This post is a continuation of my last one, which discussed the theory behind Qaul BLE support for Linux where I explained the theoretical concepts and jargon and high-level system design for Bluetooth low energy flow using GATT. Please refer to that blog before proceeding.
In this blog, we will explore the implementation and low-level design of the BLE support in greater depth.
Introduction
We will begin with creating a GATT server serving two Bluetooth services “main_service” and “msg_service” with their respective UUIDs and characteristics for our qaul device. Then we will use the adapter to advertise the “main_service_uuid” for another qaul device to connect to the GATT server. Afterward, we will create a GATT client by starting a scanning service to discover nearby advertisements. The last thing to do is to set a few functionalities and callbacks like read and write characteristic data methods and request_read and request_write callbacks for characteristics to set a basic GATT prototype with message transfer support.
GATT Server
If you remember the GATT data hierarchy diagram from the last blog shows the GATT server is made up of services and each service is made up of characteristics and each characteristic of descriptors. Here, we will dive into details of how to implement it using BlueR.
GATT Service: GATT services are a set of similar attributes in one common section of the GATT server. For Qaul GATT Server implementation, we have two services – mainService, and msgService and their UUIDs (99xxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxx23). Below is a struct for Service where you specify uuid, characteristics and whether the service is primary or secondary.
GATT Characteristics: Characteristics are the containers for user data. For the Qaul Gatt Server, we have one characteristic for each service ➖ mainChar, msgChar, and their UUIDs with respective configurations. You can specify the uuid, descriptors(if any), read permission, and callbacks under the read attribute, and write permissions and conditions under the write attribute.
Now, you can create a new session to get an instance of your Bluetooth adapter and start the service GATT application using the above configurations. Below is a sample example of a GATT server with read characteristics functionality enabled.
Advertisements and Device discovery
We will use the above instance of the adapter to start advertising the main_service_uuid to be discovered by other devices. (Refer to sample code).
We can start scanning for nearby devices advertising the same UUID for the presence of qaul. For this, BlueR gives us adapter.set_discovery_filter(get_filter()).await; a feature for filter-based UUID scanning. and stream the discovered device adapter.discover_devices().await.
Till now we have created a Ble GATT server and client, the next thing we will implement is confirming the discovered devices for a qaul node, maintaining a list of discovered devices to save the throughput of scanning devices again and again, and keeping track of devices out of range.
For Qaul ble architecture, we have stored the qaul_id in the main_service’s characteristics, Now, whenever any device discovers this node, it will connect to it, check for the presence of both services along with their characteristics, and mark that qaul_id as discoverable. If the conditions are satisfied, it adds the node and its properties to a lookup map. To keep track of the device’s availability after discovery, Qaul spawns a new task out_of_range_checker to ping recently discovered devices at regular intervals to confirm their presence.
Any message to be sent can be written into msg_service’s characteristics using CharacteristicWriteMethod::Io and receive any write request using CharacteristicControlEvent::Write(write). The message read and write process is a bit more complex than function calls. We will discuss it in more depth in the next blog.
So, in the current blog, we are able to create a GATT server and a GATT client, advertise our UUIDs, and scan for nearby BLE devices. We also learned to identify the correct device and maintain the lookup table for better connectivity and more stability.
Concluding Thoughts
The first half of the GSoC coding period had more code implementation for ble connectivity. The major part was into integrating the prototype to work with async rust on multiple threads. The next half will be implementing loss-less transmission of data and experimenting with bluer for better stability. If you want to know more about my project, please feel free to reach out. Thanks for reading!
