21-12-2012, 02:35 PM
Improving Quality-of-Service in Wireless Sensor Networks by Mitigating “Hidden-Node Collisions
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Abstract
Wireless sensor networks (WSNs) emerge as underlying
infrastructures for new classes of large-scale networked
embedded systems. However, WSNs system designers must fulfill
the quality-of-service (QoS) requirements imposed by the applications
(and users). Very harsh and dynamic physical environments
and extremely limited energy/computing/memory/communication
node resources are major obstacles for satisfying QoS metrics
such as reliability, timeliness, and system lifetime. The limited
communication range of WSN nodes, link asymmetry, and the
characteristics of the physical environment lead to a major source
of QoS degradation in WSNs—the “hidden node problem.” In
wireless contention-based medium access control (MAC) protocols,
when two nodes that are not visible to each other transmit
to a third node that is visible to the former, there will be a collision—
called hidden-node or blind collision. This problem greatly
impacts network throughput, energy-efficiency and message
transfer delays, and the problem dramatically increases with the
number of nodes. This paper proposes H-NAMe, a very simple yet
extremely efficient hidden-node avoidance mechanism for WSNs.
H-NAMe relies on a grouping strategy that splits each cluster of
a WSN into disjoint groups of non-hidden nodes that scales to
multiple clusters via a cluster grouping strategy that guarantees no
interference between overlapping clusters. Importantly, H-NAMe
is instantiated in IEEE 802.15.4/ZigBee, which currently are the
most widespread communication technologies for WSNs, with only
minor add-ons and ensuring backward compatibility with their
protocols standards. H-NAMe was implemented and exhaustively
tested using an experimental test-bed based on “off-the-shelf”
technology, showing that it increases network throughput and
transmission success probability up to twice the values obtained
without H-NAMe. H-NAMe effectiveness was also demonstrated
in a target tracking application with mobile robots over a WSN
deployment.
INTRODUCTION
A. Research context
I NDUSTRIAL applications such as factory automation,
process control, quality control, or smart energy can
greatly benefit from or even impose the use of wireless/mobile
Manuscript received December 05, 2008; revised April 23, 2009 and
communication capabilities. Due to the growing tendency for
continuously monitoring/controlling everything, everywhere,
computing systems tend to be ubiquitous, largely distributed
and tightly embedded in their physical environments [1]. To
be cost-effective, these systems must be mainly composed
of tiny resource-constrained embedded devices with wireless
communication capabilities, forming Wireless Sensor/Actuator
Networks, usually simply referred asWireless Sensor Networks
(WSNs).
WSN applications can be of many different types and can impose
different quality-of-service (QoS) requirements [2], e.g.,
an air quality monitoring application gathering air parameters
measurements has less stringent timing requirements than a mobile
robot navigation application. However, all WSN applications
benefit from higher network throughput, lower message
delay, and longer system lifetime.
The provision of QoS in WSNs is very challenging due
to two main problems, though: 1) the usually severe limitations
of WSN nodes, such as the ones related to their energy,
computational and communication capabilities, in addition to
the large-scale nature of WSNs and 2) most QoS properties
are interdependent, in a way that improving one of them may
degrade others, e.g., increasing throughput (by increasing WSN
nodes duty-cycle or increasing bit rate) will decrease system
lifetime or providing time-bounded (real-time) communications
may imply worst-case resource reservation, leading to
lower network throughput and lifetime. These negative facts
force system designers to try to achieve the best tradeoffs
between QoS metrics. In this paper, a mechanism that enables
to improve several QoS properties of a WSN system at the
same time is proposed, as it will be presented hereafter.
RELATED WORK
The hidden-node problem is known to be a serious source
of performance degradation in wireless communication networks.
In [7] and [8], the authors derived a mathematical
analysis based on queueing theory and quantified the impact
of the hidden-node problem on the performance of
small-scale linear wireless networks. Many research works
have addressed solutions for eliminating or reducing the
impact of the hidden-node problem in wireless networks,
roughly categorized as: 1) busy tone mechanisms; 2) Request-
To-Send/Clear-To-Send (RTS/CTS) mechanisms; 3)
carrier-sense tuning mechanisms; 4) interference cancellation
mechanisms; and 5) node grouping mechanisms. These are
briefly described next.
CONCLUDING REMARKS
This paper proposes a simple but effective solution to the
hidden-node problem, which is a fundamental impairment to
QoS in wireless communication networks and particularly for
WSNs. Our solution is very attractive for WSN applications
with more stringent QoS requirements, as the hidden-node
problem represents one of the major causes of QoS degradation,
particularly in what concerns network throughput,
message delay, energy-consumption, and reliability.
The proposed mechanism—H-NAMe—eliminates hiddennode
collisions in synchronized single or multiple cluster
WSNs using contention-based MAC protocols. It follows a
proactive approach, since it avoids hidden-node collisions
before occurring, through the creation of hidden-node interference-
free node groups and node cluster groups.
One of the most important contributions of our work is the integration
of H-NAMe in the IEEE 802.15.4/ZigBee protocols,
which currently are the dominant communication technologies
for WSNs. This integration is shown to be very simple and in
a way that WSN nodes implementing H-NAMe are fully and
transparently interoperable with the default WSN nodes (not
implementing H-NAMe). Also important, the implementation
of H-NAMe will be available as an open-source, within our
open-ZB tool suite [35].