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How Would a Tag Behave If Two Readers Attempted to Read It at the Same Time in Different Sessions?
What would happen in this scenario?
With the first generation of Electronic Product Code (EPC) RFID tags, when one reader was communicating with a batch of tags, it would run through an algorithm to singulate each one. Essentially, the reader would say: "If your serial number starts with a 0, please respond." If more than one tag responded, the device would then say: "If your serial number starts with 00, please respond." It would keep doing this until only a single tag responded. Then it would go through the same process again with different starting digits, and keep doing so until all tags were interrogated.
But a problem arose during real-world deployments. Sometimes, a company would have fixed readers communicating with tags and someone would come along with a handheld and start asking the same question: "If your serial number starts with a 0, please respond." A tag that had already responded would do so again, but the fixed reader would also pick up this response and become confused. So neither interrogator would be able to communicate with a bunch of tags at the same time.
To address this scenario, the second-generation EPC air-interface protocol standard introduced something called sessions. Tags could respond to one reader using session A. If another reader came along and started interrogating those same tags, each tag would respond to the second device in session B, while still responding to the first one in session A. So the tag would respond to one reader in each session as though it were communicating with only one device at a time.
Consider if I were talking to someone on the phone and face-to-face at the same time. If I interrupted the person in front of me to take the call, I would ask him or her to hold on while I said something to the person on the phone, and vice versa. Only EPC Gen 2 readers and tags have the ability to do this.
—Mark Roberti, Founder and Editor, RFID Journal
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