Standardisation - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 241

which means that the response may only be transmitted at defined moments in a fixed
time grid (Table 9.9).
For the response to a REQA, WakeUp or SELECT command N = 9. For all other

commands (e.g. application commands) N must be greater than or equal to 9 ( N =
9 , 10 , 11 , 12 ,... ).
Type B cards If a Type B smart card is brought within the interrogation field of a
reader, the smart card, after the performance of a few initialisation routines, is initially
put into IDLE mode and waits to receive a valid REQB (REQUEST-B) command (see
Figure 9.20).
The transmission of a REQB command immediately initiates the anticollision algo-
rithm in Type B cards. The procedure used here is a dynamic slotted ALOHA proce-
dure , 5 in which the number of slots can be dynamically changed by the reader. The
number of slots currently available is encoded in a parameter of the REQB command.
In order to facilitate a preselection during the selection of a card, the REQB command
has a further parameter, the Application Family Identifier (AFI), which allows a certain
application group to be entered as a search criterion (Table 9.10).
After a card has received a valid REQB command it checks whether the application
group preselected in the parameter AFI is present in the applications stored on the card.
If so, the parameter M of the REQB command is evaluated to detect the number of
slots available for anticollision (Table 9.11). If the number of available slots is greater
than one, a random-check generator in the card is used to determine the number of the
slot in which the card wishes to transmit its response to the reader. In order to guar-
antee the synchronisation of the cards with the slots, the reader transmits its own slot
marker at the beginning of each slot. The card waits until the slot marker of the previ-
ously determined slot is received (Ready Requested State) and responds to the REQB
command by sending an ATQB (Answer To Request B) See Figures 9.21 and 9.22.
A short time after the transmission of a slot marker (Figure 9.23) the reader can
determine whether a smart card has begun to transmit an ATQB within the current
slot. If not, the current slot can simply be interrupted by the transmission of the next
slot marker in order to save time.
The request response ATQB sent by the smart card provides the reader with a
range of information about important parameters of the smart card (Figure 9.22). In
order to be able to select the card, the ATQB first of all contains a 4-byte serial num-
ber. In contrast to Type A cards, the serial number of a Type B card is not necessarily
permanently linked to the microchip, but may even consist of a random number, which
is newly determined after every Power-on reset (PUPI, pseudo unique PICC identifier).

Table 9.9 Required time grid for the transponder response
during anticollision

Last received byte Required behaviour

= (n · +

84 ) · t 0 '1' t RESPONSE 128

'0' t RESPONSE = (n · 128 + 20 ) · t 0

5 Knowledge of this procedure is a prerequisite at this point. A step-by-step introduction into the method
of functioning can be found in Section 7.2.4.2.