Data Integrity - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 213

Command: Request 10110 111 11111111 11111111 11111111

011 10011001 01011001 00101011

Response: 10110

Bit: N... X (X − 1) .............................................. 0

Figure 7.24 Reader's command ( n th iteration) and transponder's response when a 4-byte serial
number has been determined. A large part of the transmitted data in the command and response
is redundant (shown in grey). X is used to denote the highest value bit position at which a bit
collision occurred in the previous iteration

• Bits ( X − 1) to 0 of the command contain no additional information for the

transponder since they are always set to 1.

• Bits N to X of the serial number in the transponder's response contain no additional

information for the reader, as they are already known and predetermined.

We therefore see that complementary parts of the transmitted serial numbers are
redundant and actually do not need to be transmitted. This quickly leads us to an
optimized algorithm. Instead of transmitting the full length of the serial numbers in
both directions, the transfer of a serial number or the search criterion is now simply
split according to bit ( X ). The reader now sends only the known part ( N - X )ofthe
serial number to be determined as the search criterion in the REQUEST command and
then interrupts the transmission. All transponders with serial numbers that correspond
to the search criterion in the bits ( N - X ) now respond by transmitting the remaining bits

1st iteration: REQUEST NVB = 0 Reader Transponder

10110010

10100011

10110011

11100011

2nd iteration: REQUEST NVB = 2 10

110010

100011

110011

3rd iteration: REQUEST NVB = 4 1010

0011

Figure 7.25 The dynamic binary search procedure avoids the transmission of redundant parts
of the serial number. The data transmission time is thereby noticeably reduced