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The problem of mutual exclusion has been extensively studied in distributed systems. The proposed solutions can be mainly classified in consensus based and token based protocols. Some of the proposed solutions consider the physical topology of the networks and try to provide optimal message exchange and minimal synchronisation delays. Others, impose a logical structure on the network like a ring or a tree. Recently, the mutual exclusion problem received an interest for mobile ad hoc networks. These networks are known as a challenging domain. To our knowledge, few algorithms have been proposed in the literature and all of them are token based approach. In this paper, we review the distributed mutual exclusion algorithms developed for mobile environments and principally for ad hoc networks and discuss some issues.

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The presence of coverage holes in a wireless sensor network has severe consequences on the performance of the network, that affects coverage, connectivity and routing paths. A number of approaches had been proposed to heal coverage holes in WSN. However, these approaches had tried to reach a full coverage, but ignored the large overlapping coverage area created. This paper presents a Healing Algorithm of Coverage Hole (HACH) involving the existing nodes to cover the hole. Our algorithm satisfies a full coverage while minimizing the overlapped area of sensing disks and deploying a minimum number of nodes. Experimental results which confirm the effectiveness of our approach are presented.

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