How to achieve max throughput with NUS service?

[Philip]

Hi All.

I have a project where my peripheral (nRF52832) connects to my central (nRF52840), and then transmits 128 byte packets continuously one after the other. It works ok, but data rate is about 5,000 bytes per second max (40,000 bps). This seems a little low. I am using the following event to trigger the message, so this should be the quickest way to send one after the other?

case BLE_NUS_EVT_TX_RDY:

            send_data();

            break;

Send data;

void send_data(void)
{
    uint8_t testData[128];
    ret_code_t err_code;

    for (uint8_t i = 0; i < 128; i++) {
       testData[i] = i;
    }
    testData[0] = P0;
    testData[1] = P1;
    testData[2] = P2;
    testData[3] = page_count;


    //TODO - Add something here so if it fails X number of times, we stop download and disconnect 
    uint16_t length = (uint16_t)128;
    err_code = ble_nus_data_send(&m_nus, testData, &length, m_conn_handle);
    if(err_code == NRF_SUCCESS)
    {
      P1++;
      if(P1 == 0) P2++;
      page_count--;
    }
}

Also here are my settings for the radio times which seem to give the best results (same on both central and peripheral);

#define MIN_CONNECTION_INTERVAL   MSEC_TO_UNITS(7.5, UNIT_1_25_MS)      /**< Determines minimum connection interval in milliseconds. */
#define MAX_CONNECTION_INTERVAL   MSEC_TO_UNITS(10, UNIT_1_25_MS)       /**< Determines maximum connection interval in milliseconds. */
#define SLAVE_LATENCY             0                                     /**< Determines slave latency in terms of connection events. */
#define SUPERVISION_TIMEOUT       MSEC_TO_UNITS(4000, UNIT_10_MS)       /**< Determines supervision time-out in units of 10 milliseconds. */

#ifndef NRF_SDH_BLE_GAP_DATA_LENGTH
#define NRF_SDH_BLE_GAP_DATA_LENGTH 251
#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 1
#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 0
#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 1
#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_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

I don't really understand the above settings to be honest, but from what I have read should give the best throughput. However 40kbps seems a bit slow considering both ends are nRF52 devices?

Thanks

Phil

[ovrebekk]

Hi Phil

The SoftDevice team wrote a nice guide on how to set connection parameters and stack configuration properly for good throughput, and I would suggest reading through that first. 

The guide is available here. 

If something is still unclear after reading through it just let me know, and I will do my best to help

Best regards
Torbjørn

[Philip]

Thanks Torbjørn, I will take a look at that guide.

I managed to boost the speed a fair bit by calling "ble_nus_data_send" in a while loop until its response doesn't equal NRF_SUCCESS rather than just once in the BLE_NUS_EVT_TX_RDY event.

Ill try some other tweeks once I have read the guide.

Thanks

Phil

[Philip]

Hi Torbjørn,

Yes it is working better now, see above message for latest graph. The packets are 240 bytes. Phy is auto so probably 1Mbps.

I figured out the issue, and set interval to 75 - 100mS, and the event length to 60. It now sends loads of packets in each interval. Maybe before the settings we such that it would only do 1 packet. Changing the settings meant I needed to increase the RAM for softdevice, so it probably has a buffer enabled somewhere.

Thanks for your help.

Phil

[ovrebekk]

Hi Philip 

Good to hear you figured it out and got better performance! 

With Nordic devices on both sides you could also switch to 2Mbps mode for even higher throughput (or lower average current), but doing so will reduce the maximum range by around 30%. 

Best regards
Torbjørn

[Philip]

Ok, might try that at some point.

Is there an example anywhere how to change to 2Mbps?

Best Regards

Phil

[ovrebekk]

Hi Philip 

Requesting 2Mbps mode once you are in a connection is quite easy. 

Just run the following code (copied from line 594 of this file):

ble_gap_phys_t phys;
phys.rx_phys = BLE_GAP_PHY_2MBPS;
phys.tx_phys = BLE_GAP_PHY_2MBPS;
err_code = sd_ble_gap_phy_update(m_conn_handle, &phys);
APP_ERROR_CHECK(err_code);

Best regards
Torbjørn

[Philip]

Great thanks Torbjørn, ill give that a go.

Best Regards

Phil