e., the countervalue is 0). By adding the selected
binary number to the countervalue, a set is split into two subsets. When tag collisions
occur, the tag which is not involved in collision (i.e., the countervalue is not 0) increases its
countervalue by 1. When the reader??™s feedback indicates no collision, all tags decrease their
countervalues by 1. The tag infers the successful transmission from the following feedback
indicating no collision. The tag recognized by a reader does not transmit any signal until
the ongoing frame is terminated. Figure 8.1b shows an example of tag identi?¬?cation of the
binary tree protocol and the number by the side of the lines indicates the binary number
selected randomly by con?¬‚icting tags.
The reader also has a counter to terminate a frame. It initializes the countervalue with 0
in every frame. The countervalue of the reader indicates the number of tag sets which are
not yet recognized in a frame. If tag collision occurs, the reader adds 1 to its countervalue
since the number of tag sets, which the reader should recognize, increases. Otherwise, it
decreases its countervalue by 1. When the countervalue is <0, the reader terminates the
frame.
8.2.2 Query Tree Protocol
The query tree (QT) protocol [26,27] uses tag IDs to split a tag set. The reader transmits a
query including a bit string. The tag whose ?¬?rst bits of ID equal the bit string of the query
responds by transmitting ID.
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