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

IDEA: An Iterative-Deepening Algorithm for Energy-Efficient Querying 201algorithm would asymptotically perform well by satisfying the query at a depthless than d. In deed, experiments show (Section 6) that for a fairly dense ad-hocnetwork with approximately 8 neighbors per node, with depth,d = 7, almost70% of all queries can be satisfied at a depth less than d.IDEA works on a network-wide rule called i-Rule which specifies the policyfor depths of iteration for searching and the time-interval between successiveiterations. Suppose the rule is, i − Rule = {i 1 ,i 2 ,i 3 ,T}, this means that thedepth of search for the first iteration is i 1 , the search depth for second iterationis i 2 and the third to depth i 3 . Intuitively, if the search reaches the third-leveliteration, then it has the same performance as BFS of depth i 3 . The intervalbetween these successive iterations known as the inter-iteration interval, T, whichis required for the source (which initiates the query) to receive and analyze theresponse messages. One advantage of such a policy is that the rule could bebased on any metric like the number of hops, time-to-search (like TTL) etc. Butin case of ad hoc networks, the most common metric is the number of hops,considering that the communication cost is proportional to the number of hops.The algorithm details are explained further below.For a i-Rule = {i,j,k,T}, a source node S initiates flooding (or BFS) tilla depth i i.e. starting from the source the query is flooded to all nodes i hopsaway from the source. When the query reaches nodes that are i hops away,the query is halted and not flooded further. During this time the source S, mayor may not receive the response messages to the query. After waiting for theinter-iteration interval, T, if it receives appropriate responses, then the sourcewould stop querying. Else if the source does not receive appropriate results orno results, it initiates the second search iteration. It is important to note that,the definition of the term ‘appropriate response’ or ‘search results’ is applicationspecific and is not addressed in this paper.To initiate the second search iteration, source S will send the Continue-jmessage, indicating nodes to flood the message, j hops from the source. It shouldbe noted that nodes which are i hops away from the source, have already processedthe query and store the query with them (they are in halted state). Henceif these nodes (within i hops from the source) were to process the query, it woulddegrade the performance of the protocol by wasting energy of this constrained adhoc network. Instead of re-processing the query, these nodes which are within ihops from the source would simply forward the received continue-j messages.Once the last node (i.e. i hops from the source) receives the message, thesenodes re-send the halted query, and flood it to all the nodes which are ‘j - i’hops from the present node (remember, j means j nodes from the source). Oneimportant point to note here is that, node will halt the query only for a timegreater than the inter-iteration interval, T; before deleting its state.It would always be a case that more than one query and consequently manycontinue messages are being flooded in the network. To match the queries withappropriate continue messages, every query is assigned a unique identifier, asin case of DSR [2]. The continue message will have the identifier for the correspondingquery, thus nodes know which query is to be re-sent.