By
using these probabilities, we could calculate the collision ratio [18], which is a fraction of
the number of collision slots to the frame size. The collision ratio can be obtained by
Comparative Performance Analysis of Anticollision Protocols in RFID Networks 169
subtracting the summation of the probability of the appearance of readable slots and the
probability of the appearance of idle slots from one. We already know the previous frame
size and the number of collision slots so that we can estimate the number of tags using the
collision ratio. The number of tags can be obtained by
DFSA: Cratio ?? 1 1
1
F Ntag
1 ??
Ntag
F 1 : (9:3)
ID ID
ID
ID
ID
ID
ID
ID
ID
ID
ID
ID
ID
ID
ID
Slot
Frame
ID
ID
ID
ID
ID
ID
ID ID
ID
ID
ID
ID
ID ID
ID
ID ID ID ID Idle Idle Idle Idle
Tag 1
Tag 2
Tag 3
Tag 4
Medium
Tag 1
Tag 2
Tag 3
Tag 4
Medium
Tag 1
Slot
Frame size=3 Frame size=4 Frame size=4
Frame
size=2
Tag 2
Tag 3
Tag 4
Medium
Collision
Collision
Colli
sion
Colli
sion
Colli
sion
Collision
Collision
(a)
(b)
(c)
Slot
FIGURE 9.7
Examples of the operation of slotted ALOHA, framed slotted ALOHA, and adaptive framed slotted ALOHA.
(a) Slotted ALOHA. (b) Framed slotted ALOHA. (c) Adaptive framed slotted ALOHA.
170 RFID Handbook: Applications, Technology, Security, and Privacy
We refer to this protocol as DFSA in this chapter.
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