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

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Comput<strong>in</strong>g 2-Hop Neighborhoods <strong>in</strong> Ad Hoc Wireless Networks 179Fig. 2. An illustration of the virtual backbone of Alzoubi, Wan, and Frieder. The solidround nodes are the MIS node, which form a dom<strong>in</strong>at<strong>in</strong>g set. Two virtually-adjacentMIS nodes are connected <strong>by</strong> paths of length at most three through connector nodes -the empty t<strong>in</strong>y circles <strong>in</strong> the figure. Nodes not <strong>in</strong> the virtual backbone are small solidsquares <strong>in</strong> the figureto an MIS node adjacent to the dest<strong>in</strong>ation of the path. This fact was noticed <strong>by</strong>Alzoubi [1], and <strong>by</strong> Wang and Li [22], which also planarize the virtual backbonewhile keep<strong>in</strong>g all its attractive properties.3 Geographic Position AvailableIn this section we describe the distributed algorithm which allows every nodeto construct the list of its 2-hop neighbors, assum<strong>in</strong>g every node knows its geographicalposition. With this <strong>in</strong>formation, every node can also easily computethe l<strong>in</strong>ks between its 1-hop and 2-hop neighbors. Our algorithm is described <strong>in</strong>the simplest version, and we do not try to optimize the constant hidden <strong>in</strong> theO notation.We start from the moment the virtual backbone is already constructed, andevery node knows the ID and the position of its neighbors. The idea of thealgorithm is for every node to efficiently announce its ID and position to asubset of nodes which <strong>in</strong>cludes its 2-hop neighbors.The responsibility for announc<strong>in</strong>g the ID and position of a node v is taken <strong>by</strong>the MIS nodes adjacent to v. Each such MIS node assembles a packet conta<strong>in</strong><strong>in</strong>g:< ID, position, counter >, with the ID and position of v, and a counter variablebe<strong>in</strong>g set to 2. The MIS node then broadcasts the packet.A connector node is used to establish a l<strong>in</strong>k <strong>in</strong> between several pairs ofvirtually-adjacent MIS nodes, and will not retransmit packets which do nottravel <strong>in</strong> between these pairs of MIS nodes. The connector node will rebroadcastpackets with nonzero counter orig<strong>in</strong>ated <strong>by</strong> one of the nodes <strong>in</strong> a pair of virtuallyadjacentMIS nodes, thus mak<strong>in</strong>g sure the packet advances towards the otherMIS node <strong>in</strong> the pair. Recall that the path <strong>in</strong> between a pair of virtually-adjacentMIS nodes has one or two connector nodes.

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