The active tags with the addition of 1 in their ASCs convert their state into wait
state. The tags in wait state increase ASC when collision occurs. When the reader sends
tags the message which means the idle slot, the tags in the wait state decrease ASC.
Figure 9.6 shows the operations of the binary tree protocol and ABS protocol. If the tag
population does not change, neither collision nor idle occurs in the current cycle under ABS
protocol. The reader remembers the number of tags in the previous cycle and informs them
of it at the start of an identi?¬?cation cycle. If there are incoming tags, they randomly set their
counters into a smaller value than the number sent by the reader. Incoming tags can cause
collisions with existing tags. When a collision occurs, those collided tags add a randomly
selected binary number (0 or 1) to ASC. If there are leaving tags, an idle slot can occur.
9.4 Probabilistic Anticollision Protocols
Probabilistic tag anticollision protocols are based on ALOHA [8]. ALOHA is one of the
basic medium access control mechanisms. In ALOHA, each tag generates a random
number and waits for its transmission time according to the number chosen. If the data
transmitted by a tag is not interfered by other data, the reader can identify the tag. A tag
continues to do the same work after its transmission; generating a new random number
and transmitting its own data after waiting for random amount of time.
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