its vicinity due to the maximum number of connections
being reached at the other nodes, it re-connects to the first
node it had contacted in the network. With the second re-
quest, the master radio in that node will drop one of its slave
node connections and accept the connection from the new
node. The disconnected node will find another node in its
vicinity to connect. The network topology formed by this
procedure is a connected tree.
Experimental results indicate that Bluetooth-based sen-
sor networks using BTnodes are suitable for applications
that are active over a limited time period with a few unpre-
dictable traffic bursts. BTnodes can achieve high through-
put; however, they consume a lot of energy even when
idle. Connection maintenance is expensive and dual radios
are needed to support multi-hop routing. Hence, Bluetooth
can only serve as an alternative to broadcast radios.
Detection-and-classification system developed in VigilNet
[35] can detect and classify vehicles, persons, and persons
carrying ferrous objects. It targets objects with a maximum
velocity error of 15%. The VigilNet surveillance system con-
sists of 200 sensor nodes which are deployed in a pre-
planned manner into the environment. Their locations
are assigned at the time they are deployed. Each sensor
node is equipped with a magnetometer, a motion sensor,
and a microphone.
A hierarchical architecture was designed for this system
in order to distribute sensing and computation tasks to dif-
ferent levels of the system. The hierarchical architecture is
comprised of four tiers: sensor-level, node-level, group-le-
vel, and base-level. The lowest level, the sensor-level, deals
with the individual sensor and its sensing algorithm to de-
tect and classify objects. Once the sensing algorithm has
processed the sensor data, the classification result is sent
to the next level, namely the node-level. At the node-level,
classification deals with the fusion of various sensor data
obtained by the individual nodes. The node-level sensing
algorithm relays the sensor data from each sensor and
forms node-level classification results. Both the sensor-le-
vel and node-level classification functions reside on the
node itself. The next level is the group-level. This level of
classification is performed by a group of nodes. A set of
nodes is organized in a group, and a group leader is elected
to perform group-level classification. The input to the
group-level classification is the node-level classification re-
sults of the aggregated attributes. At group-level classifica-
tion, group leaders can accomplish more advanced tasks
and gain better knowledge of the location of the targets.
The highest level is the base-level classification. At this le-
vel, the results from the group-level classification are
transmitted via multi-hop to the base station. The base-le-
vel classification algorithm finalizes the results collected
and reduces false positives among the reported results.
VigilNet was deployed and tested in an outdoor site.
The system was able to accurately detect targets and re-
duce false negatives with a dense deployment of sensor
nodes.
5.2. Standards
Wireless sensor standards have been developed with
the key design requirement for low power consumption.
The standard defines the functions and protocols necessary
for sensor nodes to interface with a variety of networks.
Some of these standards include IEEE 802.15.4 [37], ZigBee
[38,39], WirelessHART [40,41], ISA100.11 [42], IETF 6LoW-
PAN [43–45], IEEE 802.15.3 [46], Wibree [47]. The follow-
ing paragraphs describes these standards in more detail.
IEEE 802.15.4: IEEE 802.15.4 [37] is the proposed stan-
dard for low rate wireless personal area networks (LR-
WPAN’s). IEEE 802.15.4 focuses on low cost of deployment,
low complexity, and low power consumption. IEEE
802.15.4 is designed for wireless sensor applications that
require short range communication to maximize battery
life. The standard allows the formation of the star and
peer-to-peer topology for communication between net-
work devices. Devices in the star topology communicate
with a central controller while in the peer-to-peer topol-
ogy ad hoc and self-configuring networks can be formed.
IEEE 802.15.4 devices are designed to support the physical
and data-link layer protocols. The physical layer supports
868/915 MHz low bands and 2.4 GHz high bands. The
MAC layer controls access to the radio channel using the
CSMA-CA mechanism. The MAC layer is also responsible
for validating frames, frame delivery, network interface,
network synchronization, device association, and secure
services. Wireless sensor applications using IEEE 802.15.4
include residential, industrial, and environment monitor-
ing, control and automation.
ZigBee [38,39] defines the higher layer communication
protocols built on the IEEE 802.15.4 standards for LR-PANs.
ZigBee is a simple, low cost, and low power wireless com-
munication technology used in embedded applications.
ZigBee devices can form mesh networks connecting hun-
dreds to thousands of devices together. ZigBee devices
use very little power and can operate on a cell battery for
many years. There are three types of ZigBee devices: Zig-
Bee coordinator, ZigBee router, and ZigBee end device. Zig-
Bee coordinator initiates network formation, stores
information, and can bridge networks together. ZigBee
routers link groups of devices together and provide mul-
ti-hop communication across devices. ZigBee end device
consists of the sensors, actuators, and controllers that col-
lects data and communicates only with the router or the
coordinator. The ZigBee standard was publicly available
as of June 2005.
WirelessHART: The WirelessHART [40,41] standard pro-
vides a wireless network communication protocol for pro-
cess measurement and control applications. The standard
is based on IEEE 802.15.4 for low power 2.4 GHz operation.
WirelessHART is compatible with all existing devices,
tools, and systems. WirelessHART is reliable, secure, and
energy efficient. It supports mesh networking, channel
hopping, and time-synchronized messaging. Network com-
munication is secure with encryption, verification, authen-
tication, and key management. Power management
options enable the wireless devices to be more energy effi-
cient. WirelessHART is designed to support mesh, star, and
combined network topologies. A WirelessHART network
consists of wireless field devices, gateways, process auto-
mation controller, host applications, and network man-
ager. Wireless field devices are connected to process or
plant equipment. Gateways enable the communication be-
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