I would like to understand how an interrogator can tell the difference between tags, even if they were not made by the same manufacturer. Can you please explain this?
Tags and readers communicate using an air-interface protocol. Think of it as the language of an RFID system. There are international standards for air-interface protocols. This is like establishing a common language for a meeting of diplomats from a dozen different countries. If tags and readers from different manufacturers all use the same air-interface protocol, then they can communicate with each other, just as diplomats could all communicate using, say, English at the meeting.
As for how the reader differentiates from among a large group of tags, this is accomplished through something called the anti-collision algorithm, which is part of the air-interface protocol. Collision occurs when signals from more than one tag confuse a reader. To avoid this problem and enable a reader to talk to many tags within the same read field, tags and readers employ a variety of algorithms. The interrogator might ask all tags that have a serial number starting with "0" to respond. If four tags respond, the device can then ask for only tags with serial numbers beginning with "00" to respond. It can keep doing this until it gets down to only one tag responding, which allows it to differentiate that tag from all others.
—Mark Roberti, Founder and Editor, RFID Journal
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