An RFID Reader or Interrogator scans using RF (Radio Frequency) waves.
RFID Interrogator is a common way of referring to Passive RFID because it physically transmits RF and juices up RFID tags located within range. Active RFID tag readers are, typically, 'listeners' -- however, they may be transmitters also.
How a Passive RFID Reader works (simple version)
The RFID Reader is connected to power or PoE and attached to a network
One to eight antennas are connected to the RFID reader
The reader continuously transmits no more the 4 Watts Effective Radiated Power (FCC requirement)
The RFID Tags harvest the RF power and begin to power up their internal capacity
The RFID Reader sends a "Who's there request"
The RFID Tags respond with their ID -- typically 300 tags per second under for an EPC Class 1 Gen 2 rfid tag
Software on the server, e.g. Nox Core Asset Tracking Software, manages thousands of incoming RFID tag reads per second to figure out where stuff is.
RFID Reader Installations
Dock Door with four antennas
How Passive RFID works (complicated version)
We would gladly make this available, however, we can not post the link publicly due to copyright. Please contact EPC Global for a detailed specification on the passive RFID air protocol.