Information Technology Reference
7.2 Active tags
Active tags are equipped with an internal power source, often a small lithium
battery. This is used to supply power to the transceiver and to keep alive the
content of a static RAM, where tag data are stored. The use of an internal
power supply enables the tag to operate at higher frequencies and higher
transmission power, thus reaching a longer operational range up to 15-20 m.
These types of tags are used for automatic identification of moving objects,
such as cars along the highway, where the detection distance can be quite
long and not fixed. Most of the existing active tags operate within the 450
and 900 MHz bands, for which there is a wide range of cheap electronic
components commonly available.
Despite its name, an RFID tag reader is a device that can read (decode) and
write (encode) data from/to a transponder. Besides these basic activities, a
reader is often part of a LAN in order to receive data to be written on the tag
or to forward data read from the tag.
A reader is normally composed of a control unit (the controller ), a network
controller with different interfaces (serial ports RS232/RS422/RS485, Ethernet,
Fielbdus, Profibus, etc.), and one or more antennas to exchange data with the
tags. An external dedicated antenna allow for better performances, but smaller
systems, such as the Compact-Flash ones, have small integrated antennas.
The reader plays a key role within an RFID system. In fact, it must deal
with all radio communications with all the tags that are within its range,
correctly managing data exchange with many tags simultaneously, and
avoiding or resolving collisions, if some occurs.
RFID tag readers may be fixed (production lines, moving objects control
and detection), transportable (installation on trucks, forklifts, etc.), or hand-
held to be integrated within a PDA for activities 'in the field'.
7.4 RFID systems
An RFID system consists of a transceiver (reader) and one or more
transponders communicating one another via a frequency-modulated radio
signal. The transponders receive the RF signal from the reader and then send
their data back to the reader by modulating an RF signal. In the case of
passive tags, the energy needed for their operation is produced by the same
RF received signal. Data sent to the reader is often a unique code, chosen
among several billion of possible combinations, which is stored on the chip
during its production.