In this section, we study sink mobility for energy efficiency in real-time networks. We consider controllable sink mobility only. We first introduce representative sink mobility strategies and then review some specialized routing algorithms for data dissemination to mobile sinks.

6.5.1 Sink Relocation

According to the energy models introduced in Section 6.2, a sink should move toward data sources so as to shorten path length and thus reduce and balance energy consumption. During relocation, the sink may keep receiving data. It will bring extra load to the nodes in its visited areas and increase their energy consumption. So it is desirable that the sink goes through energy-intense area rather than energy-sparse areas. On the basis of this consideration, Bi et al. (2007) suggested that the sink temporarily changes its moving direction by a certain policy before entering an energy-sparse area and later turns back toward the relocation destination.

Optimal multisink placement is a NP-complete problem, as it is equivalent to the NP-complete dominating set problem on unit disk graphs (Bogdanov et al., 2004). In the following sections we will introduce some representative sink relocation strategies presented in the literature.

Cluster-Based Approach

Banerjee et al. (2009) studied sink relocation in a WSN clustered with multiple mobile sinks. They assumed that each sensor joins one and only one nearest cluster and sends data to the corresponding ...

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