RFID has been used for taking attendance, but there is the possibility that two cards will have the same frequency, which could cause a problem. How can one avoid this?
The issue you are concerned about, I believe, is called collision. There can be tag collision or reader collision, when two tags talk back to an interrogator simultaneously, or when two readers use the same frequency. Most interrogators currently have built-in systems for avoiding collision. Tags are asked to respond in sequence, based on predetermined algorithms. I'll explain how this works in a very simplified way.
Each transponder has a unique serial number. For instance, the reader might ask all tags with a serial number starting with one to respond. Several tags might respond simultaneously, so the interrogator would then signal all tags with a serial number that begins with one, two to respond. If more than one tag responds, the device might then ask for tags that start with one, two, three. In this way, the algorithm can ensure that all tags are read without interference.
Readers also have systems for ensuring they do not interfere with one another. One is called "listen before talk." Interrogators can use a portion of the radio spectrum, which is divided into channels. In Europe, for example, readers operate from 865 to 868 MHz, which is divided into 10 channels. With "listen before talk" enabled, each reader listens for any signals on a channel before utilizing that channel to communicate with tags. If there is a signal, it chooses another channel. Another option is to use frequency hopping, in which readers randomly jump between channels. If one channel is being used, the device can then jump to another channel and broadcast a signal to the tags.
These are generally issues users don't have to worry much about. The equipment is sophisticated enough to enable readers to interrogate all of the tags in the read field without interference.
—Mark Roberti, Editor, RFID Journal
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