Vehicular Sensor Networks: General Aspects and Implementation Issues - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 222

10.6.2.2 Active Tags

An RFID tag is an active tag when it is equipped with a battery that can be used as
a partial or a complete source of power for the tag's circuitry and antenna. Some
active tags contain replaceable batteries for years of use, others are sealed units. It
is also possible to connect the tag to an external power source. The major advan-
tages of active RFID tags are that it can be read at distances of 100 ft or more,
greatly improving the utility of the device. However, active RFID tags are typically
more expensive and physically larger, which may limit applications [ 34 ].
Both tagging technologies, i.e., active and passive, are needed for RFID to release
its full potential. The cost savings afforded by passive tags makes RFID tagging
possible at a much lower price point than would be possible with active tags alone.
On the other hand, active tags add functionality not possible with passive tags.
RFID tags can be also classified according to the memory capabilities. Namely,
read-only tags have a unique ID code which is permanently stored on the tag, while
the read / write tags are allowing a user to change the ID and add additional data to
the tag's memory. In addition, these tags can be combined with a permanent tag ID
and some storage space for the user's data. Passive tags typically have anywhere
from 64 bits to 1 kB of non-volatile memory. Active tags tend to have larger memo-
ries with a range of, typically, between 16 bytes and 128 kB [ 32 ].

10.6.3 RFID Reader

An RFID reader or a scanning device typically contains a module (transmitter and
receiver), a control unit, and a coupling element (antenna). They vary in size, weight,
and power, and may be mobile or stationary. The reader has three main functions:
energizing, demodulating, and decoding. In addition, readers can be fitted with an
additional interface that converts the radio waves returned from the RFID tag into a
form that can then be passed on to another system, like a computer or any program-
mable logic controller. Anti-collision algorithms permit the simultaneous reading of
large numbers of tagged objects, while ensuring that each tag is read only once. This
is any device, such as a laptop computer, an electronic scale head, or a handheld
computer, that is capable of communicating with a transceiver and accepting the
information from it. Usually, the RFID-tagged information is then stored in an RFID
database used to tie electronic identity to production and management information,
message the data, and share the information with others [ 35 ].
There are two main classes of RFID readers, i.e., read-only and read / write . The
main difference is that the latter ones can write new information back to a tag that
has been equipped with a read/write memory. The readers are becoming increas-
ingly sophisticated, acting as gateways into the network-centric communication
systems of modern enterprises by supporting communication protocols such as
TCP/IP and network technologies such as DHCP, UDP/IP and Ethernet, or 802.11×
(for wirelessly sending data back to the enterprise). Many models of readers are