Advertising

Advertising

Advertising is the process by which a BLE peripheral broadcasts short data packets on dedicated channels to announce its presence, capabilities, and available data. It is the entry point for all BLE communication -- devices cannot be discovered or connected to without advertising.

Advertising Channels and Packets

BLE reserves three channels for advertising: channel 37 (2402 MHz), channel 38 (2426 MHz), and channel 39 (2480 MHz). These frequencies are chosen to minimize overlap with the three most common Wi-Fi channels (1, 6, 11). During each advertising event, the peripheral transmits on all three channels in rapid succession.

A legacy advertising packet contains up to 31 bytes of payload, structured as a sequence of AD (Advertising Data) structures. Common AD types include Local Name, TX Power Level, Service UUIDs, Manufacturer Specific Data, and Flags. An additional 31 bytes can be provided via scan response if a central performing active scanning sends a SCAN_REQ.

Advertising PDU Types

BLE defines several advertising PDU types that control how the peripheral responds to scanners and connectors:

• ADV_IND: Connectable, scannable undirected advertising (most common type)
• ADV_DIRECT_IND: Connectable directed advertising to a specific device
• ADV_NONCONN_IND: Non-connectable, non-scannable (beacon mode)
• ADV_SCAN_IND: Scannable but non-connectable

Bluetooth 5.0 introduced extended advertising, which supports payloads up to 255 bytes and uses secondary advertising channels from the data channel pool for larger payloads.

Connectionless Data Distribution

Not all advertising leads to connections. Beacons (iBeacon, Eddystone) use non-connectable advertising to continuously broadcast location or sensor data without ever establishing a connection. Periodic advertising (BLE 5.0+) provides precisely timed broadcasts for synchronized data distribution. PAwR (BLE 5.4) extends this further with bidirectional communication for electronic shelf labels and other large-scale connectionless IoT networks.

The advertising interval controls how often events occur, ranging from 20 ms (high discoverability, high power) to 10.24 seconds (low discoverability, minimal power). A random 0--10 ms delay is added to each interval to reduce collision probability when multiple devices advertise simultaneously in the same environment.