Low Throughput Issues

[willdavies]

According to some benchmarks shown here: https://www.novelbits.io/bluetooth-5-speed-maximum-throughput/, I should be able to get a connection speed of around 120,000 bytes / second. However, in my testing, I can only get up to 20,000 bytes / second.

I have a nR52 2832 S132 chip that I connect to from an Android (galaxy s9) phone. The following settings are enabled:

-MTU: 247

-Data length extension: 251

-PHY: 2M

-TX Power: 4

-Connection interval: 7.5 min, 7.5 max

My code is available here in full: https://github.com/wdavies973/MagiCoil-microcode

Gatt init: (set mtu & data length extension)

static void gatt_init(void) {
ret_code_t err_code = nrf_ble_gatt_init(&m_gatt, gatt_evt_handler);
printf("GATT SUCCESS: %d\n",err_code);

conn_evt_len_ext_set();

printf("nrf_ble_mut_max: %d, mtu_default: %d\n", NRF_SDH_BLE_GATT_MAX_MTU_SIZE, BLE_GATT_ATT_MTU_DEFAULT);

err_code = nrf_ble_gatt_att_mtu_periph_set(&m_gatt, 247);
printf("mtu: %d\n",err_code);

err_code = nrf_ble_gatt_data_length_set(&m_gatt, BLE_CONN_HANDLE_INVALID, 251);
printf("data_length: %d\n",err_code);
}

PHY & MTU update requests:

static void ble_evt_handler(ble_evt_t const * p_ble_evt, void * p_context)
{
uint32_t err_code;

switch (p_ble_evt->header.evt_id)
{
case BLE_GAP_EVT_CONNECTED:
printf("Connected");
err_code = bsp_indication_set(BSP_INDICATE_CONNECTED);
APP_ERROR_CHECK(err_code);
m_conn_handle = p_ble_evt->evt.gap_evt.conn_handle;
err_code = nrf_ble_qwr_conn_handle_assign(&m_qwr, m_conn_handle);
APP_ERROR_CHECK(err_code);

err_code = sd_ble_gap_tx_power_set(BLE_GAP_TX_POWER_ROLE_CONN, m_conn_handle, 4);

break;

case BLE_GAP_EVT_DISCONNECTED:
NRF_LOG_INFO("Disconnected");
// LED indication will be changed when advertising starts.
m_conn_handle = BLE_CONN_HANDLE_INVALID;
advertising_start(true);
break;
case BLE_GAP_EVT_CONN_PARAM_UPDATE_REQUEST:
{
ble_gap_conn_params_t params;
params.max_conn_interval = MAX_CONN_INTERVAL;
err_code = sd_ble_gap_conn_param_update(p_ble_evt->evt.gap_evt.conn_handle, &params);
} break;
case BLE_GAP_EVT_CONN_PARAM_UPDATE:
{
uint16_t max_con_int = p_ble_evt->evt.gap_evt.params.conn_param_update.conn_params.max_conn_interval;
uint16_t min_con_int = p_ble_evt->evt.gap_evt.params.conn_param_update.conn_params.min_conn_interval;

printf("conn updated\n");

//m_ble_params_info.con_interval = max_con_int;
//ble_its_ble_params_info_send(&m_its, &m_ble_params_info);
//NRF_LOG_INFO("Con params updated: CI %i, %i", (int)min_con_int, (int)max_con_int);
} break;

case BLE_GAP_EVT_PHY_UPDATE_REQUEST:
{
printf("PHY update request.");
ble_gap_phys_t const phys =
{
.rx_phys = BLE_GAP_PHY_2MBPS,
.tx_phys = BLE_GAP_PHY_2MBPS,
};
err_code = sd_ble_gap_phy_update(p_ble_evt->evt.gap_evt.conn_handle, &phys);
printf("PHY update to 2MBPS: %d\n",err_code);
} break;

case BLE_GAP_EVT_PHY_UPDATE:
//m_ble_params_info.tx_phy = p_ble_evt->evt.gap_evt.params.phy_update.tx_phy;
// m_ble_params_info.rx_phy = p_ble_evt->evt.gap_evt.params.phy_update.rx_phy;
// ble_its_ble_params_info_send(&m_its, &m_ble_params_info);
printf("Phy update: %i, %i", (int)p_ble_evt->evt.gap_evt.params.phy_update.tx_phy, (int) p_ble_evt->evt.gap_evt.params.phy_update.rx_phy);
break;

case BLE_GAP_EVT_SEC_PARAMS_REQUEST:

// Pairing not supported
err_code = sd_ble_gap_sec_params_reply(m_conn_handle, BLE_GAP_SEC_STATUS_PAIRING_NOT_SUPP, NULL, NULL);
APP_ERROR_CHECK(err_code);
break;

/*case BLE_GATTS_EVT_SYS_ATTR_MISSING:
// No system attributes have been stored.
err_code = sd_ble_gatts_sys_attr_set(m_conn_handle, NULL, 0, 0);
APP_ERROR_CHECK(err_code);
break;*/

case BLE_GATTC_EVT_TIMEOUT:
// Disconnect on GATT Client timeout event.
err_code = sd_ble_gap_disconnect(p_ble_evt->evt.gattc_evt.conn_handle,
BLE_HCI_REMOTE_USER_TERMINATED_CONNECTION);
APP_ERROR_CHECK(err_code);
break;

case BLE_GATTS_EVT_TIMEOUT:
// Disconnect on GATT Server timeout event.
err_code = sd_ble_gap_disconnect(p_ble_evt->evt.gatts_evt.conn_handle,
BLE_HCI_REMOTE_USER_TERMINATED_CONNECTION);
APP_ERROR_CHECK(err_code);
break;

default:
// No implementation needed.
//NRF_LOG_INFO("BLE event not handled by app: %i", p_ble_evt->header.evt_id);
break;
}
}

And finally, the send method:

void ble_cus_custom_value_update(ble_cus_t * p_cus)
{
uint8_t data[244];
uint16_t payload_len = 244;

ble_gatts_hvx_params_t hvx_param;

memset(&hvx_param, 0, sizeof(hvx_param));

hvx_param.handle = p_cus->custom_value_handles.value_handle;
hvx_param.type = BLE_GATT_HVX_NOTIFICATION;
hvx_param.p_len = &payload_len;
hvx_param.p_data = data;

uint32_t err_code = NRF_SUCCESS;

while(err_code == NRF_SUCCESS) {
(void)uint32_encode(0, data);

err_code = sd_ble_gatts_hvx(p_cus->conn_handle, &hvx_param);

if (err_code == NRF_ERROR_RESOURCES)
{
// Wait for BLE_GATTS_EVT_HVN_TX_COMPLETE.
busy = true;
break;
}
else if (err_code != NRF_SUCCESS)
{
printf("sd_ble_gatts_hvx() failed: 0x%x", err_code);
}
}

}

Android Code:

@Override
            public void onCharacteristicChanged(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic) {
                super.onCharacteristicChanged(gatt, characteristic);

               // if(status == BluetoothGatt.GATT_SUCCESS) {
                    // characteristic available for reading
                    //final byte[] data = characteristic.getValue();
                   // System.out.println("Receiving characteristic read"+characteristic.getValue().length);

                if(receivedBytes > 10_000) {

                    double elapsedSeconds = (System.nanoTime() - startTime) / 1000000000.0;

                    double tp = 10_000.0 / elapsedSeconds;

                    System.out.println("TP: "+tp);
                    receivedBytes = 0;
                    startTime = System.nanoTime();
                } else {
                    receivedBytes+=244;
                }

//                    if (data != null && data.length > 0) {
//                        final StringBuilder stringBuilder = new StringBuilder(data.length);
//                        for(byte byteChar : data)
//                            stringBuilder.append(String.format("%02X ", byteChar));
//                        intent.putExtra(EXTRA_DATA, new String(data) + "\n" +
//                                stringBuilder.toString());

              //  }
            }

[haakonsh]

"Connection interval: 7.5 min, 7.5 max" As far as i know the Android BLE controller does not support a lower Connection Interval than 15ms, but I've not seen the spec for the galaxy 9. 

Have you enabled Connection Event Length Extension?


We need a sniffer trace of the connection establishment and connection parameter negotiation. 

[willdavies]

As far as I know, the Android device successfully connects with the connection interval as 7.5 min, 7.5 max.

The Android device receives a MTU update of 247, and a PHY update of rx=2, tx=2 as well.

I currently call the following function to enable event length extension: https://github.com/wdavies973/MagiCoil-microcode/blob/943c757fee94b1141a1ee2b6c8374bcdf40c6453/main.c#L642

I do also receive data packets of length 244 on the Android device.

[haakonsh]

And what Connection interval, MTU length, Data length extension, and Connection event length are you getting? 

[willdavies]

The Android device reports MTU length 247 and I receive packets of length 244 (data payload). The connection interval is defined as 7.5 min, 7.5 max when setting up the connection. I'm not sure how to read these values on the chip to see if they were successfully setup.

[haakonsh]

There's a difference between what parameters you tell the SoftDevice that you would like to get and what the actual outcome of the negotiations between the Link Layers are. You need to verify those parameters, I suggest you set up a sniffer (nRF Sniffer User Guide v2.1) to see what the negotiated parameters are. 

My guess is that you get a 100ms connection interval because the android stack does not support 7.5ms and you have not given it any other option but 7.5ms. 

See Data Length Update Procedure and GATTC ATT_MTU Exchange for examples.

[willdavies]

The Android device is actually reporting the interval:

D/BluetoothGatt: onConnectionUpdated() - Device=FC:88:C7:C9:5C:82 interval=9 latency=0 timeout=400 status=0

The interval is 9 ms, so it seems like something else might be causing the issue.

[willdavies]

I was able to resolve the issue, thanks for the help! 

Changed connection interval to 400 ms and event length to:

#define NRF_SDH_BLE_GAP_EVENT_LENGTH 400