R
EM4223
10. COLLISION ARBITRATION
2. The Reader detects a collision between two or more
Transponder replies. Collisions may be detected
either as contention from the multiple transmissions
or by detecting an invalid CRC. After waiting until the
channel is clear, the Reader sends a Close_Slot
command to increment the Transponder slot
counter.
The EM4223 implements the ISO 18000-6 Type A anti-
collision scheme as described in CD ISO-IEC 18000 part
6 Type A. Additionally, the EM4223 implements the Fast
Supertagꢀ anti-collision protocol.
The basic collision arbitration scheme is based on slots.
The ISO implementation uses regimented slots that are
controlled by the Reader. Fast Supertagꢀ uses pseudo-
slots (non-synchronised slots) by virtue of the fact that
transmissions are initiated in integer multiples of a slot
time. However because Transponder clocks will not be
identical and because the Reader does not synchronize
slots at the start of each slot, there will be a natural drift
and the timing of slots between individual Transponders
will diverge.
3. The Reader receives a Transponder Reply without
error, i.e. with a valid CRC. The Reader sends a
Next_slot
command
synchronized
to
the
Transponder timing window, containing the
signature of the Transponder just received.
When Transponders in the ROUND_ACTIVE state that
have not transmitted in the current slot receive a
Next_slot command or a Close_Slot command, they
increment their slot counters by one. When the slot
counter equals the slot number previously selected by
the Transponder, the Transponder transmits according to
the rules above otherwise the Transponder waits for
another command.
Refer to the state diagram, Fig. 19.
General explanation of the collision arbitration
mechanism
The collision arbitration uses a mechanism, which
allocates Transponder transmissions into rounds and
slots. A round consists of a number of slots. A
Transponder will only transmit once in a round unless the
Transponder is in ISO mode and the WUS bit= 0, in
which case the Transponder will reply in the first slot as
well as in its chosen slot, or only in the first slot if the first
slot was selected as the Reply slot by the Transponder.
The time position where it transmits in a round is
determined randomly.
The Reader keeps track of the slot count each time it
issues a Next_slot command or Close_Slot command.
When the number of slots used equals the round_size
issued in the Init_round command, the round has
completed and the Reader may issue a round initializing
command. (Note: A Reader may issue a round initializing
command at any time).
Transponders that have not been acknowledged (by a
synchronous Next_Slot command with a valid signature)
during the current round, will enter a new round on
determining the end of the current round or at any time
ISO COMPLIANT SYSTEMS
on receiving
a
round initializing command. The
Each slot has a duration at least as long as a
Transponder transmission or as long as the Reader
requires to identify an unproductive (empty) slot and
send the CLOSE_SLOT command to the Transponder
population. The Reader determines the duration of the
slot by closing slots with CLOSE_SLOT or NEXT_SLOT
commands in response to successful data replies from
Transponders or clashing replies from Transponders or
in response to identifying an empty slot.
Transponders will select a slot at random and transmit in
the new round when the slot counter value and the slot
selected are equal.
If at any time the Transponder receives a wake_up (FST)
command whether in the READY state or in the ISO
ROUND_ACTIVE or ROUND_STANDBY states, it will
immediately switch to the FST mode of operation.
On receiving an Init_round command, Transponders
randomly select a slot in which to respond. If a
Transponder has selected the first slot it will transmit its
Reply. The Transponder includes its four-bit
Transponder signature in its Reply. If the Transponder
has selected a slot number greater than one, it will retain
its slot number and wait for a further command.
FST SYSTEMS
In the absence of an RF field, the Transponders are in
the RF_field_off state. When the Transponders enter the
energizing field of a Reader, they go through a power on
reset sequence. If the FST bit = 0 and the WUS bit = 0,
then the Transponder moves to the ROUND_ACTIVE
State it is therefore in a Tag Talks First mode and
commences
a Fast Supertagꢀ collision arbitration
After the Reader has sent the Init_round command there
are three possible outcomes:
sequence. If the FST bit = 0 and the WUS bit = 1, then
the Transponder moves to the ROUND_STANDBY state
until it receives a Next_Slot, Close_Slot, New_Round or
Wake_up_FST command, at which time it commences a
Fast Supertagꢀ collision arbitration sequence.
1. The Reader does not receive a Reply because
either no Transponder has selected slot one or the
Reader has not detected a Transponder Reply. The
Reader then issues a Close_Slot command because
it has not received a Reply.
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