Journal of Emerging Technologies in Web Intelligence Contents

82 JOURNAL OF EMERGING TECHNOLOGIES IN WEB INTELLIGENCE, VOL. 2, NO. 2, MAY 2010messages are handled may differ among differentprotocols, but their functionality remains the same: arequest is relayed until it reaches a node with a valid routeto the destination or the destination itself, which triggers areply message sent back to the originator. Severalparameters (such as how long to keep requests in a cache,timeouts for requests, timeouts for hellos) are subject totuning, and the choices made may result in improvementsin the protocol performance. However, RREQs arepropagated using either an unrestricted broadcast or anexpanding ring search. In either case, the resultingflooding operation causes considerable collisions ofpackets in wireless networks using contention-basedchannel access.In addition to applying DP to reduce the number ofnodes that need to propagate RREQs transmitted onbroadcast mode, information regarding prior routes to adestination is used to unicast RREQs to a region close tothe intended destination, so that broadcast RREQs arepostponed as much as possible and occur (if necessary)only close to the destination, rather than on network-widebasis. This RREQ Algoritm presents the pseudo-code forthe modified RREQ. A route request (RREQ) is handledas follows:• If the source of a RREQ does not have any previousknowledge about the route to the destination or is retryingthe RREQ, it calculates its forwarder list using DP, andbroadcasts the packet (Lines 8, 9, and 14).• On the other hand, if the source of a RREQ hasknowledge about a recently expired route to thedestination, and there is a valid route to the next hoptowards the destination (Lines 2, 3, and 4), the nodecalculates the forwarder list using DP (Line 9), but insteadof broadcasting the RREQ packet, the node unicasts thepacket to the last known next hop towards the destination(Line 12).Upon receiving a route request, a forwarder thatcannot respond to this request calculates its ownforwarder list using the information provided in theRREQ packet (i.e., forwarder list, second to previousforwarder list, and source node) and broadcasts or unicaststhe packet (depending on which one of the two first casesapply) after updating it with its own forwarder list.RREQ AlgorithmData : n i, destination D, B i , U iResult : Unicast the RREQ, or Broadcast the RREQBegin1 if recently expired route to D and not retryingthen2 NextHop ← previous_nextHop(D)3 if validRoute(NextHop) then4 result←Unicast5 else6 result←Broadcast7 else8 result←Broadcast9 F i ←DP( n i, B i , U i )10 Update RREQ packet with F i11 if result== Unicast then12 Unicast the RREQ packet to NextHop13 else14 broadcast the RREQ packetendEventually, the RREQ reaches a node with a route tothe destination or the destination itself. Our approachattempts to reduce the delay of the route discovery byunicasting a RREQ towards the region where thedestination was previously located. The success of thisapproach depends on how fresh the previous known routeto the destination is, and how fast the destination node ismoving out of the previous known location. If anintermediate node has completely removed any route tothe destination, the RREQ is then broadcasted. Theintended effect is to postpone the broadcast of a RREQ tothe region closest to the destination. In the case that theunicast approach fails, or there is no previous route to thedestination, the source broadcasts by default.Because of topology changes, nodes may not havecorrect two-hop neighborhood information, which mayresult in forwarding lists that do not cover all nodes inthe neighborhood. However, this is not a major problemwhen the request is broadcasted, because a nodeincorrectly excluded from the forwarder list may alsoreceive the request and is able to respond in the case it hasa route to the destination.C. Enhanced Dominant Pruning AlgorithmIn their paper [6], wei Lou and Jie Wu proposed twoenhanced dominant pruning algorithms: the TotalDominant pruning (TDP) algorithm and ParatialDominant Pruning (PDP).In TDP algorithm, if node v can receive a packetpiggybacked with N(N(v)) from node u , the 2-hopneighbour set that needs to be covered by v’s forwardnode list F is reduced to U = N(N(v)) – N(N(u)). Themain objective of the TDP algorithm is that 2-hopneighbourhood information of each sender is piggybackedin the broadcast packet which results in consumption ofmore bandwidth.D. Partial Dominant Pruning Algorithm:Just like the DP algorithm, in PDP, noneighbourhood information of the sender is piggybackedwith the broadcast packet. Apart from excluding N (u)and N (v) from N (N (v)) as in the DP algorithm, we canhere exclude some more nodes from neighbours of eachnode in N (u) ∩ N (v). Such a node set is denoted by P(u,v) (or simply P) = N(N(u) ∩ N(v)). Then 2-hopneighbour set U in the PDP algorithm can be given by U= N(N(v)) – N(u) – N(v) – P since P is a subset ofN(N(u)), we can easily see P can be excluded fromN(N(v)). Also it can be proved that U is subset of N(B),when P = N(N(u)∩N(v)), U = N(N(v)) – N(u) – N(v)-Pand B = N(v) – N(u).E Adaptive Partial Dominant Pruning Algorithm[19]Adaptive dominant pruning algorithm (APDP) issimilar to PDP. However, besides excluding N (u), N (v)and P from N (N (v)) as mentioned in PDP algorithm,adjacent nodes of U are eliminated from U.© 2010 ACADEMY PUBLISHER