Vehicular Sensor Networks: General Aspects and Implementation Issues - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 221

wave. This radio wave may be received by an RFID tag, which, as a result,
“reflects” some of the energy it receives in a particular way. While this reflection
occurs, the RFID reader is also acting as a radio receiver so that it can detect and
decode the reflected signal in order to identify the tag. The reader is larger, more
expensive, and it needs more power compared to the RFID tag. There are a number
of different types of RFID systems, but one basic classification is based on the
power source used by the tag [ 33 ].

10.6.2 RFID Tags

An RFID tag is a microchip combined with an antenna in a compact package. The
packaging is structured to allow the RFID tag to be attached to an object that is to
be tracked. The tag's antenna picks up signals from the RFID reader and then
returns the signal, usually with some additional data (like a unique serial number or
other customized information). Some RFID tags can be very small, the size of a
large rice grain, while others may be the size of a small paperback book. In RFID
systems, the tags that hold the data are divided into two different types, i.e., active
and passive tags.

10.6.2.1 Passive Tags

Passive tags have no battery and do not initiate communication. Instead, they draw
power from a reader, which sends electromagnetic waves that induce a current in
the tag's antenna and powers the microchip's circuits. Consistent with any appli-
cable authorization, authentication, and encryption, the tag will then respond to the
reader, sending via radio waves the data stored on it.
The advantage of a passive tag is the fact that these tags have a useful life of
20 years or more and are typically much less expensive to manufacture. Also, the
tag is of very small size (rice grain proportions). These tags have almost unlimited
applications in consumer goods and other areas. But, there are some disadvantages
of the passive tags. For example, the tag can be read only at very short distances,
approximately 30 ft. This greatly limits the device for certain applications. Also, it
may not be possible to include sensors that can use electricity for power. The tag
remains readable for a very long time, even after the product to which the tag is
attached has been sold and is no longer being tracked [ 34 ].
Semi-passive tags . These tags do not initiate communication with readers
(like the passive ones); however, they do possess batteries providing on-board
power used for storing information, e.g., ambient temperature. The semi-passive
tags can be combined with other sensors to create “smart dust”, tiny wireless
sensors that can monitor environmental factors. For example, a vineyard might
use smart dust to measure incremental weather changes that could critically
affect grapes [ 32 ].