11-05-2013, 04:08 PM
A Survey on Sensor Networks
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ABSTRACT
Recent advancement in wireless communications
and electronics has enabled the development
of low-cost sensor networks. The sensor
networks can be used for various application
areas (e.g., health, military, home). For different
application areas, there are different technical
issues that researchers are currently resolving.
The current state of the art of sensor networks is
captured in this article, where solutions are discussed
under their related protocol stack layer
sections. This article also points out the open
research issues and intends to spark new interests
and developments in this field.
INTRODUCTION
Recent advances in wireless communications and
electronics have enabled the development of lowcost,
low-power, multifunctional sensor nodes that
are small in size and communicate untethered in
short distances. These tiny sensor nodes, which
consist of sensing, data processing, and communicating
components, leverage the idea of sensor
networks. Sensor networks represent a significant
improvement over traditional sensors.
A sensor network is composed of a large
number of sensor nodes that are densely
deployed either inside the phenomenon or very
close to it. The position of sensor nodes need
not be engineered or predetermined. This allows
random deployment in inaccessible terrains or
disaster relief operations. On the other hand,
this also means that sensor network protocols
and algorithms must possess self-organizing
capabilities. Another unique feature of sensor
networks is the cooperative effort of sensor
nodes. Sensor nodes are fitted with an onboard
processor. Instead of sending the raw data to the
nodes responsible for the fusion, they use their
processing abilities to locally carry out simple
computations and transmit only the required and
partially processed data.
SENSOR NETWORKS
COMMUNICATION ARCHITECTURE
The sensor nodes are usually scattered in a sensor
field as shown in Fig. 1. Each of these scattered
sensor nodes has the capabilities to collect data
and route data back to the sink. Data are routed
back to the sink by a multihop infrastructureless
architecture through the sink as shown in Fig. 1.
The sink may communicate with the task manager
node via Internet or satellite. The design of the
sensor network as described by Fig. 1 is influenced
by many factors, including fault tolerance,
scalability, production costs, operating environment,
sensor network topology, hardware constraints,
transmission media, and power consumption.
DESIGN FACTORS
The design factors are addressed by many
researchers as surveyed in this article. However,
none of these studies has a fully integrated view
of all the factors driving the design of sensor
networks and sensor nodes. These factors are
important because they serve as a guideline to
design a protocol or an algorithm for sensor networks.
In addition, these influencing factors can
be used to compare different schemes.
Hardware Constraints — A sensor node is
made up of four basic components, as shown in
Fig. 2: a sensing unit, a processing unit, a transceiver
unit, and a power unit. They may also have
additional application-dependent components
such as a location finding system, power generator,
and mobilizer. Sensing units are usually composed
of two subunits: sensors and analog-to-digital converters
(ADCs). The analog signals produced by
the sensors based on the observed phenomenon
are converted to digital signals by the ADC, and
then fed into the processing unit. The processing
unit, which is generally associated with a small
storage unit, manages the procedures that make
the sensor node collaborate with the other nodes
to carry out the assigned sensing tasks. A
transceiver unit connects the node to the network.
One of the most important components of a sensor
node is the power unit. Power units may be
supported by power scavenging units such as
solar cells.
Power Consumption
The wireless sensor
node, being a microelectronic device, can only
be equipped with a limited power source (< 0.5
Ah, 1.2 V). In some application scenarios,
replenishment of power resources might be
impossible. Sensor node lifetime, therefore,
shows a strong dependence on battery lifetime.
In a multihop ad hoc sensor network, each node
plays the dual role of data originator and data
router. The malfunctioning of a few nodes can
cause significant topological changes and might
require rerouting of packets and reorganization
of the network. Hence, power conservation and
power management take on additional importance.
It is for these reasons that researchers are
currently focusing on the design of power-aware
protocols and algorithms for sensor networks.
MEDIUM ACCESS CONTROL
The MAC protocol in a wireless multihop selforganizing
sensor network must achieve two
goals. The first is the creation of the network
infrastructure. Since thousands of sensor nodes
are densely scattered in a sensor field, the MAC
scheme must establish communication links for
data transfer. This forms the basic infrastructure
needed for wireless communication hop by hop
and gives the sensor network self-organizing
ability. The second objective is to fairly and efficiently
share communication resources between
sensor nodes
Hybrid TDMA/FDMA-Based
This centrally
controlled MAC scheme is introduced in [8]. In
this work, the effect of nonideal physical layer
electronics on the design of MAC protocols for
sensor networks is investigated. The system is
assumed to be made up of energy-constrained
sensor nodes that communicate to a single nearby
high-powered base station (< 10 m). Specifically,
the machine monitoring application of
sensor networks with strict data latency requirements
is considered, and a hybrid TDMA-frequency-
division multiple access (FDMA)
medium access scheme is proposed. While a
pure TDMA scheme dedicates the full bandwidth
to a single sensor node, a pure FDMA
scheme allocates minimum signal bandwidth per
node. Despite the fact that a pure TDMA
scheme minimizes the transmit-on time, it is not
always preferred due to the associated time synchronization
costs. An analytical formula is
derived in [8] to find the optimum number of
channels which gives the lowest system power
consumption.
POWER SAVING MODES OF OPERATION
Regardless of which type of medium access
scheme is used for sensor networks, it certainly
must support the operation of power saving
modes for the sensor node. The most obvious
means of power conservation is to turn the
transceiver off when it is not required. Although
this power saving method seemingly provides
significant energy gains, an important point that
must not be overlooked is that sensor nodes
communicate using short data packets. As
explained in an earlier section
NETWORK LAYER
Sensor nodes are scattered densely in a field
either close to or inside the phenomenon, as
shown in Fig. 1. As discussed in the first section,
special multihop wireless routing protocols
between the sensor nodes and the sink node are
needed. Traditional ad hoc routing techniques do
not usually fit the requirements of the sensor networks
due to the reasons explained earlier. The
networking layer of sensor networks is usually
designed according to the following principles:
• Power efficiency is always an important consideration.
• Sensor networks are mostly data-centric.
• Data aggregation is useful only when it does
not hinder the collaborative effort of the
sensor nodes.
• An ideal sensor network has attribute-based
addressing and location awareness.
SENSOR MANAGEMENT PROTOCOL
Designing an application layer management protocol
has several advantages. Sensor networks
have many different application areas, and
accessing them through networks such as the
Internet is aimed at in some current projects [6].
An application layer management protocol
makes the hardware and software of the lower
layers transparent to the sensor network management
applications.
System administrators interact with sensor
networks using SMP. Unlike many other networks,
sensor networks consist of nodes that do
not have global identifications and are usually
infrastructureless.
CONCLUSION
The flexibility, fault tolerance, high sensing fidelity,
low cost, and rapid deployment characteristics
of sensor networks create many new and exciting
application areas for remote sensing. In the
future, this wide range of application areas will
make sensor networks an integral part of our
lives. However, realization of sensor networks
needs to satisfy the constraints introduced by factors
such as fault tolerance, scalability, cost, hardware,
topology change, environment, and power
consumption. Since these constraints are highly
stringent and specific for sensor networks, new
wireless ad hoc networking techniques are
required. Many researchers are currently engaged
in developing the technologies needed for different
layers of the sensor networks protocol stack
shown in Fig. 3. A list of current sensor network
research projects is given in Table 3. Along with
the current research projects, we encourage
more insight into the problems and intend to
motivate a search for solutions to the open
research issues described in this article.