Maintaining High-bandwidth Communication
Between Mobile Groups of Nodes
Kamin Whitehouse, Ying Zhang
UC Berkeley, Palo Alto Research Center
Motivation
Objects or events in a sensor network will often be
identified by groups of nodes, whose locations and
member nodes will change as the object/event moves.
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As the groups
move, the current
route becomes
invalid. How can
we repair this
route without
consuming the
bandwidth of the
route we are
trying to
maintain?
1.Route Discovery
4. Route Capture
Route discovery is expensive. The source broadcasts to
the entire network and the destination replies, forming
a reverse route along the broadcast tree.
Because this algorithm maintains an explicit route, it is
extremely fragile to node/link failure. To solve this
problem, the route captures nearby nodes in the case
of a single node failure.
2. Route Append
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When the leader of a group changes, the old route is no
longer valid. Assuming each successive leader is
within communication radius of the old leader (ie. there
is a clean handoff) the new leader’s parent is simply
set to be the old leader.
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For groups to communicate, a route must be maintained
between the two group leaders. However, as the group
leaders change this may require message passing and
data propagation, which consumes valuable bandwidth.
Our goal is to maintain a route between two mobile
groups with a minimal number of maintenance messages.
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Because the new
leader is always
within
communication
range of the old
leader, it reuses
the old route by
appending a link
to the old leader,
node number 9.
This creates a
suboptimal route.
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3. Route Pruning
Overall Idea
Assuming that the group leaders are elected such that
each successive leader is within communication range
of the previous leader, we can simply append a new
leg onto the existing route.
By adding more state to each node indicating where it is
on the route, a node further down the route can usurp
a message from the beginning of the route if it can
hear it. This eliminates any extraneous nodes by
shortcutting them.
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8
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As the orange
objects moves, 8
becomes the new
leader of the
orange group. In
order to maintain
connectivity with
1, the new leader
can try to
connect with the
old route.
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Node 8 can be
pruned from the
new route by
node 7, who
knows that it is
downstream but
can still overhear
node 9’s
messages.
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Note that all candidates are almost guaranteed to be
within radio range of each other and that the receiving
nodes can break ties by multiple volunteers.
5. Route Optimization
This routing algorithm is far from optimal in terms of
finding shortest path routes. While pruning takes care
of many sub-optimalities, it is still possible to use
unnecessarily long paths.
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1. Loops
1. Loops: prune the cycle if a leader moves in a circle
2. Long routes
2. Turns: prune redundant path if a leader back-tracks
3. Baby steps: prune redundant nodes if multiple
successive nodes are within communication range,
(perhaps caused by the sensing range being much
smaller than the communication range).
When node 20
dies, the link
between 9 and 5
is broken. Those
nodes that can
hear both 5 and
9 are candidates
to replace 20,
and self-elect
themselves when
they see that 20
is not
responding.
Each components of this algorithm besides route
discovery are done implicitly, not requiring extra
messages but instead utilizing eavesdropping and
maintaining state. While this makes the route fragile, this
is ameliorated in part by having nodes that neighbor the
route in the network also maintain state. In total, only a
single extra message is required for route maintenance
after the route is initially discovered. In practice, this
allows an order of magnitude higher bandwidth between
the groups than competitive methods; the bandwidth of
the leaders is not consumed by messages from their
neighbours.
All nodes that can hear a blocked message and that can
forward the message to other half of the route are
candidates to be captured. Nodes randomly elect
themselves to be in the route and forward the
message, thereby suppressing all other candidates
from joining the route.
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This simple algorithm runs into several problems:
We will try to solve these without using extra bandwidth
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This technique automatically prunes:
4. Redundant Links
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3. Broken Links
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Analysis
Conclusion
This algorithm was developed for the NEST
pursuer/evader game Mid-term demo. One main
problem with the demo was the large amount of network
traffic required for each tracking update. By using a
higher-bandwidth routing algorithm, the network can
provide more tracking updates faster.
Without
sacrificing
bandwidth,
nodes far from
source and
destination can
optimize the
route during
quite periods,
thereby finding
non-local
shortcuts.
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Nodes not near the source or destination can propagate
a cost-to-go estimate to other nodes in the network,
similar to the distributed Bellman-ford algorithm. By
restricting this to the center of the route while the route
is inactive, we can optimize for path length while not
reducing bandwidth.
Contact
Kamin Whitehouse
[email protected]
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