Data Integrity - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 206

SELECT
10111010

Downlink REQUEST

1 2 3

Uplink

Collision Collision 10111010

Transponder 1

10110010

Transponder 2

10100011

Transponder 3

10110011

Transponder 4

11110101

Transponder 5

10111010

t

T

Figure 7.17 Transponder system with slotted ALOHA anticollision procedure

there are slots available. If many further transponders are added, then the throughput
quickly falls to zero. In the worst case, no serial numbers can be detected even after
an infinite number of attempts because no transponder succeeds in being the only one
to transmit in one slot. This situation can be eased by the provision of a sufficient
number of slots. However, this reduces the performance of the anticollision algorithm,
since the system has to listen for possible transponders for the duration of all time
slots — even if only a single transponder is located in the interrogation zone of the
reader. Dynamic S-ALOHA procedures with a variable number of slots can help here.
One possibility is to transmit the number of slots (currently) available for the
transponders with each REQUEST command as an argument: in wait mode the reader
transmits REQUEST commands at cyclical intervals, which are followed by only one
or two slots for possible transponders. If a greater number of transponders cause a
bottleneck in both slots, then for each subsequent REQUEST command the number
of slots made available is increased (e.g. 1, 2, 4, 8, ... ) until finally an individual
transponder can be detected.
However, a large number of slots (e.g. 16, 32, 48, ... ) may also be constantly
available. In order to nevertheless increase performance, the reader transmits a BREAK
command as soon as a serial number has been recognised. Slots following the BREAK
commands are 'blocked' to the transmission of transponder addresses (Figure 7.18).

Downlink REQUEST
Uplink

1 2 3 4 BREAK! 6 7 8

Transponder 1

10110010

Transponder 2

Figure 7.18 Dynamic S-ALOHA procedure with BREAK command. After the serial number
of transponder 1 has been recognised without errors, the response of any further transponders
is suppressed by the transmission of a BREAK command