08-08-2012, 01:28 PM
EWANT: The Emulated Wireless Ad Hoc Network Testbed
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INTRODUCTION
A wireless ad hoc network consists of autonomous mobile
nodes that spontaneously spawn a communication network
between each other. The nodes are usually PDA’s or laptops
equipped with wireless ethernet cards. Because these nodes
are constantly moving, the link states are never stable. Wired
routing protocols do not have to account for rapidly changing
topology or link quality and thus cannot be used efficiently
over such a network. Hence ad hoc wireless specific protocols
such as the Ad hoc On demand Distance Vector (AODV) [1]
and the Dynamic Source Routing (DSR) have been developed
[2]. While a large amount of work has gone into developing
these protocols, there has never been a standard way to test
their implementations. This creates the need for a standard,
reliable, low cost test bed that allows for direct comparison
between mobile wireless ad hoc routing protocol implementations.
HARDWARE IMPLEMENTATION
This section lists the requirements of a hardware testbed
and then details our approach in satisfying the same. We also
describe the manner in which mobility is achieved.
A. Testbed requirements and our approach
Based on prior work and our initial testing experiences we
derived the following requirements for our testbed.
• Tests must be controlled and repeatable.
• Shared RF media effects such as interference and channel
capture must be present.
Simulation of Mobility
It is difficult to simulate a continuous change in position
without physically moving the antennas. We decided to ignore
the smooth node movement and assumed that the worst case
scenario is a sudden change in the signal strength. This can be
seen in practice too, for example, when a node moves behind
a building or a similar obstruction.
A computer controlled variable attenuator can be used to
gradually vary the attenuation. For example, a stepper motor
based attenuator can be controlled, remotely, via PC and then
the attenuation can be varied in small steps repeatedly. This
approach thus emulates a limited sort of continuous change in
position. However we decided not to include this in our setup,
mostly because the cost of such attenuators is substantially
high. Also as mentioned above we were mainly interested in
observing effects sudden changes in the RF environment.
CONCLUSION AND FUTURE WORK
We have thus successfully demonstrated EWANT as an
accessible wireless testbed. This testbed has realistic features
and allows rapid development. Mobility, which was our main
goal was successfully demonstrated. Although the testbed does
not allow a gradual movement of nodes, this effect could be
simulated by slowly “hopping” the node over short distances
or by using variable attenuators.
Our aim here was not to test some specific code, but
provide a level playing field for all wireless routing protocols.
To achieve this goal we have designed a testbed which is
independent of the underlying operating system or hardware
used. Although we have implemented this testbed for 802.11b,
it could easily be modified to suit similar wireless protocols.
Inspite of these advantages, there is still room for improvement.
We feel a more elaborate central controller is needed.
This must allow remote access to all nodes.