Data Integrity - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 204

0.4

0.3

0.2

0.1

0 0

1 2 3 4 5

Offered load G

ALOHA
S-ALOHA

Figure 7.15 Comparison of the throughput curves of ALOHA and S-ALOHA. In both proce-
dures the throughput tends towards zero as soon as the maximum has been exceeded

than the others that transponder may be able to override the data packets from other
transponders as a result of the greater signal strength at the reader. This is known as the
capture effect . The capture effect has a very beneficial effect upon throughput behaviour
(Figure 7.16). Decisive for this is the threshold b , which indicates the amount by which
a data packet must be stronger than others for it to be detected by the receiver without
errors (Borgonovo and Zorzi, 1997; Zorzi, 1995).

b · G
1 b +

S = G · e ( 7 . 6 )

The practical application of a slotted ALOHA anticollision procedure will now be
considered in more detail on the basis of an example.
The transponder used must also have a unique serial number (i.e. one that has been
allocated only once). In this example we use an 8-bit serial number; this means that a
maximum of 256 transponders can be put into circulation if the uniqueness of serial
numbers is to be guaranteed.
We define a set of commands in order to synchronise and control the transponders
(Table 7.2).
A reader in wait mode transmits a REQUEST command at cyclical intervals. We
now bring five transponders into the interrogation zone of a reader at the same time
(Figure 7.17). As soon as the transponders have recognised the REQUEST command,
each transponder selects one of the three available slots by means of a random-check
generator, in order to send its own serial number to the reader. As a result of the
random selection of slots in our example there are collisions between the transponders
in slots 1 and 2. Only in slot 3 can the serial number of transponder 5 be transmitted
without errors.