Next: Heartbeat Period and Awareness
Up: Simulation Results
Previous: Effect of Leadership Handoff
Next, we analyze the overhead of the algorithm by exploring the point
at which it saturates the underlying network. This saturation point
depends on the setting of the heartbeat period and the awareness horizon.
It is fairly obvious that decreasing the heartbeat period results in
more frequent communication between the nodes and therefore the ability
to track faster targets. This is
based simply on the speed of response necessary for faster targets.
However, the bandwidth limitations in the
wireless medium place limits on our timer settings and
constrain our architecture's ability to track migrating events.
To determine the bandwidth needs of our algorithm, we start with a
very small leader heartbeat period (that saturates the network),
then increase it gradually. We plot the resulting
connection delay, which is
the time it takes to send a message from the moving entity to the
friendly-force entity. As the heartbeat period increases to the point
when the network is no longer saturated with heartbeat traffic, we observe
a sharp decrease in the connection delay.
Figure 6 shows this effect.
The experiment is
repeated for different awareness horizons, expressed in the number of
hops that leader heartbeats are propagated to. It is seen that
when the horizon is increased, the onset of overload occurs earlier as
more messages are communicated.
Figure 6:
Effect of timer settings on message delay (speed = 12 m/s)
|
From Figure 6 we can see that the bandwidth
of the wireless medium is fully saturated when the leader heartbeat period is
reduced to approximately
2.9, 5.9, and 11.7 ms for an awareness horizon of = 1, 2, and 3
hops respectively. To conservatively
avoid this saturation point and ensure enough bandwidth is left for
alternate local traffic, we multiply these numbers by 5 (i.e., limit the
worst case overhead of tracking to 20%). Hence, in the rest of the
evaluation section, we consider only those
leader heartbeat periods that are above 12.5, 25, and 50
ms for 1, 2, and 3 hops respectively.
We next turn our attention to the selection of the leader
heartbeat period and the awareness horizon, the
two key parameters of the algorithm, subject to the above
constraints.
Next: Heartbeat Period and Awareness
Up: Simulation Results
Previous: Effect of Leadership Handoff
root
2003-03-05