If the random space is larger than the number of tags in the reader??™s
range, there exist many collision slots. On the other hand, if it is smaller than the number of
tags, there are many idle slots in the frame. It is important to set suitable random space
based on the predicted number of tags.
Under the framed slotted ALOHA protocols, the frame size is the size of random space.
It is easy to change the frame size at the start of a frame. There have been many proposed
protocols which improve framed slotted ALOHA, called adaptive framed slotted ALOHA.
Figure 9.7 shows examples of the operation of slotted ALOHA, framed slotted
ALOHA, and adaptive framed slotted ALOHA. An optimal frame size happens when
the number of tags is the same as the frame size. Many protocols have been proposed that
estimate the number of tags, and use the number of readable slots, collided slots, and idle
slots. We now introduce three of them. The symbol notions and their descriptions used
throughout this chapter are summarized in Table 9.1.
9.4.1 Dynamic Slot Allocation
A readable slot which occurred in the previous frame is the one that contains a single
tag. A collided slot has at least two tags. By using these two facts, the lower bound of the
number of tags can be estimated. The lower bound of the number of tags can be
estimated as
Vogt1: Ntag ?? S ?? 2C: (9:1)
In Ref.
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