Setup Guide

Concepts (Performing an Inventory)
(Pointing to the next child.) she's moved on to the next child. She puts her hand down and sets
her state to “Inventoried”.
“Mikey who?” “Jones!”
“Pardon me.” If the reader doesn't understand the reply it can issue a NAK and
try again.
“Mikey who?” “Jones!”
“You!”
(On to the next child.)
Mikey puts his hand down, too and sets his state to “Inventoried”.
And off they go...
When the teacher reaches the end of the round because she sees no more raised hands, she is
done.
This is clearly contrived and an oversimplification of both the teacher's real-life protocol and
Gen2, but it does captures some of the important features:
1. Inventories of the field need an anti-collision protocol to prevent multiple tags from
talking at the same time.
2. An inventory can begin with one or more Select commands that establish who will
participate in the inventory. (Teacher: “Ok, only the boys, put your hands up!”)
3. The state of whether or not a tag has been inventoried is maintained in the tag.
4. In the process of singulating a tag, the reader gets a handle (the child's first name in this
example) that it can use for additional operations with that tag (more on this below).
The analogy breaks down when you realize that unlike the teacher, the reader cannot see the
inventoried state of the tags (hands in the air). If the teacher tried to take attendance of the
class from behind a curtain, it would be a lot more difficult. Rather than pointing at a child and
saying, “You!” to keep them from talking at once, a different protocol would be needed.
In Gen2, this is accomplished with the Query command. When the reader issues a Query
command, it includes in the message a parameter called Q that the tags use to determine if
they will respond immediately, or after some number of subsequent QueryRep commands. The
number of Query or QueryRep commands the tag will wait to hear is determined randomly
and can vary from 1 to 2
Q
.
By adjusting the Q parameter used in its Query commands, the reader can prevent multiple
tags from responding simultaneously, most of the time. If there is a collision, the reader can
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