These two discoveries form the basis of far-?¬?eld passive
RFID tags.
There are many types of RFID tags in existence, and although this chapter focuses on
passive RFID tags, a brief introduction of the other types of RFID tags is presented in this
section. EPCglobal designates four classes of RFID tags. Class 1 and Class 2 tags are purely
3
passive tags with Class 2 being a Class 1 tag with additional memory or supporting
additional protocol commands. Semipassive tags, designated as Class 3 and Class 4, are
active tags.
Semipassive tags have an onboard power source and may have onboard sensors. The
onboard power source serves two purposes: (1) it provides continuous power for the sensors
and (2) it allows the intelligence contained in the chip to function without harvesting
energy. When monitoring an asset, it is critical to take sensor readings at the required
intervals to obtain a complete history of the asset with respect to a given phenomenon. The
onboard power supply ensures that the semipassive tag can take these readings even in the
absence of a reader to power the tag. Another use of semipassive tags is to increase the read
range or to read the tag in an unfriendly environment. Because the chip in a semipassive tag
is powered by the battery, the semipassive tag is not required to harvest energy for operation
from the reader signal.
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