Civil Engineering Reference
In-Depth Information
There are two types of RFID systems: active and passive. In a passive RFID system,
an antenna transmits a radio signal to the RFID tag, which then disturbs the signal
in an identified expected way and returns the modified signal to the radiating
antenna. The biggest advantages of passive RFID systems are the low cost and long
useful life of tags. The biggest disadvantage is that their read distance is very limited;
the tag can be read at very short distances. On the other hand, active RFID tags have
batteries and transmit data either at a regular rate or when activated by other
transmitters. They have the advantage of being able to transmit longer distances
with smaller antennae, but aren't true location solutions since the distances are still
typically only 2-3 meters (Aksoy
et al
., 2004, 2005).
Other researchers, such as Skibniewski and Jang (2006), explored the use of
ZigBee wireless networks for use in civil infrastructure systems. A prototype
application was developed for object tracking and monitoring on construction
sites in order to provide insights on industrial practices in sensor and network-
based ubiquitous computing. The Zigbee protocol is less complex than Bluetooth,
has superior power management (with two AA batteries a ZigBee module can last
over years), supports many more nodes per network, has lower latency, lets devices
join the network more quickly and wakes up in milliseconds instead of seconds.
Tiny ZigBee sensors can be used to replace RFID tags, which transmit data.
However, the ZigBee protocol is designed to operate over a radio defined by
the IEEE 802.15.4 standard for the physical and data link protocol layers. As such,
ZigBee also inherits the disadvantages of this 802.15.4 standard, which is a
2.4 GHz direct sequence spread spectrum radio. A good general purpose radio
standard, 802.15.4 is not particularly well suited to applications with significant
distances between nodes, operation within buildings or other high blockage
environments, or operation in high interference environments. There are
proprietary products that implement the ZigBee networking protocol over
another type of radio, but these variations are entirely proprietary and discount
all of the advantages.
In the last few years, Wireless Local Area Network (WLAN) radio-signal-based
positioning systems, supported by underlying Radio Frequency (RF) and Infrared
(IR) transmission technologies, have seen enormous expansion. This trend is
expected to continue because it is an economical solution providing convenient
connectivity and high speed links, and can be implemented with relative ease in
software (Hightower and Borriello, 2001). The distance over which RF and IR
waves can communicate depends on product design (including transmitted power
and receiver design) and the propagation path, in particular in indoor environ-
ments. IR, blocked by solid objects, such as walls, metal, and even people,
constitutes a main limitation. For this reason, most WLAN systems use RF, because
radio waves can penetrate many indoor walls and surfaces. RF-based WLAN covers
a large area and is not restricted by line of sight issues. The range of a typical WLAN
node is about 100 meters (Wang and Liu, 2005). A WLAN system can support a
large number of nodes and vast physical areas by adding access points to extend
coverage. This means using access points to cover an area in such a way that their
coverage overlaps each other. This can allow users to navigate around and move
from the coverage area of one access point to another without even knowing they
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