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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.

• Route Discovery
• Route discovery is expensive. The source
  broadcasts to the entire network and the
  destination replies, forming a reverse route
  along the broadcast tree.
• 2. Route Append
• 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  is no longer valid.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
  leaders parent is simply set to be the old
  leader.
• 3. Route Pruning
• 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.
• This technique automatically prunes

• Route Capture
• 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.
• 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.
• 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

Analysis 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.
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?
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.
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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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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.
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.

• 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.
• This simple algorithm runs into several problems
• Loops
• Long routes
• Broken Links
• Redundant Links
• We will try to solve these without using extra
  bandwidth

Node 8 can be pruned from the new route by node
7, who knows that it is downstream but can still
overhear node 9s messages.
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.
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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Contact Kamin Whitehouse kamin_at_cs.berkeley.edu
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