In a situation in which multiple passive RFID tags are in close proximity to one another, how does a reader pick out just one tag from many?
—Matthew (Reading, Mass.)
What you are referring to is known as singulation. Part of the air-interface protocol—the language that tags and readers use to communicate—is the anti-collision algorithm. This is the method used to avoid two tags talking simultaneously to a reader. Each serial number is made up of a string of binary digits—ones and zeroes.
Based on the anti-collision algorithm in the protocol, a reader might communicate with tags by asking any tag with a 0 as its first digit to respond. If 10 tags responded, the device might say, "All tags that start with 00, please respond." If two tags responded, it might then ask to hear from all tags that start with 000 or 001. It would then keep going through this algorithm until it received no more response from the tags.
So, for instance, let's say you have four tags with these serial numbers:
Tag 1: 00010101000100
Tag 2: 00100101101010
Tag 3: 10101001101110
Tag 4: 01010101000100
The reader might ask for all tags that start with a 1 to respond, in which case tag 3 would be immediately identified. If it asked for all tags that start with a 0 to respond, tags 1, 2 and 4 would do so. The device would not be able to differentiate these signals, so it would ask for tags that begin with 00 to respond. In that case, only tags 1 and 2 would reply. But the reader would need to talk to only one tag at a time, so it would ask for tags that begin with 000 to respond, and this time, only tag 1 would do so. It could then communicate with this tag, and move on to tags that start with 001.
Interestingly, early uses of the EPC Gen 1 protocol found that if two readers spoke to the same tags, that would mess up the anti-collision algorithm. Reader 1 might be going through the singulation process, but if reader 2 said, "All tags that begin with 0 please respond," the tags would start communicating with that reader. So the Gen 2 protocol has something called "sessions." A tag can communicate with one reader in one session, and if it were interrogated by a different reader, it would begin a new session and go through the responses, but not forget where it was with the first device.
—Mark Roberti, Editor, RFID Journal
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