Data Integrity - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 195

When CRC algorithms were first developed for disk controllers, priority was given
to the realisation of a simple CRC processor in the form of a hardware circuit. This
gave rise to a CRC processor made up of backcoupled shift registers and XOR gates
that is very simple to implement (Figure 7.6).
When calculating CRC 16 using shift registers, the 16-bit shift register is first
set to its starting value. The calculation is then initiated by shifting the data bits,
starting with the lowest in value, into the backcoupled shift register one after the other.
The backcoupling or polynomial division is based upon the XOR logic gating of the
CRC bits (Figure 7.7). When all the bits have been shifted through the register, the
calculation is complete and the content of the 16-bit CRC register represents the desired
CRC (Rankl and Effing, 1996).

7.2 Multi-Access Procedures - Anticollision

The operation of RFID systems often involves a situation in which numerous transpon-
ders are present in the interrogation zone of a single reader at the same time. In such a
system — consisting of a 'control station', the reader, and a number of 'participants',
the transponders — we can differentiate between two main forms of communication.
The first is used to transmit data from a reader to the transponders (Figure 7.8).
The transmitted data stream is received by all transponders simultaneously. This is
comparable with the simultaneous reception by hundreds of radio receivers of a news
programme transmitted by a radio station. This type of communication is therefore
known as broadcast (abramson, n.d.).
The second form of communication involves the transmission of data from many
individual transponders in the reader's interrogation zone to the reader. This form of
communication is called multi-access (Figure 7.9).
Every communication channel has a defined channel capacity, which is determined
by the maximum data rate of this communication channel and the time span of its
availability. The available channel capacity must be divided between the individual

Trans-
ponder 1

Trans-
ponder 2

Trans-
ponder 3

Reader

Trans-
ponder 4

Trans-
ponder 5

Trans-
ponder 6

Figure 7.8 Broadcast mode: the data stream transmitted by a reader is received simultaneously
by all transponders in the reader's interrogation zone