26-11-2012, 11:42 AM
Medium Access Control With Coordinated Adaptive Sleeping for Wireless Sensor Networks
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Abstract
This paper proposes S-MAC, a medium access
control (MAC) protocol designed for wireless sensor networks.
Wireless sensor networks use battery-operated computing and
sensing devices. A network of these devices will collaborate for a
common application such as environmental monitoring.We expect
sensor networks to be deployed in an ad hoc fashion, with nodes
remaining largely inactive for long time, but becoming suddenly
active when something is detected. These characteristics of sensor
networks and applications motivate a MAC that is different from
traditional wireless MACs such as IEEE 802.11 in several ways:
energy conservation and self-configuration are primary goals,
while per-node fairness and latency are less important. S-MAC
uses a few novel techniques to reduce energy consumption and
support self-configuration. It enables low-duty-cycle operation
in a multihop network. Nodes form virtual clusters based on
common sleep schedules to reduce control overhead and enable
traffic-adaptive wake-up. S-MAC uses in-channel signaling to
avoid overhearing unnecessary traffic. Finally, S-MAC applies
message passing to reduce contention latency for applications
that require in-network data processing. The paper presents
measurement results of S-MAC performance on a sample sensor
node, the UC Berkeley Mote, and reveals fundamental tradeoffs
on energy, latency and throughput. Results show that S-MAC
obtains significant energy savings compared with an 802.11-like
MAC without sleeping.
INTRODUCTION
WIRELESS sensor networking is an emerging technology
that has a wide range of potential applications including
environment monitoring, smart spaces, medical systems, and
robotic exploration. Such networks will consist of large numbers
of distributed nodes that organize themselves into a multihop
wireless network. Each node has one or more sensors,
embedded processors, and low-power radios, and is normally
battery operated. Typically, these nodes coordinate to perform a
common task.
RELATED WORK
MAC is a broad research area, including work in the new area
of low-power and wireless sensor networks [10]–[13]. Current
MAC design for wireless sensor networks can be broadly divided
into contention-based and TDMA protocols.
a) Contention-Based MACs: The standardized IEEE
802.11 distributed coordination function (DCF) [1] is an
example of the contention-based protocol, and is mainly built
on the research protocol MACAW [14]. It is widely used
in ad hoc wireless networks because of its simplicity and
robustness to the hidden terminal problem. However, recent
work [2] has shown that the energy consumption using this
MAC is very high when nodes are in idle mode. This is mainly
due to the idle listening. 802.11 has a power-save mode, and
we will discuss it shortly. PAMAS [8] made an improvement
on energy savings by trying to avoid the overhearing among
neighboring nodes. Our paper also exploits the same idea. The
main difference of our work with PAMAS is that we do not use
any out-of-channel signaling, whereas in PAMAS, it requires
two independent radio channels, which in most cases indicates
two independent radio systems on each node. PAMAS does not
attempt to reduce idle listening.
S-MAC DESIGN OVERVIEW
S-MAC includes approaches to reduce energy consumption
from all the sources of energy waste that we have identified,
i.e., idle listening, collision, overhearing and control overhead.
Before describing the components in S-MAC, we first summarize
our assumptions about the wireless sensor network and its
applications.
Sensor networks will consist of large numbers of nodes to
take advantage of short-range, multihop communications to
conserve energy [4]. Most communications will occur between
nodes as peers, rather than to a single base station. In-network
processing is critical to network lifetime [5], and implies that
data will be processed as whole messages in a store-and-forward
fashion. Packet or fragment-level interleaving from
multiple sources only increases overall latency. Finally, we
expect that applications will have long idle periods and can
tolerate latency on the order of network messaging time.
COORDINATED SLEEPING
Periodic sleeping effectively reduces energy waste on idle listening.
In S-MAC, nodes coordinate their sleep schedules rather
than randomly sleep on their own. This section details the procedures
that all nodes follow to set up and maintain their schedules.
It also presents a technique to reduce latency due to the
periodic sleep on each node.
Message Passing
This subsection describes how to efficiently transmit a long
message in both energy and latency. A message is the collection
of meaningful, interrelated units of data. The receiver usually
needs to obtain all the data units before it can perform in-network
data processing or aggregation.
The disadvantages of transmitting a long message as a single
packet is the high cost of re-transmitting the long packet if only a
few bits have been corrupted in the first transmission. However,
if we fragment the long message into many independent small
packets, we have to pay the penalty of large control overhead
and longer delay. It is so because the RTS and CTS packets are
used in contention for each independent packet.
Our approach is to fragment the long message into many
small fragments, and transmit them in a burst. Only one
RTS and one CTS are used. They reserve the medium for
transmitting all the fragments. Every time a data fragment is
transmitted, the sender waits for an ACK from the receiver.
If it fails to receive the ACK, it will extend the reserved
transmission time for one more fragment, and re-transmit the
current fragment immediately.
CONCLUSION
This paper presents S-MAC, a medium access control
protocol specifically designed for wireless sensor networks.
Energy efficiency is the primary goal in the protocol design.
Low-duty-cycle operation of each node is achieved by periodic
sleeping. Together with overhearing avoidance and message
passing, S-MAC obtains significant energy savings compared
with 802.11-like protocols without sleeping. It is able to greatly
prolong the network lifetime, which is critical for real-world
sensor network applications.
Periodic sleeping increases latency and reduces throughput.
However, adaptive listening largely reduces such cost for energy
savings. It enables each node to adaptively switch mode according
to the traffic in the network.
S-MAC has been implemented on the Mote hardware, and
the source code is freely available to the research community.
Experimental results have verified our design principles.