RF Wireless World

Browse articles, tutorials, tools, and vendors.

Bluetooth LE Scanning: Active vs. Passive Scan Explained

By RF Wireless Expert Team

In the Bluetooth LE Link Layer, Scanning is a specific operational state. When a device is in the Scanning state, it acts as a “scanner” and actively listens on the physical radio channels for advertising packets being broadcast by other devices (i.e. “advertisers”).

Scanning is the primary method devices use to discover each other, read broadcasted data (like beacons), and find devices they may want to connect to.

Scanning behavior is primarily governed by two timing parameters set by the Host as follows.

  • Scan Interval: How frequently the controller starts a new scan cycle.
  • Scan Window: How long the controller actually listens during that interval. (Note: The Scan Window must always be less than or equal to the Scan Interval. If they are equal, the device scans continuously).

There are two distinct types of scanning in Bluetooth LE viz. Passive Scanning and Active Scanning.

Bluetooth LE Passive Scanning

In Bluetooth Passive Scanning, the scanner acts strictly as a listener.

  • How it works: The scanner tunes its radio to the advertising channels and listens for incoming Protocol Data Units (PDUs) from advertisers. When it hears an advertisement, it processes the data and passes the relevant information up to the Host.
  • No Transmissions: The defining characteristic of passive scanning is that the scanner never transmits any packets back to the advertiser. It does not send Scan Requests.
  • Use Case: This mode is highly power efficient. It is ideal for simply discovering nearby devices, reading sensor data broadcasted in advertising packets, or listening for Bluetooth beacons where two-way communication is unnecessary.

Bluetooth LE Active Scanning

In Bluetooth Active Scanning, the scanner not only listens but also actively requests additional information from the advertiser.

  • How it works: When an active scanner receives an advertisement that is marked as “scannable” (such as an “ADV_SCAN_IND” or a standard “ADV_IND” packet), it pauses its listening and transmits a Scan Request PDU (“SCAN_REQ” for legacy advertising, or “AUX_SCAN_REQ” for extended advertising) directly to the advertiser.
  • The Response: Upon receiving this request, the advertiser transmits back a Scan Response PDU (“SCAN_RSP” or “AUX_SCAN_RSP”). This response contains additional data that could not fit into the initial advertising packet.
  • Use Case: Active scanning is used when a device needs more context before deciding to connect. For example, the initial advertisement might only contain the device’s MAC address and a flag, while the Scan Response contains the device’s human-readable name and its supported services.

Additional Scanning Features

To make scanning more efficient, the Bluetooth Technology also includes features as follows.

  • Duplicate Filtering: The scanner can be configured to filter out duplicate advertising reports. Instead of notifying the Host every single time an advertisement is received from the same device, the Link Layer can filter these out so the Host only hears about the device once per scan period.

  • Scanning Filter Policy: A scanner doesn’t have to process every advertisement it hears. It can use a “Filter Accept List” to instruct the controller to only process advertisements (and only send Active Scan Requests) to specific, pre-approved devices, ignoring all unknown traffic to save power and processing time.

References & Further Reading

  1. Bluetooth SIG : Bluetooth Core Specification Version 6.3, May 5, 2026.
  2. Bluetooth SIG : Bluetooth Core Specification change history
  3. Bluetooth SIG : Bluetooth Technology Overview

Continue Learning Bluetooth Basic Concepts

Continue Learning Bluetooth Technology

Compare Bluetooth With Other Technologies

Explore Deep Insight Bluetooth Technology

Keep Reading

Related Posts