If during the
interval two or more tags transmit, a complete or partial collision occurs. In order to
solve partial collision problems, transmission time is divided into discrete time intervals
in the slotted ALOHA [9]. All tags try to transmit their data after random back-off. If there
are no partial collisions under the slotted ALOHA protocol, the slotted ALOHA doubles
the channel utilization. A framed slotted ALOHA [10??“19] groups some slots into a frame,
each frame having N slots. In a frame, each tag transmits its data only once. Under the
framed slotted ALOHA, collisions caused by backlogged tags can be prevented.
FIGURE 9.6
An example of tag identi?¬?cation using
the adaptive binary splitting protocol with
four tags.
D D 3
2
1
0
Set of tags is changed
C
B
A
C F
B
A
Adaptive binary splitting
Progressed-slot
number
Splitting
Collision handling
Idle handling
Reader Reader Reader
D D D Reader Reader Reader
D D D C C C
F F F C F C C
F F B B B B
A A A A A A
1
0
Allocated-slot number
1
2
3
4
2 3 4 5 6Slot
168 RFID Handbook: Applications, Technology, Security, and Privacy
Under ALOHA, slotted ALOHA, and framed slotted ALOHA, the waiting time for a tag
is determined by a random function. The important factor which in?¬‚uences performance is
the relationship between the number of tags and random space and the maximum value of
the back-off timer.
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