Why too much data is lost when there is 11 peripheral connected to 1 central via NUS connection

[laumung]

Hello, I have a big problem now. I set up a BLE network with 11 peripheral boards connected to 1 central board via NUS protocol. the peripheral board is based on NRF52840 and the central board is NRF52840 DK board. The peripheral board read the sensor at 50Hz and 16 bytes per reading. Every 14 readings, the sensor board will send the data as s packet to the central board, and the central board transfers data to the laptop via the UART port. When there is only one peripheral, it is fine.

But when the peripheral boards increase, the data will be lost and most data cannot transfer to the laptop. What causes the problem? Thanks. Any help will be appreciated.

[Einar Thorsrud]

Hi,

You are sending quite long packets (sensor data per packet is 16 byte * 14 readings = 224 byte), so it takes a bit of time on air. Ignoring any overhead and just looking at sensor data this is about 9 kbps with 11 peripherals ,which in itself is not too much. However, the central will need to schedule time for all connections. Depending on connection parameters and number peripherals the SoftDevice scheduling may be difficult. You can see more about SoftDevice scheduling here, and specifically Connection timing as a Central is relevant.

With your fairly low total data rate you should be able to get everything through with sensible connection parameters. Which connection parameters do you use? It may make sense to send less data more often, using shorter event lengths and shorter packets. That way each connection can exchange data more frequency and you can add some margin for retransmissions (which will happen more seldom with shorter packets).

[laumung]

Hi Einar, I reduced the data size in the packet to 8 and 4 tonight. Unfortunately the problem were still there. When up to 8 or 9 peripherals added to the BLE network, the central board will loose all the connection and re-connect to the peripherals after reset all the peripherals. BTW, can we find out what cause the problem by the debug information? Thanks.

[Einar Thorsrud]

Hi,

You are right that for maximum throughput you should use long packets in general. I was thinking it might not be a good idea in this case but that is not the major concern. The major point is that you should adapt the event length to the length of the packet you send, and ensure that you can fit an event for each connection within each connection interval, as shown in Connection timing as a Central. What packet event length do you use with which packet length? You should make sure that fits in the connection interval you use. What is that (I just see min and max in your peripheral)?

[laumung]

Hi Einar, I understood what you said in your poster. Which parameter should be modified to make more peripherals? I changed the APP_ADV_INTERVAL from 64(default) to 600 and the peripherals can add up to 15. But the central board will reset every 6 or 7 seconds after the change even the number of peripherals increase. The debug information of the central board is shown below.

Which parameter else should I change to stop the reset?

[Einar Thorsrud]

Please let me know your complete configuration. The relationship between connection interval, event length, packet length and number of connections is essential as you can see here. Also, what is the cause for the reset?

[laumung]

Hi, here is the configuration for connection in central board.

// <o> NRF_SDH_BLE_GAP_EVENT_LENGTH - GAP event length. 
// <i> The time set aside for this connection on every connection interval in 1.25 ms units.

#ifndef NRF_SDH_BLE_GAP_EVENT_LENGTH
#define NRF_SDH_BLE_GAP_EVENT_LENGTH 6
#endif

// <o> NRF_BLE_SCAN_MIN_CONNECTION_INTERVAL - Determines minimum connection interval in milliseconds. 
#ifndef NRF_BLE_SCAN_MIN_CONNECTION_INTERVAL
#define NRF_BLE_SCAN_MIN_CONNECTION_INTERVAL 7.5
#endif

// <o> NRF_BLE_SCAN_MAX_CONNECTION_INTERVAL - Determines maximum connection interval in milliseconds. 
#ifndef NRF_BLE_SCAN_MAX_CONNECTION_INTERVAL
#define NRF_BLE_SCAN_MAX_CONNECTION_INTERVAL 30
#endif


// <o> NRF_SDH_BLE_GAP_EVENT_LENGTH - GAP event length. 
// <i> The time set aside for this connection on every connection interval in 1.25 ms units.

#ifndef NRF_SDH_BLE_GAP_EVENT_LENGTH
#define NRF_SDH_BLE_GAP_EVENT_LENGTH 6
#endif


// <o> NRF_SDH_BLE_PERIPHERAL_LINK_COUNT - Maximum number of peripheral links. 
#ifndef NRF_SDH_BLE_PERIPHERAL_LINK_COUNT
#define NRF_SDH_BLE_PERIPHERAL_LINK_COUNT 0
#endif

// <o> NRF_SDH_BLE_CENTRAL_LINK_COUNT - Maximum number of central links. 
#ifndef NRF_SDH_BLE_CENTRAL_LINK_COUNT
#define NRF_SDH_BLE_CENTRAL_LINK_COUNT 16
#endif

// <o> NRF_SDH_BLE_TOTAL_LINK_COUNT - Total link count. 
// <i> Maximum number of total concurrent connections using the default configuration.

#ifndef NRF_SDH_BLE_TOTAL_LINK_COUNT
#define NRF_SDH_BLE_TOTAL_LINK_COUNT 16
#endif

[laumung]

Is the packet length parameter defined as below?

// <o> NRF_SDH_BLE_GATT_MAX_MTU_SIZE - Static maximum MTU size. 
#ifndef NRF_SDH_BLE_GATT_MAX_MTU_SIZE
#define NRF_SDH_BLE_GATT_MAX_MTU_SIZE 247
#endif

BTW, there is no error code shown for the resetting. How can I program to send debug information when resetting occurs?