 2, RX: 2               //2MBPS
<info> app: Data len is set to 0xF4(244)            //huge MTU
<info> app: LL packet size is set to 251            //Data Length Extension

#define NRF_SDH_BLE_GAP_EVENT_LENGTH 20             //Event Length Extension

hvn_tx_queue_size = 20

I spent a lot of time with testing different configurations of the connection parameter, but nothing seems to work. I always get NRF_ERROR_RESOURCES immediately. Normally I should be able to transfer huge amounts of data with this high throughput parameters, as I experienced it in the throughput example.

Can you please help, to get this 64 kBit/s per peripheral to work. 

1. Which connection parameters do I need?

2. Where is the problem?

Thank you very much in advance. I really need some help at this point.

Best regards,

Michael

[Kenneth]

Hi,

I recommend to first check out these two pages if you haven't already: 
https://infocenter.nordicsemi.com/topic/sds_s140/SDS/s1xx/multilink_scheduling/central_connection_timing.html 
https://www.novelbits.io/bluetooth-5-speed-maximum-throughput/ 

To update the queue size you should follow the code example you can find at the bottom of this reply, this will add a few more lines to nrf_sdh_ble_default_cfg_set() to update the hvn_tx_queue_size:
https://devzone.nordicsemi.com/f/nordic-q-a/64837/change-queue-length-of-handle-value-notifications-sdk-15-3-0/265091#265091

If you still have problems, I suggest you also look at the scan parameters you are using while maintaining a central link(s). The scan parameters can affect the central link(s). For instance try to set NRF_BLE_SCAN_SCAN_INTERVAL and NRF_BLE_SCAN_SCAN_WINDOW both to 8 (=5ms), which should schedule scanning between active links.

Kenneth