Select Command
The process by which an interrogator selects a tag population for inventory and access. Interrogators may use one or more Select commands to select a particular tag population prior to inventory.
It can look for matching bits in the tag identifications, electronic product codes (EPCs), or in the user memory. So, if pallets stacked with tagged cases are coming through a dock door, the reader could issue a Select command to tags with an EPC that indicates they are pallet tags. That way, only the pallet tags respond or the reader could select only case tags. The reader can use several Select commands for more complex operations.
EPC Gen2 protocol adopted slotted aloha-based probabilistic algorithm to solve the collision problem. The reader first issues a Select command to pick particular portion of the tag population. Next, the reader issues a Query command, which contains Q-parameter to specify theframe size (equal to 2Q-1), then each selected tag will pick a random number between 0 and 2Q-1 and put it into its slot counter.
If the reader often gets back no response to its queries, it automatically decreases the Q value. If it gets more than one tag responding, it increases the Q value, thereby increasing the range of numbers that can be generated by the tags.