Reader Session Control
The session defines the duration of time and conditions that an inventoried tag remains in either of two 'states,' A or B. Think of A as being the state in which a tag has not been inventoried, and B as the state in which the tag has been inventoried. Of course, you could mask tags and place all tags not of interest in state B as well, but in general, let's assume the tag's default state is A. So as you go through the inventory process, reading one tag at a time, you would instruct the reader to switch the tag's state to B once the read has been confirmed. Since tags in the B state have presumably already been accounted for, you would then only focus on those in the A state.
Which session you choose to use plays an important role in reading many tags over a short period of time, because the session defines the duration and condition for which that tag remains in the B state. For large tag populations, you don't want to waste time counting tags more than once, so you want to retain the B state for as long as possible, and focus only on tags in the A state.
In session 0, the tag persists in the B state only as long as it is within a continuous RF field. So when the reader power is removed, the tags in the B state revert back to the A state and are candidates to be read again. This state is good for reading a small number of tags quickly and repeatedly, but it is not good for dense tag populations. Keep in mind that a tag loses power when antennas are multiplexed (that is, when you shut down one antenna and power up another), or when the reader cycles through its frequencies (e.g., every 400 microseconds).
In session 1, the tag persists in the B state for a limited duration (persistence), even after being removed from the RF field. The persistence in the B state is defined between a minimum of 500 microseconds and a maximum of 5 seconds. Persistence may vary from IC manufacturer to manufacturer, IC model to model, and IC to IC, but should fall within these boundaries. When the persistence time elapses, the tag reverts to the A state.
Session 2 or 3
In sessions 2 and 3, the tag persists in the B state for least 2 seconds after being removed from the RF field. The sessions are similar in duration, but operate independently. So, if I were to set up a system for large tag populations and wanted to minimize duplicate reads (they only need to be read once), I would select session 2 or 3.
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