Protecting bridges: Reorganizing sensor networks after catastrophic events

The worst case scenario for the life cycle of a sensor network is the fragmentation of a network which still has many functional and well-powered nodes. The loss of connectivity renders even the functional nodes useless as the nodes are not able to transmit their observations to the sink. A well-engineered sensor network will not fragment due to the energy consumption occurring during normal functioning. However, catastrophic events, which unpredictably destroy a large subset of the nodes, can transform a well engineered network into a heavily unbalanced one. Very often, even if the network is not yet fragmented, the connectivity is relying on one or more bridge nodes, which survived the catastrophic event accidentally. If the network operates as before, the bridge nodes will soon exhaust their power resources by having to route an unexpectedly large number of packets. This paper describes the Bridge Protection Algorithm (BPA), a combination of techniques which, in response to a catastrophic event, change the behavior of a set of topologically important nodes in the network. These techniques protect the bridge node by letting some nodes take over some of the responsibilities of the sink. At the same time, they relieve some other overwhelmed nodes and prevent the apparition of additional bridge nodes. To achieve this, BPA sacrifices the length of some routes in order to distribute routes away from critical areas. Through a simulation study we show that the application of these techniques can significantly decrease the load of the nodes in the critical areas, while only minimally affecting the performance of the network.