BLE GAP Roles: Peripheral, Central, Broadcaster and Observer

Peripheral and Central

Peripheral usually advertises and provides data or service. Central scans, finds the appropriate device, and initiates the connection. After connection, the role of the Link Layer is not the same as the role of GATT Client and Server; Often Central is the client, but this is not an absolute requirement.

Broadcaster and Observer

Broadcaster broadcasts data without connecting; Beacon is a common example. Observer only receives and analyzes Advertising and does not necessarily make connections. These two roles are for simple telemetry, attendance and low-cost environment scanning.

Phone and sensor

In a thermometer, the sensor advertises, the phone scans it and connects, then discovers services. If the phone activates Notify, the sensor will send a new value when the temperature changes. It is responsible for starting the connection of the phone, but the sensor produces the useful data.

Two ESP32 and multiple roles

An ESP32-C3 can be a peripheral and provide SHT31 data; The second ESP32 is Central and shows it on the screen. Some chips support multiple connections or simultaneous roles, but RAM, radio time, and state management complexity increase.

See two independent roles in one table

This arrangement is common, but there is no "Central is always Client" rule. The role of GAP is related to detection and binding; The role of GATT is to provide or consume Attributes. A device can operate in both GATT directions if the stack supports it.

Do not confuse an unconnected Beacon with a connected GATT Server: receiving Advertising for an Observer does not constitute a connection or service discovery.