How to send 25 packets of data each second

RE: How to send 25 packets of data each second

ovrebekk — Tue, 21 May 2024 11:43:00 GMT

It's true that the L2CAP_TX_MTU is not limited to 251 bytes, but the data length is:

config BT_CTLR_DATA_LENGTH_MAX
	int "Maximum data length supported"
	depends on BT_CTLR_DATA_LENGTH
	default BT_BUF_ACL_RX_SIZE if BT_BUF_ACL_RX_SIZE < 251
	default 27
	range 27 BT_BUF_ACL_RX_SIZE if BT_BUF_ACL_RX_SIZE < 251
	range 27 251
	help
	  Set the maximum data length of PDU supported in the Controller.

Once you send packets longer than this they will have to be segmented by the link layer, and the throughput will suffer as a result. The ideal situation is to negotiate a 251 byte data length, and send data packets of 244 bytes (leaving 7 bytes for the L2CAP and ATT headers).

[quote user=""]I am reading the sensor data continuosly and every 40 ms i am sending the data to bluetooth using gatt_notify[/quote]

Rather than sending the packets every 40ms I would recommend sending them as fast as possible, then you will be able to more efficiently fill the buffers in the Bluetooth stack, which will increase overall throughput.

Best regards
Torbjørn

RE: How to send 25 packets of data each second

IvanR — Tue, 21 May 2024 11:30:19 GMT

Where did you find that its maxed out at 291 bytes?

https://developer.nordicsemi.com/nRF_Connect_SDK/doc/2.6.0/kconfig/index.html#CONFIG_BT_L2CAP_TX_MTU

RE: How to send 25 packets of data each second

Madhu Mohan Reddy — Tue, 21 May 2024 11:11:54 GMT

In documentation and L2CAP_TX_MTU, it is showing that we can update upto 251 bytes only

At a time i can send 494 bytes of array to receiver?

In my application i want to send 6000 bytes of array with in one second so this is not possible thats why every 40 ms i am sending 240 bytes of array

RE: How to send 25 packets of data each second

IvanR — Tue, 21 May 2024 10:43:26 GMT

Devices can agree on what MTU they can use. Im checking it using code bellow. Ive not tried setting it manually as this fits my purpose. I then use that negotiatedMTU as size of the packet sent via notify.

void exchange_mtu_result(struct bt_conn* conn, uint8_t err, struct bt_gatt_exchange_params* params) {
  if (err)
    printErr("MTU exchange failed (Error: %d)", err);
  else {
    negotiated_MTU = bt_gatt_get_mtu(conn) - 4; // INFO negotiated MTU is reduced by 4 as 3 bytes are ATT and 1 for null termination
  }
}

BT_CONN_CB_DEFINE(conn_callbacks) = {
  .connected           = connected,
  .disconnected        = disconnected,
  .le_param_req        = param_req,
  .le_param_updated    = param_updated,
  .le_phy_updated      = phy_updated,
  .le_data_len_updated = data_length_updated
};

 MTU : 494 bytes

RE: How to send 25 packets of data each second

Madhu Mohan Reddy — Tue, 21 May 2024 10:15:52 GMT

Hi IvanR,

By default MTU for notify is 23 but we can update the MTU to 251 right?

Thank you in advance

RE: How to send 25 packets of data each second

IvanR — Tue, 21 May 2024 06:24:03 GMT

The typical MTU for notify is 23 bytes.

Try setting auto update connection parms to y or set the desired length via

    struct bt_conn_le_data_len_param data_len = {
        .tx_max_len = 247,
        .tx_max_time = 1000,
    };

    err = bt_conn_le_data_len_update(NULL, &data_len);
    if (err) {
        printk("LE Data Length update failed (err %d)\n", err);
    }