Hardware Reference
In-Depth Information
CHAPTER 3
GAP (Advertising and Connections)
The Generic Access Profile (GAP) is the cornerstone that allows Bluetooth Low Energy
devices to interoperate with each other. It provides a framework that any BLE imple‐
mentation must follow to allow devices to discover each other, broadcast data, establish
secure connections, and perform many other fundamental operations in a standard,
universally understood manner. It's important to understand GAP thoroughly, because
many BLE protocol stacks use it as one of the lowest-level entry points when providing
a functional API to application developers.
As mentioned previously, the sections of the GAP chapter in the core specification that
apply to Bluetooth Low Energy define the following different aspects of device inter‐
action:
Roles
Each device can operate in one or more roles at the same time. Each role imposes
restrictions and enforces certain behavioral requirements. Certain combinations of
roles allow devices to communicate with each other, and GAP establishes the in‐
teractions between those roles precisely. Although not always, roles tend to be as‐
sociated with specific device types, and for many (though not all) implementations,
they are also tightly bound with their use case and do not change at all.
Modes
Further refining the concept of a role, a mode is a state in which the device can
switch to for a certain amount of time to achieve a particular goal or, more espe‐
cifically, to allow a peer to perform a particular procedure. Switching modes can be
triggered by user interface actions or automatically when required, and devices tend
to switch modes more frequently than roles.
