Verifying connection event length extension and data length extension

[Justin Gasprom]

Hi, NRF52832, SDK 15.2, SD 132 6.1.0

1. How to verify if connection event length extension is enabled (or supported)?

In ble_app_att_mtu_throughput in function conn_evt_len_ext_set you are using err_code = sd_ble_opt_set(BLE_COMMON_OPT_CONN_EVT_EXT, &opt). It is mentioned in documentation that sd_ble_opt_get is not supported for BLE_COMMON_OPT_CONN_EVT_EXT. I tried sd_ble_opt_set method with smartphones supporting and not supporting BLE 4.2 and on both phones this function returns without any errors. Also in ble_app_att_mtu_throughput example you are just setting the new value with sd_ble_opt_set and saying that connection event length was enabled without any read back.

Not sure if this is related but in In BLE 5 specification (BLUETOOTH SPECIFICATION Version 5.0 | Vol 6, Part F) page 2781 I found there is some LE test status event returning byte if packet length extension  as well as 2M PHY is supported, but I was unable to find something like this in SDK.

2. Do I need to enable Data length extension (DLE) separately? How to verify it is supported?

In the same ble_app_att_mtu_throughput  you are setting connection event length extension but naming it Data length extension:

    NRF_CLI_CMD(conn_evt_len_ext,   &m_cele_cmd,           "Enable or disable Data Length Extension (on or off)", default_cmd),

even though it is clear that it is connection event length extension.

In nrf528xx-ble-throughput-demo-master which is for SDK 13 you had two different parameters: connection event length extension and data length extension.

static test_params_t m_test_params =
{
    .att_mtu                  	= 247,
    .conn_interval            	= 400.0f,
    .data_len_ext_enabled     	= true,
    .conn_evt_len_ext_enabled	= true,

I know the difference between those two, but I am very confused if these two options are still separate in SDK 15 and how to enable and verify DLE.

[Vidar Berg]

Hi,

It's recommended to use sdk_config.h to set desired MTU and DLE length. These settings are applied when calling nrf_sdh_ble_default_cfg_set() in ble_stack_init().  Data length (MTU/DLE) is then negotiated with the peer device after the connection is established, typically trough the GATT module. 

Initialize the GATT module as follows if you want to request the max.  MTU size:

/**@brief Function for initializing the GATT library. */
void gatt_init(void)
{
    ret_code_t err_code;

    err_code = nrf_ble_gatt_init(&m_gatt, gatt_evt_handler);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_ble_gatt_att_mtu_periph_set(&m_gatt, NRF_SDH_BLE_GATT_MAX_MTU_SIZE);
    APP_ERROR_CHECK(err_code);
}

You can then check the result of the negotiation by monitoring the callback from the GATT module. E.g., 

/**@brief Function for handling events from the GATT library. */
void gatt_evt_handler(nrf_ble_gatt_t * p_gatt, nrf_ble_gatt_evt_t const * p_evt)
{
    if ((m_conn_handle == p_evt->conn_handle) && (p_evt->evt_id == NRF_BLE_GATT_EVT_ATT_MTU_UPDATED))
    {
        NRF_LOG_INFO("MTU size is set to 0x%X(%d)", p_evt->params.att_mtu_effective, p_evt->params.att_mtu_effective);
    }

    if ((m_conn_handle == p_evt->conn_handle) && (p_evt->evt_id == NRF_BLE_GATT_EVT_DATA_LENGTH_UPDATED))
    {
         NRF_LOG_INFO("LL packet size is set to %d", p_gatt->data_length);
    }
}

Connection event length extension enables the softdevice to overstay the configured event length in sdk_config.h (see NRF_SDH_BLE_GAP_EVENT_LENGTH), and should be enabled if you want to maximize throughput. Excerpt from SDS:

"To get the maximum bandwidth on a single link, it is recommended to enable Connection Event Length Extension and increase the connection interval. This will allow the SoftDevice to send more packets within the event and limit the overhead of processing between connection events. For more information, see Suggested intervals and windows."

Unfortunately, I'm not aware of any method for checking if  whether event length extension is supported by the peer device except for measuring the throughput or by analyzing the packet transfer with a  Bluetooth sniffer.