Fundamental Operating Principles - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 28

Fundamental Operating
Principles
This chapter describes the basic interaction between transponder and reader, in par-
ticular the power supply to the transponder and the data transfer between transponder
and reader (Figure 3.1). For a more in-depth description of the physical interactions
and mathematical models relating to inductive coupling or backscatter systems please
refer to Chapter 4.
3.1 1-Bit Transponder
A bit is the smallest unit of information that can be represented and has only two states:
1 and 0. This means that only two states can be represented by systems based upon a
1-bit transponder : 'transponder in interrogation zone' and 'no transponder in interro-
gation zone'. Despite this limitation, 1-bit transponders are very widespread — their
main field of application is in electronic anti-theft devices in shops ( EAS , electronic
article surveillance).
An EAS system is made up of the following components: the antenna of a 'reader'
or interrogator, the security element or tag , and an optional deactivation device for
deactivating the tag after payment. In modern systems deactivation takes place when the
price code is registered at the till. Some systems also incorporate an activator ,which
is used to reactivate the security element after deactivation (Gillert, 1997). The main
performance characteristic for all systems is the recognition or detection rate in relation
to the gate width (maximum distance between transponder and interrogator antenna).
The procedure for the inspection and testing of installed article surveillance
systems is specified in the guideline VDI 4470 entitled 'Anti-theft systems for
goods — detection gates. Inspection guidelines for customers'. This guideline contains
definitions and testing procedures for the calculation of the detection rate and false
alarm ratio. It can be used by the retail trade as the basis for sales contracts or
for monitoring the performance of installed systems on an ongoing basis. For the
product manufacturer, the Inspection Guidelines for Customers represents an effective
benchmark in the development and optimisation of integrated solutions for security
projects (in accordance with VDI 4470).