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Because only transponders of interest to the application
will be selected any other Transponders in the Reader
field will not degrade Reader performance by needing to
be read and acknowledge to send them to the QUIET
state – they virtually do not exist if they have not been
selected.
8. TRANSPONDER SELECTION OPERATION
– INIT_ROUND AND BEGIN_ROUND
COMMANDS
The INIT_ROUND and BEGIN_ROUND commands have
the ability to move only a selected sub-set of the
Transponder population from the READY to the
ROUND_ACTIVE states. Transponders that are already
in the ROUND_ACTIVE or ROUND_STANDBY states
will be removed from the active Transponder population
and moved to the READY state if they do not match the
selection parameters sent with the INIT_ROUND or
BEGIN_ROUND command.
The selection capabilities also allow the Transponder
population to be “Tree-Walked” allowing fully
“Deterministic” arbitration of a Transponder population.
By adding more and more bits to the selection criteria,
the population can be resolved down to a single
Transponder. (See the explanatory note below).
This allows the population to be “thinned”, thus
increasing
the
effective
read
rate
achieved.
EXPLANATION OF “DETERMINISTIC” OPERATION BASED ON “TREE-WALKING”
Transponders that use randomly selected reply slots in order to transmit their data to a Reader have a
very small risk of more than one Transponder selecting the same slot several times, which could mean
that such tags may not be read before they move out of the active population. This is known as
“Probabalistic” operation and must be balanced against the many advantages of this mode of operation.
“Tree Walking” is a method of resolving Transponder populations by effectively issuing a series of “tests”
or “challenges” in which the Reader would request a response from all tags containing say “0” in the 1st bit
position of the Transponder data (or in an encrypted version of the data). If the Reader received a non-
clashing response (only 1 transponder responding) it could request that Transponder to send its full data.
If the Reader received a clashing response (more than 1 transponder responding) it would know that it
had identified a productive “branch” and would extend its test by requesting a response from all tags
containing say “00” in the 1st two bit positions of the Transponder data. It would continue testing and
requesting responses until it had resolved the entire tag population in this manner. If the Reader received
no response it would know that it had identified an unproductive “branch” and would temporarily abandon
further testing for Transponders with “0” in the 1st bit position. The Reader would then test for
Transponders with “1” in the first bit position. This would continue until all Transponders had been
identified, or moved out of the Reader’s RF field.
INIT_ROUND COMMAND SELECTION OPERATION
(see Fig. 19)
If the AFI value contained in the INIT_ROUND command
is 0x00, the Transponders will ignore the parameter in
the command and all Transponders will move to the
ROUND_ACTIVE state from the ROUND_ACTIVE or
ROUND_STANDBY or READY states. With an AFI
parameter of 0x00, the command will perform identically
to an INIT_ROUND_ALL command.
The INIT_ROUND command contains a single fixed
length (8 bit) selection parameter. This parameter
represents the AFI (Application Family Identifier
according to ISO18000-6A) value which will be matched
with the AFI value contained in the Transponders
memory. Transponders with a matching AFI value will
move from the ROUND_ACTIVE or ROUND_STANDBY
or READY states to the ROUND_ACTIVE state and
commence participation in the Arbitration process.
Transponders that do not match the AFI value sent in the
command will remain in the READY state or they will
move to the READY state if they are already in the
ROUND_ACTIVE or ROUND_STANDBY states.
Tags in the QUIET state will ignore the INIT_ROUND
command.
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