Standardisation - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 240

S b0 .... b7 P b0 .... b7 P b0 .... b7 P E

LSB MSB LSB MSB LSB MSB

Figure 9.18 With the exception of the REQA command and data transmitted during the anti-
collision routine, all data sent between reader and card (i.e. command, response and useful data)
is transferred in the form of standard frames. This always begins with a start-of-frame signal (S),
followed by any desired number of data bytes. Each individual data byte is protected against
transmission errors by a parity bit. The data transmission is concluded by an end-of-frame
signal (E)

PCD:

SEL '93' NVB '25' 1111 0

NVB: 2 Byte, 5 Bit

PICC:

000 1111 0000 1111 0000 1111 0000 1111 0000

UID size: triple SEL '93' = CL1

CT2 '08' UID 0 UID 1 UID 2 UID 3

SEL '95' = CL2

UID 4 UID 5 UID 6 UID 7 UID 8

SEL '97' = CL3

UID 9 UID 10 UID 11 RFU BCC

UID size: double SEL '93' = CL1

SEL '95'

CT1 '88' UID 0 UID 1 UID 2 UID 3

= CL2

UID 4 UID 5 UID 6 UID 7 BCC

UID size: single SEL '93' = CL1

UID 0 UID 1 UID 2 UID 3 BCC

Figure 9.19 A dynamic binary search tree algorithm is used for the determination of the serial
number of a card. The serial numbers can be 4, 7 or 10 bytes long, so the algorithm has to be
run several times at different cascade levels (CL)

the card's SAK, by a set cascade bit (b3 = 1), with the card remaining in the READY
state. This results in the anticollision algorithm being restarted in the reader so that
it can detect the second part of the serial number. In a triple size serial number the
anticollision algorithm must even be run a third time. To signal to the card which
part of the serial number is to be detected by the algorithm that has been initiated,
the SELECT command differentiates between three cascade levels (CL1, CL2, CL3)
(Figure 9.19). However, the process of detecting a serial number always begins with
cascade level 1. In order to rule out the possibility of fragments of a longer serial
number corresponding by coincidence with a shorter serial number, so-called cascade
tags (CT = 88 h) are inserted at a predetermined position in the double or triple size
numbers. This value may therefore never occur at the corresponding byte positions in
the shorter serial numbers.
Precise timing between a reader's command and the smart card's response should
also be ensured. The standard prescribes a synchronous behaviour of the smart card,