30-05-2012, 04:01 PM
Introduction to Wireless Sensor Networks
Introduction to Wireless Sensor Networks.pdf (Size: 454.7 KB / Downloads: 184)
Overview
With the popularity of laptops, cell phones, PDAs, GPS devices, RFID, and
intelligent electronics in the post-PC era, computing devices have become
cheaper, more mobile, more distributed, and more pervasive in daily life.
It is now possible to construct, from commercial o-the-shelf (COTS) com-
ponents, a wallet size embedded system with the equivalent capability of a
90's PC. Such embedded systems can be supported with scaled down Win-
dows or Linux operating systems. From this perspective, the emergence of
wireless sensor networks (WSNs) is essentially the latest trend of Moore's
Law toward the miniaturization and ubiquity of computing devices.
Enabling Technologies
Hardware
The hardware basis of WSNs is driven by advances in several technologies.
First, System-on-Chip (SoC) technology is capable of integrating complete
systems on a single chip. Commercial SoC based embedded processors from
Atmel, Intel, and Texas Instruments have been used for sensor nodes such as
UC Berkeley's motes [48; 173], UCLA's Medusa [120] and WINS [197], and
MIT's AMPS-1 [187]. Several research groups, such as the PicoRadio team
from UC Berkeley [139], have been trying to integrate prototype sensor
nodes (PicoNode I) onto a few chips (PicoNode II).
Wireless Networking
Besides hardware technologies, the development of WSNs also relies on
wireless networking technologies. The 802.11 protocol, the rst standard
for wireless local area networks (WLANs), was introduced in 1997. It was
upgraded to 802.11b with an increased data rate and CSMA/CA mech-
anisms for medium access control (MAC). Although designed for wire-
less LANs that usually consist of laptops and PDAs, the 802.11 proto-
cols are also assumed by many early eorts on WSNs. However, the
high power consumption and excessively high data rate of 802.11 pro-
tocols are not suitable for WSNs. This fact has motivated several re-
search eorts to design energy ecient MAC protocols [109; 145; 189;
206]. Recently, the 802.15.4-based ZigBee protocol was released, which was
specically designed for short range and low data rate wireless personal
area networks (WPAN). Its applicability to WSNs was soon supported by
several commercial sensor node products, including MicaZ [48], Telos [140],
and Ember products [58].
Collaborative Signal Processing
Collaborative signal processing algorithms are another enabling technology
for WSNs. While raw data from the environment are collected by sensor
nodes, only useful information is of importance. Hence, raw data need to
be properly processed locally at sensing nodes, and only processed data
is sent back to the end users. Since computation is much more energy
ecient than wireless communication, this avoids wasting energy on sending
large volumes of raw data. Such signal processing is often required to be
performed by a set of sensor nodes in proximity, due to the weak sensing
and processing capabilities of each individual node.
Military Networks of Sensors
Since the early 1950s, a system of long-range acoustic sensors (hy-
drophones), called the Sound Surveillance System (SOSUS), has been de-
ployed in the deep basins of the Atlantic and Pacic oceans for submarine
surveillance. Beams from multiple hydrophone arrays are used to detect
and locate underwater threats. Recently, SOSUS has been replaced by the
more sophisticated Integrated Undersea Surveillance System.
Networks of air defense radars can be regarded as an example of net-
worked large scale sensors. Both ground-based radar systems and Airborne
Warning and Control System (AWACS) planes are integrated into such
networks to provide all-weather surveillance, command, control, and com-
munications. The radar dome on AWACS planes is 30 feet in diameter
and six feet thick. It can detect
ying targets in a range of more than 200
miles. In the 1980s and 1990s, the Cooperative Engagement Capability
(CEC) [33] was developed as a military sensor network, in which informa-
tion gathered by multiple radars was shared across the entire system, to
provide a consistent view of the battle eld.