28-11-2012, 04:32 PM
Sleep Scheduling for Critical Event Monitoring in Wireless Sensor Networks
Sleep Scheduling for Critical Event Monitoring.pdf (Size: 1.11 MB / Downloads: 40)
INTRODUCTION
IN mission-critical applications, such as battlefield reconnaissance,
fire detection in forests, and gas monitoring in
coal mines, wireless sensor networks (WSNs) are deployed
in a wide range of areas, with a large number of sensor
nodes detecting and reporting some information of urgencies
to the end-users. As there may be no communication
infrastructure, users are usually equipped with communicating
devices to communicate with sensor nodes. When a
critical event (e.g., gas leak or fire) occurs in the monitoring
area and is detected by a sensor node, an alarm needs to be
broadcast to the other nodes as soon as possible, which is
shown in Fig. 1 as an example. Then, sensor nodes can warn
users nearby to flee or take some response to the event.
As sensor nodes for event monitoring are expected to
work for a long time without recharging their batteries,
sleep scheduling method is always used during the
monitoring process. Obviously, sleep scheduling could
cause transmission delay because sender nodes should
wait until receiver nodes are active and ready to receive the
message. The delay could be significant as the network scale
increases. Therefore, a delay-efficient sleep scheduling
method needs to be designed to ensure low broadcasting
delay from any node in the WSN.
THE PROPOSED SCHEDULING METHOD
It is known that the alarm could be originated by any node
which detects a critical event in the WSN. To essentially
reduce the broadcasting delay, the proposed scheduling
method includes two phases: 1) any node which detects a
critical event sends an alarm packet to the center node along
a predetermined path according to level-by-level offset
schedule; 2) the center node broadcasts the alarm packet to
the entire network also according to level-by-level offset
We define the traffic paths from nodes to the center
node as uplink and define the traffic path from the center
node to other nodes as downlink, respectively. Each node
needs to wake up properly for both of the two traffics.
Therefore, the proposed scheduling scheme should contain
two parts: 1) establish the two traffic paths in the
WSN; 2) calculate the wake-up parameters (e.g., time slot
and channel) for all nodes to handle all possible traffics.
To minimize the broadcast delay, we establish a breadth
first search (BFS) tree for the uplink traffic and a colored
connected dominant set for the downlink traffic, respectively.
Wake-Up Patterns
After all nodes get the traffic paths, sending channels and
receiving channels with the BFS and CCDS, the proposed
wake-up pattern is needed for sensor nodes to wake-up and
receive alarm packet to achieve the minimum delay for both
of the two traffic paths.
CONCLUSIONS
In this paper, we proposed a novel sleeping scheme for
critical event monitoring in WSNs. The proposed sleeping
scheme could essentially decrease the delay of alarm
broadcasting from any node in WSN. The upper bound of
the delay is 3D þ 2L, which is just a linear combination of
hops and duty cycle. Moreover, the alarm broadcasting
delay is independent of the density of nodes in WSN.
Theoretical analysis and conducted simulations showed
that the broadcasting delay and the energy consumption
of the proposed scheme is much lower than that of
existing methods.