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2 DESMAC
We introduce a novel cross layer design to reduce
latency caused by periodic sleeping of the nodes.
Routing and application are used to help the MAC
protocol better adjust itself. We use S-MAC as the
basic framework for our design and adaptively
change its duty cycle. We assume that the reader is
familiar with S-MAC and Directed Diffusion (for
further reading please refer to (Ye, Heidemann and
Estrin, 2004) and (Intanagonwiwat et al., 2003)).
In this approach we use routing layer control
messages to adaptively reduce the sleep period of
the MAC layer of the nodes that take part in the
routing process. We decrease the sleep period of the
nodes that are reinforced by Directed Diffusion for
routing purposes. Nodes that are not reinforced
preserve their current sleep schedule.
In Directed Diffusion sink periodically sends
interest messages to all nodes in the network. When
a node detects an event which matches the diffused
interests it becomes a data source and starts sending
exploratory data messages. These messages are
forwarded towards the sink. When the sink receives
a positive reinforcement it issues a positive
reinforcement.
Positive reinforcements are similar to interest
messages but have lower interval. Each node
compares this interest message with the fields in its
own cache and if a lower interval is detected, it
updates its gradient toward that node to the new
value. In our approach positive reinforcements also
trigger a sleep period reduction in the MAC layer.
Negative Reinforcements have the exact opposite
effect on the sleep period. Negative reinforcements
in Directed Diffusion are used to reduce the number
of reinforced paths and path repairs. Several
mechanisms for negative reinforcement are
introduced in directed diffusion (timeouts, gradient
reductions, etc). Any mechanism for negative
reinforcement used by Directed Diffusion also
triggers an increase in the sleep period of the MAC
layer.
The goal is to benefit from energy saving
features of S-MAC and have much lower latency in
comparison to it. Nodes that are not reinforced by
the Directed Diffusion have the same duty cycle as
S-MAC, But nodes that are reinforced increase their
active period exponentially therefore upon path
establishment nodes on the path are almost always
active which results in very low latency that is
comparable to IEEE802.11.
Nodes that are not part of routing preserve their
original duty cycle and have similar energy
consumption to S-MAC. But routing nodes have
high duty cycle which provide very low latency and
can meet the application criteria.
In case a path failure or degradation occurs, the
path is negatively reinforced by Directed Diffusion
which will reduce the duty cycle of the nodes
previously involved in the routing. Therefore nodes
that are no longer involved in routing have low duty
cycle and energy saving is maximized.
S-MAC works on the basis that neighbor nodes
wake up at the same time therefore they can hear
each other’s broadcast messages (SYNC, RTS/CTS).
If neighbor nodes don’t have synchronous schedules,
communication between them becomes impossible.
Dynamic reduction of sleep period can disrupt
the synchronization done in the SYNC period of S-
MAC.
To address this problem we reduce the sleep
period in a manner that the SYNC and listen period
of the new schedule is still synchronized with that of
neighbor nodes. In order to achieve synchronized
wakeups we increase the duty cycle exponentially.
The sleep period in S-MAC is much longer than the
listen period therefore it is possible to reduce it so
that the frame size is divided in half. Each frame
turns into two frames with SYNC, RTS/CTS, and
sleep period which means that each positive
reinforcement message doubles the duty cycle of the
node until it achieves maximum possible duty cycle
This is shown in Figure 1.
Figure 1: Change in Duty Cycle as a node receives a
reinforcement message. Beginning of Listen periods is still
synchronized.
Increasing the duty cycle in this fashion does not
disrupt the synchronized wakeup of the neighbor
nodes. Nodes that are on the same path have the
same duty cycle. These nodes wakeup more often
and have more time for transmitting data therefore
provide lower latency and higher throughput. These
nodes are still able to communicate with the nodes
that have different duty cycles. Since the duty cycle
is increased exponentially the neighbor nodes still
have the synchronized wakeup. Nodes active in the
routing process have very small sleep periods and
show similar behavior to 802.11. But nodes not
involved behave similar to S-MAC.
Listen
Sleep
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Sleep Listen Listen
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DELAY EFFICIENT MAC PROTOCOL FOR DIFFUSION BASED ROUTING IN WIRELESS SENSOR NETWORKS
89