User's Manual

CAEN RFID API– User Manual
13
Optimizing the inventory process
EPC Class1 Gen2 protocol defines a set of parameters useful for the optimization of the tags' detection, in the current
paragraph we give a brief explanation of the most useful ones and we show how to work with those parameters using
the CAEN RFID API.
The Q parameter
The Gen2 protocol inventory method is based on the so called “Slotted ALOHA” algorithm; without going into the
details of the algorithm, it is important to know that it foresees a division of the time in discrete slots. Only one tag can
be detected for each slot, if two or more tags reply on the same slot a collision is generated and the tags are not
detected so a further iterations of the algorithm is needed.
The number of time slots is defined in the Gen2 protocol as 2Q where Q is a parameter ranging from 0 to 15 that can
be set by the user.
The optimal Q value for a certain application is related to the average number of tags that are simultaneously present
in the reading zone. A few tags require only a few slots, whereas many tags require many slots. Left to its own, the
reader doesn’t have any way of knowing how many tags are under the antenna's field until it counts them, which may
be difficult if its initial “guess” is wildly wrong. CAEN RFID reader will work faster and more efficiently if you provide an
accurate starting value for Q corresponding to the expected tag population: the more are the collisions the worse are
the performance of the reader in detecting the tags. The detection method implemented inside CAEN RFID readers has
also an auto-adaptive mechanism in order to generate more or less time slots when needed but, starting with an
adequate number of time slots, helps this mechaism to avoid wasting time.
For reference, the following table gives recommended values of Q that produce reasonably efficient inventories for
varying numbers of tags in the read zone.
Estimated number of tags
Starting Q value
1
0
2
1
3 – 6
2
7 – 15
3
16 – 30
4
30 – 50
5
50 – 100
6
100 – 200
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Tab. 3.1: Recommended Q values
CAEN RFID API provides the methods to set and read back the starting Q value: SetQ_EPC_C1G2 and GetQ_EPC_C1G2.