Differentiation Features of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 10

Differentiation Features of
RFID Systems
2.1 Fundamental Differentiation Features
RFID systems exist in countless variants, produced by an almost equally high number
of manufacturers. If we are to maintain an overview of RFID systems we must seek out
features that can be used to differentiate one RFID system from another (Figure 2.1).
RFID systems operate according to one of two basic procedures: full duplex (FDX)/
half duplex (HDX) systems, and sequential systems (SEQ).
In full and half duplex systems the transponder's response is broadcast when the
reader's RF field is switched on. Because the transponder's signal to the receiver
antenna can be extremely weak in comparison with the signal from the reader itself,
appropriate transmission procedures must be employed to differentiate the transpon-
der's signal from that of the reader. In practice, data transfer from transponder to
reader takes place using load modulation, load modulation using a subcarrier, but also
(sub)harmonics of the reader's transmission frequency.
In contrast, sequential procedures employ a system whereby the field from the reader
is switched off briefly at regular intervals. These gaps are recognised by the transponder
and used for sending data from the transponder to the reader. The disadvantage of
the sequential procedure is the loss of power to the transponder during the break in
transmission, which must be smoothed out by the provision of sufficient auxiliary
capacitors or batteries.
The data capacities of RFID transponders normally range from a few bytes to several
kilobytes. So-called 1-bit transponders represent the exception to this rule. A data
quantity of exactly 1-bit is just enough to signal two states to the reader: 'transponder
in the field' or 'no transponder in the field'. However, this is perfectly adequate to
fulfil simple monitoring or signalling functions. Because a 1-bit transponder does not
need an electronic chip, these transponders can be manufactured for a fraction of a
penny. For this reason, vast numbers of 1-bit transponders are used in Electronic Article
Surveillance (EAS) to protect goods in shops and businesses. If someone attempts to
leave the shop with goods that have not been paid for the reader installed in the exit
recognises the state 'transponder in the field' and initiates the appropriate reaction. The
1-bit transponder is removed or deactivated at the till when the goods are paid for.