USER'S MANUAL, VERSION 2.0
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Appendix D: Tag Population and Q
Tag Population is the RFID tag population that is to be inventoried. To be more precise, it is the
population of tags that can be “seen” by the RFID reader.
Q is an EPC Global Standard concept related to the way a group of tags is inventoried.
When a reader broadcasts its desire to inventory tags, it sends out a Q value. The tag will,
based on that Q, calculate a certain number and define that as the number of repeated
inventories the reader will do. Basically, the relationship of Inventory Repeats and Q is:
Inventory Repeats = 2
Q
The tag will then choose by random a certain number less than this Inventory Repeats. When
the reader starts doing inventory, the tag will then respond at that repeat number.
In other words, the Inventory Repeats should correspond to Tag Population:
Tag Population = Inventory Repeats = 2
Q
For example, if there are 8 tags, then in theory the Q can be 3, and if each tag chooses a
number different from that of the other 7 (miraculously, of course), then the 8 tags will be
inventoried in an orderly manner in turn.
Of course this will never happen
, as the tags will easily choose a number the same as that of
another one, and a collision will occur.
Therefore, it is a normal practice to have a bigger Q, such as 4 in this case, so that the 8 tags
would have a lower chance of choosing the same number.
Therefore, reversing the equation, ideally, we can have:
Q = INTEGER(LOG
2
(Tag Population))
But in reality, we need some headroom, so that:
Q = INTEGER(LOG
2
(Tag Population x 2)+1
