An Adaptive Energy-Efficient MAC Protocol
for Wireless Sensor Networks
Tijs van Dam
tijsvd@xs4all.nl
Koen Langendoen
K.G.Langendoen@its.tudelft.nl
Faculty of Information Technology and Systems
Delft University of Technology
The Netherlands
ABSTRACT
In this paper we describe T-MAC, a contention-based Me-
dium Access Control protocol for wireless sensor networks.
Applications for these networks have some characteristics
(low message rate, insensitivity to latency) that can be ex-
ploited to reduce energy consumption by introducing an
active/sleep duty cycle. To handle load variations in time
and location T-MAC introduces an adaptive duty cycle in
a novel way: by dynamically ending the active part of it.
This reduces the amount of energy wasted on idle listening,
in which nodes wait for potentially incoming messages, while
still maintaining a reasonable throughput.
We discuss the design of T-MAC, and provide a head-to-
head comparison with classic CSMA (no duty cycle) and
S-MAC (fixed duty cycle) through extensive simulations.
Under homogeneous load, T-MAC and S-MAC achieve simi-
lar reductions in energy consumption (up to 98 %) compared
to CSMA. In a sample scenario with variable load, however,
T-MAC outperforms S-MAC by a factor of 5. Preliminary
energy-consumption measurements provide insight into the
internal workings of the T-MAC protocol.
Categories and Subject Descriptors
C.2.1 [Network Architecture and Design]: Wireless
communication—MAC protocol; C.2.5 [Local and Wide-
Area Networks]: Access schemes; D.4.4 [Communica-
tions Management]: Message sending
General Terms
Design, Experimentation, Measurement, Performance
Keywords
Ad-hoc, sensor networks, MAC protocol, energy-efficiency
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1. INTRODUCTION
Communication in wireless sensor networks can, like most
network communication, be divided into several layers. One
of those is the Medium Access Control (MAC) layer. This
layer is described by a MAC protocol, which tries to ensure
that no two nodes are interfering with each other’s trans-
missions, and deals with the situation when they do.
Wireless sensor networks have an additional aspect: as
sensor nodes are generally battery-operated, energy con-
sumption is very important. The radio on a sensor node
is usually the component that uses most energy. Not only
transmitting costs energy; receiving, or merely scanning the
ether for communication, can use up to half as much, de-
pending on the type of radio [8].
While traditional MAC protocols are designed to maxi-
mize packet throughput, minimize latency and provide fair-
ness, protocol design for wireless sensor networks focuses
on minimizing energy consumption. The application deter-
mines the requirements for the (modest) minimum through-
put and maximum latency. Fairness is usually not an issue,
since the nodes in a wireless sensor network are typically
part of a single application and work together for a common
purpose.
1.1 Communication patterns
It is important to design and test the behavior of MAC
protocols based on the kind of traffic they have to handle.
We have identified two main communication patterns in sen-
sor applications:
Local uni-/broadcast When a real-world event in the net-
work occurs, we expect nodes to perform some in-
network processing. This will generally involve local
messages being exchanged between neighbors.
Nodes to sink reporting After processing a local event,
or just periodically, nodes may want to report some-
thing. We expect messages to be directed to one or a
few sink nodes, which are hooked up to a fixed network
or a computer. Messages from different nodes may,
or may not, be aggregated along the way. We do
not specify an exact routing protocol, but we expect
some random variation in message paths—messages
flow ‘roughly’ in the correct direction. In this commu-
nication pattern, we see a more or less unidirectional
flow of messages through the network.