Page 2 Lecture Notes in Computer Science 2865 Edited by G. Goos ...

IDEA: An Iterative-Deepening Algorithm for Energy-Efficient Querying 203at a very low rate (thus main energy is drained by staying in active mode); inthis case it can broadcast a t-token to declare a new token of its availability.Thus the generation of these tokens is primarily driven by the energy constraintsof respective nodes. Section 5 describes the energy-based token generation modelused during experiments.One point of discussion in the token generation algorithm is the primarymetric to be used. In the discussion so far energy has been the primary factor,and it would be the case for majority of applications. But suppose the networkconsists of mobile computers or handhelds which can be charged frequently,then energy might not be most important criteria. Another metric which playsan important role in searching is the connectivity and the ability to forward tonodes that lead to appropriate results. For example, nodes which have previouslyreplied to the queries at a high frequency could be used to declare tokens ofavailability. Another criterion would be to use neighbors that have respondedwith messages with least number of hops. This is a very application specificmetris, which is beyond the scope of this paper.The T-IDEA search forwards the query message(s) to just a subset of itsneighbors, based on the token-declaration made by the neighboring mobile nodes.By sending the query to a small subset of neighbor nodes, we will likely reducethe costs incurred by the nodes during query processing and forwarding. Onthe other hand, by selecting neighbors which wold produce many results, we canmaintain quality of results to a large degree, even though fewer nodes are visited.4.2 Resilient SearchThis search algorithm is a controlled flooding approach which depends on nodesof the ad-hoc network identifying themselves to be a part of the searching network.One drawback of such a scheme, isthe increased probability of failure thanflooding (where even if a node dies the query can propagated by some part ofthe network). This is possible in cases described earlier when a node experiencesheavy energy drainage than predicted during the token declaration (dueto increase in query processing rate).This problem is alleviated by use of keep-alive messages. If a query is forwardedfor certain number of hops, as declared in the i-Rule, we need to sendexplicit keep-alive messages. In case if the source node receives query responsemessages, they are considered to be implicit keep-alive messages. But if thesource does not receive either of them it can re-issue the query.4.3 Distributed Token Declaration AlgorithmA more distributed token declaration scheme for the nodes in the ad hoc network,would be to assign tokens to neighbors in proportion to their capacities. Based onthe previous tokens received from your neighbors, the neighboring node whichdeclares high capacity is assigned more tokens for querying the current node.This distributed token-algorithm is similar to fair queuing policies used for flowfairness in networks.