How Does Anti-Collision Work?

The interrogator basically asks tags to respond depending on their serial number or some other number. If two tags respond, the reader asks again in a slightly different way, and then continues to do this until only a single tag responds.

Imagine that the reader is a teacher, and the tags are a room full of pupils. The teacher might say, "If your last name begins with A, stand up." If more than one pupil stands, the teacher might then state, "If your last name begins with A and your first name also starts with A, stand up." If no one stands, the teacher could say, "If your last name begins with A and your first name starts with B, stand up." And so forth.

A reader can ask all tags with a serial number beginning with 0 to respond. If more than one tag responds, the device might ask all tags that have serial numbers starting with 00 to respond. And so on, until it isolates one tag.

One problem with this tree-walking algorithm based on serial numbers is it means that if two tags were to have the same serial number (which could happen either accidentally, or on purpose), the tags and reader would never be able to communicate since there would be no way to isolate a single tag.

To avoid this problem, the EPC Gen 2 air-interface protocol standard employs a unique anti-collision protocol based on a tag's ability to generate random numbers. The anti-collision technique used, known as the "Q Algorithm," is explained in non-technical terms in our article Part 1: Understanding the EPC Gen 2 Protocol.

In case you are interested in reading a detailed technical explanation of the Q Algorithm, IEEE offers a paper titled A Novel Anti-Collision Algorithm for EPC Gen2 RFID Systems.