Sending more than 1 packet per connection with nRF52840

[DL_November]

As I'm new to the nRF52840 and Bluetooth, I read documents and DevZone posts related to the bluetooth maximum throughput.

According to the references like Jimmy and Sigurd and so on, it seems that I can get higher throughput if I set MTU to the maximum value and send multiple packets per connection.

The problem is that it was not able for me to find options to enable sending multiple packets.

Currently, I mixed ble_app_uart and saadc examples to get signal and send them via bluetooth. And the data length extension and MTU size are set to the maximum as I know.

And I send data using the following code which is written by another team member

static void nus_data_handler(ble_nus_evt_t * p_evt)
{

    if (p_evt->type == BLE_NUS_EVT_RX_DATA)
    {
        uint32_t err_code;

        NRF_LOG_HEXDUMP_DEBUG(p_evt->params.rx_data.p_data, p_evt->params.rx_data.length);

        for (uint32_t i = 0; i < p_evt->params.rx_data.length; i++)
        {
            do
            {
                err_code = app_uart_put(p_evt->params.rx_data.p_data[i]);
                if ((err_code != NRF_SUCCESS) && (err_code != NRF_ERROR_BUSY))
                {
                    APP_ERROR_CHECK(err_code);
                }
            } while (err_code == NRF_ERROR_BUSY);
        }
        if (p_evt->params.rx_data.p_data[p_evt->params.rx_data.length - 1] == '\r')
        {
            while (app_uart_put('\n') == NRF_ERROR_BUSY);
        }
    }

}

Are there any examples sending multiple packets per connection so I can try to implement to mine?

[Einar Thorsrud]

Hi,

Generally, to achieve a high throughput you want to send longer packets, and often send multiple packets per connection event. So you should ensure you set a long NRF_SDH_BLE_GATT_MAX_MTU_SIZE and NRF_SDH_BLE_GAP_DATA_LENGTH, and that you set a high NRF_SDH_BLE_GAP_EVENT_LENGTH (for highest throughput the event length should be equal to the connection interval). The event length is specifically wha tyou need to adjust in order to send multiple packet per connection event, as you need more time to send more packets.

There is no specific API for sending more packets per connection event, you just queue up notifications (for instance using ble_nus_data_send() in this case, which calls sd_ble_gatts_hvx()). As long as the SoftDevice has queued data, there is enough time left in the connection event and previous packet is acked, the next packet will also be transmitted, and so on.

There is no specific performance issue with using NUS, as this uses notifications, which is what gives best performance. But that assumes that you do not combine it with physical UART, which is really completely independent.

[DL_November]

Fisrt of all, thanks for the answer!

Einar Thorsrud said:

for highest throughput the event length should be equal to the connection interval

So, in the case of 10 ms(=8 units of 1.25 ms) connection internal, does it mean that I can get the highest throughput by setting the NRF_SDH_BLE_GAP_EVENT_LENGTH to 8 in sdk_config.h?

Einar Thorsrud said:

you just queue up notifications (for instance using ble_nus_data_send() in this case, which calls sd_ble_gatts_hvx()).

Is this mean that I don't need to define some variables, but only need to use ble_nus_data_send()? And then the notifications are automatically queued by the SoftDevice?

Einar Thorsrud said:

the next packet will also be transmitted, and so on

And then, maybe I get the different number of packets per connection, right?

[Einar Thorsrud]

Hi,

DL_November said:

So, in the case of 10 ms(=8 units of 1.25 ms) connection internal, does it mean that I can get the highest throughput by setting the NRF_SDH_BLE_GAP_EVENT_LENGTH to 8 in sdk_config.h?

Yes. Thogh in practice you may get higher throughput with a longer connection interval, assuming you also increase the event length correspondingly. (You can experiment with the effect of this using the throughput example in the SDK).

DL_November said:

Is this mean that I don't need to define some variables, but only need to use ble_nus_data_send()? And then the notifications are automatically queued by the SoftDevice?

Yes. Up to when there is no room left in the queue, and in that case you will get NRF_ERROR_RESOURCES returned (then you typically wait for a BLE_GATTS_EVT_HVN_TX_COMPLETE event before trying again).

DL_November said:

And then, maybe I get the different number of packets per connection, right?

Yes. The actual number of packets per connection event depends on a lot of factors.

[DL_November]

Hi, Einar!

 Though the problem was solved a month ago, I found some weird thing happens related to this issue.

 Thanks for your help, now I set the GAP_EVENT_LENGTH to 400 which is enough for the 7.5~15 ms of connection interval.

 But when I check the number of packets per connection interval with an nRF connect the application with an Android device, the maximum number of packets does not exceed 6.

 According to the throughput demo, I think the 1365 kbps speed appears when 8 packets are sent during a single connection interval.

 Am I missing something which limits the packet transmission? 

[DL_November]

P.S. If I reduce the size of data from 240 Bytes to 100 Bytes, 8 packets are sent during a connection. Still far lower than the expected number of packets considering transmission time...

[Einar Thorsrud]

Hi,

The actual parameters you get will depend on the configuration and capabilities of both peer devices. So you should expect significant differences between phone models and Android versions, and also other factors that could affect the scheduling on the phone (other BLE devices, and even WiFi traffic etc).

[DL_November]

Einar Thorsrud said:

The actual parameters you get will depend on the configuration and capabilities of both peer devices.

 As the Android device does not use any WiFi or other Bluetooth device, I cannot expect the cause of sending fewer packets. Does the demo also show a decrease in throughput if it transmits with an Android?