Installation guide

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Wi-Fi Location-Based Services—Design and Deployment Considerations
OL-11612-01
RFID Tag Considerations
Active tags can contain 512 KB of RAM (or more), which makes them ideal for access to telemetry
systems of attached assets. This enables the active tag to store information from these devices for
transmission at the next beacon interval or when polled by an active RFID reader. This large memory
capacity also makes active RFID preferable to passive RFID in situations when the RFID tag cannot
simply be used as a license plate for immediate lookup in a host database. A good example of this might
be a military installation where a host database may or may not be available at all times. By storing
critical asset data directly on the tag itself, this information can be used regardless of the availability of
the host system.
Active RFID tags can be found operating at frequencies including 303, 315, 418, 433, 868, 915, and
2400 MHz with read ranges that range from 60 to 300 feet. A distinguishing feature of active RFID tag
technology are very high read rates because of their higher transmitter output, optimized antenna, and
reliable source of onboard power. Active RFID tag cost can vary significantly depending on the amount
of memory, the battery life required, and whether the tag includes added value features such as onboard
temperature sensors, motion detection, or telemetry interfaces. The durability of the tag housing also
affects price, with the more durable or specialized housings required for specific tag applications coming
at increased cost. As with most electronic components of this nature, prices for active tags can be
expected to decline as technological advances, production efficiencies, and product commoditization all
exert a downward influence on active tag market pricing.
Active RFID tags can be sub-categorized into those that operate as transponders and those that operate
as beacons. Of special interest are the active RFID tags that operate in the unlicensed ISM bands and
abide by IEEE 802.11 protocols. These special active RFID tags are known as 802.11 (Wi-Fi) active
RFID tags and are covered in detail starting in 802.11 Active RFID Tags, page 92.
Beaconing Active RFID Tags
Beaconing active RFID tags are used in many RTLS systems and are of primary use when the precise
location of an asset needs to be tracked anywhere and anytime without the need for pre-positioned
interrogators or tag exciters to trigger tag transmission. With a beaconing active RFID tag, a short
message payload known as a “beacon” is emitted at pre-programmed intervals with the unique identifier
of the RFID tag. This interval is programmed into the tag by the tag owner or user and can be set
depending on the degree of criticality associated with tag location updates. For example, the beaconing
interval might be set for as little as every ten seconds or as much as twice a day, with the price paid for
more frequent beaconing being a reduction in the tag battery life along with an increase in RF network
traffic.
Transponder Active RFID Tags
Transponder active RFID tags contain their own power source that is switched on by an integral tag
circuit that is activated from a specialized type of reader/interrogator known as a tag exciter. Unlike
semi-passive tags that can only transmit responses when in the energy field of an interrogator, the
transponder active tag uses its onboard battery to directly power the transmission of tag responses.
Transponder active RFID tags are also sometimes referred to as semi-active RFID tags.
Many of the ubiquitous “toll tags” found in toll payment collection, checkpoint control, and other
systems are actually transponder active RFID tags. Figure 63 shows one such type of tag commonly seen
on U.S. highways.