Maximum bytes sent in a characteristic indicate function

RE: Maximum bytes sent in a characteristic indicate function

Hung Bui — Thu, 12 Nov 2020 09:24:16 GMT

Hi,

To get a high throughput you should try to achieve the maximum ATT_MTU and longest data length extension CONFIG_BT_CTLR_DATA_LENGTH_MAX (usually set = ATT_MTU+4) .

If the size is still smaller than the amount of data you want to send, you would need to segment it to multiple trunks and send one by one.

RE: Maximum bytes sent in a characteristic indicate function

JPSvanderWalt — Thu, 12 Nov 2020 05:57:56 GMT

Hi Hung Bui. Thank you for your reply. I will be dealing with large amounts of sample outputs from sensors. If notifications/indications are not meant for large amounts of data, how would I go about sending large amounts of data?

I did double-check and the maximum amount of bytes that I can send over a notification is indeed 20 bytes. I will try and increase the MTU size.

RE: Maximum bytes sent in a characteristic indicate function

Hung Bui — Wed, 11 Nov 2020 15:41:43 GMT

Hi,

As defined in Bluetooth spec, you can't do "long" notification/indication. So indication/notification can only sent as one single BLE packet. The ATT_MTU would decide the max size of a notification/indication you can send. For example if you set CONFIG_BT_L2CAP_TX_MTU = 23 bytes, you can send maximum CONFIG_BT_L2CAP_TX_MTU - 3 = 20 bytes payload (3 bytes overhead including opcode & handle value).

What you mentioned about 16 bytes maximum sounds a little bit strange, most likely it should be 20 bytes if the CONFIG_BT_L2CAP_TX_MTU is minimum at 23 bytes.

If you want to send larger indication/notification you would need to increase the CONFIG_BT_L2CAP_TX_MTU size. But please note that the actual ATT_MTU used in a connection depends on both sides. So if your peer device doesn't support large ATT_MTU you will receive "No ATT channel for MTU" warning when you send larger than limit indication as you already observed.

You can use bt_gatt_get_mtu() to get the actual mtu of a connection.