How do RFID systems deal with the likelihood of two or more tags in a read field responding to a reader simultaneously and thus not being interrogated?
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There are two basic ways in which RFID systems deal with tag collision. One is deterministic, while the other is probabilistic.
With the deterministic algorithm, a reader instructs tags to respond if their serial number begins with, say, a zero. If two tags respond, it asks them to respond if the first number is a zero and the second is a one. It keeps asking the tags to respond until all have been identified. A useful analogy is a teacher asking all students in a classroom to stand if their last name starts with an A. If five students stand, the teacher might say to remain standing only if the last name starts with an A and the second letter is B. If two students remain standing, the teacher may ask those with the first two letters A and B and a third letter to remain standing. This is known as the tree-walking algorithm.
With the probabilistic method, tags respond to reader commands at random intervals. If a collision occurs, the tags that experienced the collision will respond to the reader again after a random period of time. If there are only a few tags in the read field, it is likely they will respond at different times. If there are many, two or more might collide again. In that case, they will wait and respond at another random interval. This keeps happening until there are no more collisions. This is known as the slotted ALOHA algorithm.
I would recommend searching for "tree-walking algorithm" and "slotted ALOHA algorithm" on the Internet to learn more about how such systems work.
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