Confused about how to set data length

[HarryGe]

During my development, I found NRF5 SDK only supports symmetric RX/TX data length settings.

Here is the on_data_length_update_request_evt in nrf_ble_gatt.c file.

static void on_data_length_update_request_evt(nrf_ble_gatt_t * p_gatt, ble_evt_t const * p_ble_evt)
{
ble_gap_evt_t const * p_gap_evt = &p_ble_evt->evt.gap_evt;
nrf_ble_gatt_link_t * p_link = &p_gatt->links[p_gap_evt->conn_handle];

// The SoftDevice only supports symmetric RX/TX data length settings.
uint8_t const data_length_requested =
p_gap_evt->params.data_length_update_request.peer_params.max_tx_octets;

NRF_LOG_DEBUG("Peer on connection 0x%x requested a data length of %u bytes.",
p_gap_evt->conn_handle, data_length_requested);

uint8_t const data_length_effective = MIN(p_link->data_length_desired, data_length_requested);
(void) data_length_update(p_gap_evt->conn_handle, data_length_effective);
}

It clearly indicates SDK only supports symmetric RX/TX data length settings.

When other Bluetooth devices send LL LENGTH RSQ, NRF will only take a minimum value to reply with the same RX TX.

This is also verified in wireshark.

But, I can still manually set an unsymmetric parameter and reply via sd_ble_gap_data_length_update.

case BLE_GAP_EVT_DATA_LENGTH_UPDATE_REQUEST:
{
ble_gap_data_length_params_t params =
{
.max_rx_octets = 251,
.max_tx_octets = 132,
.max_rx_time_us = 2120,
.max_tx_time_us = 2120,
};
err_code = sd_ble_gap_data_length_update(conn_handle,&params,NULL);
APP_ERROR_CHECK(err_code);}

It also can works and verified in wireshark.

So I'm wondering if it's a wrong behavior that I manually reply the unsymmetric parameters? 

Does it take effect inside the Bluetooth stack? Does the data transfer really follow this unsymmetric parameter?

 Any reply will be appreciated.

[Vidar Berg]

Hello,

I was not able to find anything in the Softdevice documentation confirming this statement. However, I would still recommend setting the RX/TX values the same way as in the GATT module to avoid potential interoperability issues or bugs with other devices.

Best regards,

Vidar

[HarryGe]

Hi Vidar,

Thank you for getting back to me. I’m not completely sure which part of my previous note you were referring to. Could you clarify whether:

1. you couldn’t find the specific API I mentioned in the SoftDevice documentation, or

2. you couldn’t find anything that confirms whether a manually supplied asymmetric RX/TX data-length response is actually applied by the stack?

The challenge I’m facing is that the third-party app I need to support sends an LL-length update with different RX and TX values (e.g. RX = 251 bytes, TX = 27 bytes). If I let the SDK handle the request automatically, it forces both directions down to 27 bytes, its unacceptable.

Best regards,
Harry