Hence, a semipassive tag does not need to harvest energy to power
the circuitry for the backscatter communication to produce a stronger signal that is easier for
the reader to detect, resulting in increased range or the capability to be read in an unfriendly
environment. Semipassive tags are currently investigated for use in the cold chain, where
items (such as frozen foods or drugs) must be kept below a given temperature.
Active tags have an onboard power supply, active receiver, distinct active transmitter,
and may talk with each other and form a network. Active tags are very similar to nodes in
a sensor network. ZigBee and IEEE 802.11 networks can be considered as containing active
tags using a very broad de?¬?nition of active tags. One standard de?¬?ning a network of active
tags, ISO 18000??“7, does not permit tag-to-tag communication, but only communications
between a tag and a reader. Because of the active transmitter, active tags can transmit a
signal to a reader several hundred meters away. Similarly, the active receiver enables the
active tag to receive a very weak signal from devices up to several hundred meters away.
The active communications hardware enables active tags to be used in places with large
amounts of metal, which is typically very unfriendly to passive and semipassive tags. As
more memory can be incorporated into an active tag, it is often used to store information
about a shipment of goods in a shipping container.
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