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Zephyr OS: General BLE API

I am trying to write a little Zephyr OS runtime system that uses BLE to communicate between my two nrf52840dk boards. I have spent the past couple of days reading up on BLE and have gotten acquainted with GATT servers/clients.

The API I would preferably have looks something like this:

/* Send a message to conn */
void send(struct bt_conn *conn, void* data, u16_t len);

/* Callback which is invoked when a message is received from conn */
void recv(struct bt_conn *conn, void* data, u16_t len);

What I have managed to do is achieve something similar by limiting my system to two devices, one being a GATT server and one a GATT client.

The server exposes one attribute which the client scans for and subscribes to. The server can 'send' a message to the client by notifying it about a change to the attribute, and it can be sent messages by the client issuing a write request to the attribute.

The client can send messages to the server by writing to the attribute, while it can receive messages by the server notifying it of an update to the subscribed attribute.

I am thinking that primitives such as these ones must exist underneath the GATT layer (in the HCI layer?), but the Zephyr documentation is quite sparse at most places. Following this, I have two questions.

  • Is my understanding of BLE correct, that most communication between BLE devices happen through such GATT server/client relationships, or through BLE mesh networks?
  • Any pointers to information regarding where I can read up on writing the more generic API I described above (or if it already exists, pointers to that) would be helpful.

edit: I've accepted Youssifs answer. His answer together with the comments beneath it adequately answers my question.

Solution

  • Is my understanding of BLE correct, that most communication between BLE devices happen through such GATT server/client relationships, or through BLE mesh networks?

    You are correct in that most applications out there will be using the GATT server/client relationship. This is because when Bluetooth Low Energy was introduced in 2010 (and later launched with iPhone 4s through CoreBluetooth in 2011), this was the only method of communication. Since then, subsequent releases of BLE introduced newer methods of communication:-

    • LE L2CAP (introduced in BT v4.1, 2013) where lower level communication channels were used for fast and direct data transfer.
    • LE Mesh (introduced in 2017) where most of the communication is based on BLE adverts and therefore any device that is on v4.0 can theoretically support it.

    The problems with both of these methods are the relative complexity and the slow adoption by vendors. As such, my recommendation is to continue using GATT examples/applications until you're more familiar with BLE and then proceed to using the other methods of communication.

    Any pointers to information regarding where I can read up on writing the more generic API I described above (or if it already exists, pointers to that) would be helpful.

    You've probably already seen this, but the Getting Started Gudie of Zephyr is quite useful. You can then use the Central HR example on one board and the Peripheral HR example on the other to get two way communication. Once this is done you can start customising your applications to match your needs. A list of all Bluetooth examples can be found here.

    Below are further resources on BLE development in Zephyr:-

    I hope this helps.