Passive Scanning

Passive Scanning

Advertising">Passive scanning is a BLE scanning mode in which the central device listens for advertising packets without transmitting any packets of its own. Unlike active scanning, the central does not send SCAN_REQ packets and therefore does not receive scan response data, but it also does not reveal its presence on the network.

How Passive Scanning Works

In passive scanning, the central configures its radio to listen on the three advertising channels (37, 38, 39) according to the scan interval and scan window parameters. The scan interval defines the total cycle time, and the scan window defines how long the radio is active within each cycle. For example, a scan interval of 100 ms and scan window of 30 ms means the radio listens for 30 ms every 100 ms, yielding a 30% duty cycle.

When the central receives an advertising packet, it passes the data to the host stack without any further radio interaction. The central does not acknowledge the packet, send a SCAN_REQ, or otherwise indicate that it received the advertisement.

Advantages of Passive Scanning

Passive scanning offers several benefits over active scanning. The central never transmits, making it invisible to advertisers and eavesdroppers -- important for security monitoring, contact tracing, and surveillance-resistant applications. Power consumption is slightly lower without transmitting SCAN_REQ packets and waiting for responses. The difference is small for a single scan event but accumulates over continuous scanning scenarios. Passive scanning also generates zero RF emissions, which can be relevant for regulatory compliance in certain deployments.

Limitations

The primary limitation of passive scanning is reduced data availability. The central only receives the 31-byte advertising payload (or the extended advertising payload for Bluetooth 5.0+ devices). Any data the peripheral places in the scan response is inaccessible, meaning the central may not receive the complete device name, full service ATT">UUID list, or supplementary manufacturer data.

Use Cases

Passive scanning is commonly used in beacon monitoring applications where the observer should remain invisible, BLE asset tracking gateways that observe without interacting, privacy-sensitive applications such as exposure GATT & ATT">notification systems, and Direction Finding deployments where the scanner collects Constant Tone Extension (CTE) data from advertising packets without needing to communicate with the advertiser.