What is RSSI?

Understanding RSSI

RSSI (Received Signal Strength Indicator) is a measurement of the power level a wireless device receives from a transmitter, such as a Wi-Fi access point or Bluetooth device. It reflects link quality: it is usually expressed in dBm (decibel-milliwatts) as a negative number, where a value closer to zero, for example -50 dBm, indicates a stronger signal than a more negative value like -80 dBm.

RSSI is measured by the radio's receiver and is fundamental to many wireless decisions. Clients use it to choose which access point to associate with and when to roam to a stronger one; access points and controllers use it for band steering and coverage planning. The exact scale is vendor-dependent, in 802.11 RSSI is a relative integer that chipset makers map to dBm differently, so RSSI is best treated as a comparative indicator rather than an absolute calibrated power reading. It is also distinct from SNR (signal-to-noise ratio), which factors in background noise.

For security, RSSI is a useful but spoofable signal. Defenders use RSSI for rogue access-point and evil-twin detection (an unexpected AP appearing with strong signal), wireless intrusion detection, and physical localization to triangulate the source of unauthorized transmitters. RSSI-based proximity is also used in some authentication and asset-tracking schemes. However, attackers can manipulate transmit power and antennas to fake proximity, so RSSI should never be a sole trust factor.

For example, a wireless security team notices a new SSID matching the corporate network broadcasting at an unusually strong -40 dBm from a part of the building with no installed AP. Using RSSI readings from several sensors, they triangulate the signal to a closet and find a rogue evil-twin access point planted to harvest credentials. The strong, anomalous RSSI was the first clue that prompted the physical hunt and removal of the device.