Although different RFID tags may operate, as defined in ISO/IEC 18000 standardization documents, Part 1 - Part
7, at several radio frequency bands - bellow 135 KHz for Low-Frequency, at 13.56 MHz for High Frequency, at 433
MHz or at 860 MHz to 960 MHz for Ultra High Frequency (UHF), at 2.45 GHz for Super High Frequency - we will
focus on UHF band tags, mainly on account of the envisaged distance between readers and tags. There are different
types of RFID tags, operating at different radio frequency bands, and those frequency bands determine, almost
directly, the feasible reading distances, that is the distance range between readers and tags that ensure tag reading.
Those distances may vary, and for passive RFID tags, from a few centimeters (when Low Frequency bands are used)
to several meters, for UHF band, as Table 1 presents. Also, tags may be classified as Read Only (RO), Write Once
Read Many (WORM) and Write Many Read Many (WMRM) corresponding the type of the access to information
that is kept in its memory structure. Furthermore, tags may be characterized as active, passive and semi-passive,
when dealing with the existence (or not) of any internal power source: active tags do have an internal power source,
used for processing and communications; passive tags have no internal power source and derive their power, needed
for communication purposes, directly from the energy transmitted by the reader antenna, by means of power
harvesting; semi-passive tags normally have a very small internal power source, used essentially for internal
processing (e.g. sensing, data logging). Semi-active tags are only active when programmed to send a signal at
previously predetermined intervals or when interrogated by the readers antenna, from which they derive power for
data transmissions.