With a passive RFID system, a reader sends out radio waves that power up a tag and elicit a response. The air-interface protocol—the rules by which tags and interrogators communicate—includes an anti-collision algorithm that enables a reader to receive responses from multiple tags in sequence. This happens quickly, so it appears all of the tags are being read simultaneously.
Active tags, on the other hand, broadcast a signal. They can be awoken by a signal from a reader, or they can broadcast at a set interval. Active tags can also use anti-collision algorithms to ensure that more than one tag in a read field can be interrogated.
—Mark Roberti, Editor, RFID Journal
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