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Acquisition Modes
The AcquireMode property defines the method used to read tags in the field.
There are two distinct methods for reading tags, and the choice of one method
over another depends on the application at hand.
You specify the Acquire Mode by issuing the
set AcquireMode
command. It
can take one of two values, "Global Scroll" or "Inventory”.
Global Scroll
Global Scroll is the most primitive of tag ID reading operations supported by the
Alien RFID Reader system. When a global scroll command is issued, the RFID
Reader sends a single command over the air to all and any tags. That command
is simply a request for any tag to immediately send back its ID to the RFID
Reader.
The simplicity of this command is both its advantage and its downfall: The
command is very quick to execute as it involves only one round trip between the
reader and the tag. However, because the command is so simple, problems may
arise if there is more than one tag in the field. At this point, multiple tags will all
receive the same command, and will all send back their IDs to the reader at
virtually the same time. A situation such as this makes it difficult for the reader to
discern individual IDs among the general noise. Typically one or two of the
loudest or closest tags will be decoded, but the majority will not be discerned.
This is analogous to walking into a dark room full of people and shouting out the
command “if anyone can hear me, shout your name back now”. If there is one
person in the room with you, you will be able to hear their name. If there are
multiple people in the room, the results will be noise. Maybe you will be able to
make out one or two names, but typically not more than that.
There are many applications where global scroll is the best tag reading method
to use. These applications typically expect just one or two tags in the field of view
at any one time, such as conveyor belt or tollbooth applications. For these
systems, global scroll outperforms a full inventory by a factor of three as far as
individual read rates are concerned.
Inventory
The inventory command is a full-featured system for discerning the IDs of
multiple tags in the field at the same time. This single high-level command
transforms itself into a complex series of reader-tag interrogations that eventually
resolve themselves into a single list of tag IDs seen by the RFID Reader. This
method of interrogation and evaluation of multiple tags is known as an anti-
collision search.
Continuing the analogy used in the global scroll description, the anti-collision sort
works in the following way: You walk into a dark room full of people and instruct
everyone to stand up. Then you start with the letter ‘A’ and tell anyone whose
name begins with this letter to shout their name back. You may get zero replies,
one reply, or multiple noisy replies. If you can clearly make out any individual
name from the noise, you shout back telling that person to sit down and be quite
from now on. Next repeat the series of events, this time telling anyone whose
name begins with ‘AA’ to shout back their name. If you can pick out one name,
tell that person to sit down and be quite. You repeat this until no names are
heard at all, each time adding a letter such as ‘AB’, ‘AC’. When it gets to the point
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