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Unformatted text preview: ves this contention by delaying coordinator announcement with a randomized backoff delay Each node selects a delay value and delays sending HELLO message indicating the desire to become coordinator for that amount of time At the end of the delay, the node reevaluates its eligibility based on the HELLO messages received from neighbors and if it is still eligible, it makes announcement SPAN
Coordinator Announcement At the end of the delay, the node reevaluates its eligibility based on the HELLO messages received from neighbors and if it is still eligible, it makes announcement Consider a case where all the nodes have the same level of energy which means that only topology is considered in the decision of becoming a coordinator Eq. 1 Consider a case where the nodes have unequal energy left Eq. 2
Er = energy remaining at node Em = maximum amount of energy Ci = number of new connections through node i Ni = number of neighbors for node i T = round-trip delay for packet R = random number in [0, 1] SPAN
Coordinator Announcement In Eq. 1, if nodes with high Ci become coordinators, total number of coordinators needed would be less to ensure that every node in the network is covered Therefore, the nodes with a high Ci values should volunteer for coordinator position quicker than those with smaller Ci In Eq. 2, the node with large value of (Er/Em) is expected to volunteer quicker to become a coordinator than the nodes with smaller ratio in order to assure the fairness SPAN
Coordinator Withdrawal Each node periodically checks whether it should withdraw as a coordinator A node withdraws if all of its neighbors can reach each other directly or with one or more coordinators For fairness, after some period of time, a coordinator withdraws and declares itself as a tentative coordinator if all neighbors can reach each other via other neighbors, even if these are not coordinators (allows neighbors to act as coordinators) A tentative coordinator is still used to forward packets and described coordinator announcement algorithm treats tentative coordinators as non-coordinator nodes A coordinator nodes gives its neighbors the opportunity to become coordinators by declaring itself as tentative coordinator A coordinator maintains its position as tentative for WT time, where WT is the maximum value of Eq. 2 which is WT = 3 x Ni x T SPAN
Coordinator Withdrawal If the coordinator has not withdrawn within WT time period, it clears its tentative bit In order to prevent node to drain its battery completely, the amount time a node acts as a coordinator before turning on its tentative bit is proportional to the amount of energy it has, indicated as (Er/Em) SPAN
Simulation Results ASCENT: Adaptive Self-Configuring sEnsor Networks Topologies
[Cerpa+ 2002] In ASCENT, the nodes coordinate to exploit the redundancy provided by high density to extend the overall system lifetime Nodes achieve self-configuration to establish a topology that provides communication and sensing coverage under energy constraints Each node examines its connectivity and adapts its participation in the multi-hop network topology based on the operating region The node Signals when it detects high message loss, requesting additional nodes to join the network to continue relaying messages Reduces its duty cycle if high messages losses are detected due to collisions Probes local communication environment and only joins to the multi-hop routing infrastructure if it is useful ASCENT Sensor nodes do...
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- Spring '08