Standardisation - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 238

9.2.2.3 Part 3 - Initialisationandanticollision

If a proximity coupling smart card enters the interrogation field of a reader, then a
communication relationship must first of all be built up between reader and smart
card, taking into consideration the fact that there may be more than one smart card
within the interrogation zone of this reader and that the reader may already be in
communication with another card. This part of the standard therefore first describes
the structure of the protocol frames from the basic elements defined in Part 2 — data
bit, start-of-frame and end-of-frame marks — and the anticollision procedure used
for the selection of an individual card. Since the different modulation procedure
for Type A and Type B also requires a different frame structure and anticollision
procedure, the divide between the two types A and B is reflected in Part 3 of the
standard.
TypeAcard As soon as a Type A smart card enters the interrogation zone of a reader
and sufficient supply voltage is available, the card's microprocessor begins to operate.
After the performance of some initialisation routines — if the card is a dual interface
card these include checking whether the card is in contactless or contact mode — the
card is put into so-called IDLE mode . At this point the reader can exchange data
with another smart card in the interrogation zone. However, smart cards in the IDLE
state may never react to the reader's data transmission to another smart card ('any
command') so that an existing communication is not interrupted.
If, when the card is in IDLE mode, it receives a valid REQA command (Request-A),
then an ATQA block (Answer to Request) is sent back to the reader in response
(Figure 9.16). In order to ensure that data destined for another card in the interrogation
field of the reader is not falsely interpreted as a REQA command, this command is
made up of only 7 data bits (Figure 9.17). The ATQA block sent back, on the other
hand, consists of 2 bytes and is returned in a standard frame.
After the card has responded to the REQA command it is put into the READY
state. The reader has now recognised that at least one card is in the interrogation
field and begins the anticollision algorithm by transmitting a SELECT command.
The anticollision procedure used here is a dynamic binary search tree algorithm . 4
A bit-oriented frame is used for the transfer of the search criterion and the card's
response, so that the transmission direction between reader and card can be reversed
after a desired number of bits have been sent. The NVB (number of valid bits)
parameter of the SELECT command specifies the current

length of the search

criterion.
The length of a single serial number is 4 bytes. If a serial number is detected by the
anticollision algorithm, then the reader finally sends the full serial number (NVB =

40 h) in the SELECT command, in order to select the card in question. The card with the
detected serial number confirms this command by an SAK (SELECT-Acknowledge)
and is thereby put into ACTIVE state, the selected state. A peculiarity, however, is that
not all cards possess a 4-byte serial number (single size). The standard also permits
serial numbers of 7 bytes (double size) and even 10 bytes (triple size). If the selected
card has a double or triple size serial number, this will be signalled to the reader in

4 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 7.2.4.3.