This identi?¬?cation process can
be considered as constructing a searching tree based on tag ID codes. The reader increases
the length of the query until the identi?¬?cation cycle is completed.
The operation of the reader can be implemented by using a data structure (e.g., queue or
stack). The queries set to 0 and 1 are stored in the data structure initially. When a collision
occurs, the reader makes two queries whose lengths are 1 bit longer than the queries
which cause the collision by concatenating query and the extra bit (0 and 1). Then the
reader inserts itself to the stack or queue. When a readable slot or an idle slot occurs,
the reader gets a next query from the data structure without any further processing. In case
of using a stack for the data structure, the search is depth ?¬?rst search like the binary tree.
When a queue is used, the search is breadth ?¬?rst search.
In Figure 9.3, we give an example of the identi?¬?cation procedure of the query tree
protocol. The queue is initialized with null query. At the ?¬?rst reading slot, all tags reply
and a collision occurs. It makes reader push query ??????0??™??™ and query ??????1??™??™ into queue. The
reader extracts the query ??????0??™??™ from the queue and transmits it to tags. Three tags (001, 010,
and 011) which start with ??????0??™??™ reply simultaneously and a collision occurs.
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