BLE5 - Make payload size 200 bytes - the right way

[Michael]

Hello,

I try to configure two nrf52832 devices work with payload of 200 bytes (data should be 200 bytes, not the all ble packet).

I took ble_app_uart_c peripheral and central examples - for 20 bytes everything looks ok.

After that, I tried to understand and implement in my code the ble_app_att_mtu_throughput example, but without luck.

Where can I find the exact algorithm for the increasing the payload size?

My algorithm now:

Common definitions:

NRF_SDH_BLE_GATT_MAX_MTU_SIZE             defined for 247 bytes (in sdk_config.h)

BLE_GATT_ATT_MTU_DEFAULT                         defined to 204 bytes (in ble_gatt.h)

Central:

in function ble_evt_handler(ble_evt_t const * p_ble_evt, void * p_context) - the function handles all BLE events

- in event BLE_GAP_EVT_PHY_UPDATE_REQUEST

-- I define the 2MBPS speed:

ble_gap_phys_t const phys =
 {
       .rx_phys = BLE_GAP_PHY_2MBPS,
       .tx_phys = BLE_GAP_PHY_2MBPS,
};
sd_ble_gap_phy_update(p_ble_evt->evt.gap_evt.conn_handle, &phys);

in function gatt_init() - the function initiates the gatt library

- nrf_ble_gatt_init(&m_gatt, gatt_evt_handler);

- nrf_ble_gatt_att_mtu_central_set(&m_gatt, BLE_GATT_ATT_MTU_DEFAULT); //to 204 bytes

in function data_len_ext_set() - the function should set the new MTU size. the function placed in main() function, after all initializations, but before scan_start() call

- nrf_ble_gatt_data_length_set(&m_gatt, BLE_CONN_HANDLE_INVALID, 200 + L2CAP_HDR_LEN);  - according to example. 200 - needed payload, L2CAP_HDR_LEN - defined as 4

in function conn_evt_len_ext_set() - not sure, but looks that the function enables length increasing option in the stack, called just after the data_len_ext_set() function

- memset(&opt, 0x00, sizeof(opt));
- opt.common_opt.conn_evt_ext.enable = 1;
- sd_ble_opt_set(BLE_COMMON_OPT_CONN_EVT_EXT, &opt);

Peripheral

in ble_evt_handler(ble_evt_t const * p_ble_evt, void * p_context) - the function handles all BLE events

- I define the 2MBPS speed:

ble_gap_phys_t const phys =
 {
       .rx_phys = BLE_GAP_PHY_2MBPS,
       .tx_phys = BLE_GAP_PHY_2MBPS,
};

- in event BLE_GAP_EVT_CONNECTED

-- sd_ble_gap_phy_update(p_gap_evt->conn_handle, &phys); //request for 2MBPS

- in event BLE_GAP_EVT_PHY_UPDATE_REQUEST

-- sd_ble_gap_phy_update(p_ble_evt->evt.gap_evt.conn_handle, &phys);

- in event BLE_GAP_EVT_DATA_LENGTH_UPDATE_REQUEST

-- memset(&dl_params, 0, sizeof(ble_gap_data_length_params_t));

-- sd_ble_gap_data_length_update(p_ble_evt->evt.gap_evt.conn_handle, &dl_params, NULL);

in function gatt_init() - the function initiates the gatt library

- nrf_ble_gatt_init(&m_gatt, gatt_evt_handler);

- nrf_ble_gatt_att_mtu_periph_set(&m_gatt, BLE_GATT_ATT_MTU_DEFAULT); //to 204 bytes

in function data_len_ext_set() - the function should set the new MTU size. the function placed in main() function, after all initializations, but before ble_advertising_start(&m_advertising, BLE_ADV_MODE_FAST); call

- nrf_ble_gatt_data_length_set(&m_gatt, BLE_CONN_HANDLE_INVALID, 200 + L2CAP_HDR_LEN);  - according to example. 200 - needed payload, L2CAP_HDR_LEN - defined as 4

in function conn_evt_len_ext_set() - not sure, but looks that the function enables length increasing option in the stack, called just after the data_len_ext_set() function

- memset(&opt, 0x00, sizeof(opt));
- opt.common_opt.conn_evt_ext.enable = 1;
- sd_ble_opt_set(BLE_COMMON_OPT_CONN_EVT_EXT, &opt);

So, is it looks ok? Some mistakes that you could point me on? Something is missing?

Thanks a lot!

[Kenneth]

Hello,

You should basically only need to add gatt_init() and this will be automatically handled for you, something like:

void nrf_ble_gatt_evt_handler(nrf_ble_gatt_t * p_gatt, nrf_ble_gatt_evt_t const * p_evt)
{
	if(p_evt->evt_id == NRF_BLE_GATT_EVT_DATA_LENGTH_UPDATED)
	{
		// p_evt->params.data_length is the max length on-air
	}	
	if(p_evt->evt_id == NRF_BLE_GATT_EVT_ATT_MTU_UPDATED)
	{
		// p_evt->params.att_mtu_effective is the max attribute mtu size		
	}	
	
	// Once both are updated, then you can start sending data that is longer than 20bytes effectively.
    // Though you should also make sure attr_char_value.max_len is set to 
    // more than 20bytes when calling sd_ble_gatts_characteristic_add().
}

/**@brief Function for initializing the GATT module.
 */
static void gatt_init(void)
{
    ret_code_t err_code = nrf_ble_gatt_init(&m_gatt, nrf_ble_gatt_evt_handler);
    APP_ERROR_CHECK(err_code);
}

[Michael]

Hello Kenneth,

I don't understand where I can define that the payload data in the packet should be 200 bytes.

In addition I cannot understand where I should use the sd_ble_gatts_characteristic_add function? My code is based on ble_app_uart example and I don't see usage of this function.

Thnaks a lot!

[Kenneth]

It is used already when peripheral is setting up the GATT database in one of the BLE modules, you should find it by searching for 'sd_ble_gatts_characteristic_add' throught the project.