Relay Placement for Two-Connectivity

Motivated by applications to wireless sensor networks, we study the following problem. We are given a set S of wireless sensor nodes, given as a multiset of points in a normed space. We must place a minimum-size (multi)set Q of wireless relay nodes in the normed space such that the unit-disk graph induced by Q ∪ S is two-connected. The unit-disk graph of a set of points has an edge between two points if their distance is at most 1. Kashyap, Khuller, and Shayman (Infocom 2006) present algorithms for the two variants of the problem: two-edge-connectivity and biconnectivity. For both they prove an approximation ratio of at most 2dMST , where dMST is the maximum degree of a minimum-degree Minimum Spanning Tree in the normed space. In the Euclidean two and three dimensional spaces, dMST = 5, and dMST = 13 respectively. We give a tight analysis of the same algorithms, obtaining approximation ratios of dMST for biconnectivity and 2dMST − 1 for two-edge-connectivity respectively.