In traditional WSNs, sensor nodes are distributed in the sensing field whereupon detecting some event of interest, nodes report the sensed event back to some static sink(s) through multi-hop or single hop communication. One major drawback of such communication infrastructures is that the sensor nodes close to the sink will consume more energy (partly for reporting their own sensed data and partly for relaying their neighbors’ data), and thus their energy will deplete quickly. Consequently, this will result in isolation of the sink and as a whole the entire network would no longer be operational. This problem is commonly known as the hot-spot or sink-hole problem in wireless communication.

To deal with this issue, the concept of mobile sink was introduced in [4,5], that not only results in balanced energy consumption among the nodes but can also be exploited to connect isolated segments of the network [6]. Another motivation for introducing a mobile sink in a WSN is that some applications explicitly require sink mobility in the sensor field. For instance, a rescuer equipped with a PDA moves around in a disaster area to look for any survivors [7], and a farmer while walking around a field would be interested in knowing which segment of the field requires watering, fertilizers, etc. Although the sink mobility improves network lifetime, at the same time it incurs additional overhead for the routing protocol for dynamic route adjustments. Due to sink mobility, the topology of a WSN becomes dynamic and to cope with such a dynamic topology, the routing algorithms specifically designed for static WSNs cannot be directly applied in mobility situations.

This has triggered the development of new routing strategies for Mobile sink-based Wireless Sensor Networks (mWSNs).In this paper, sink mobility is covered from different perspectives with the main aim of critically discussing the performance of existing mobile sink-based data collection schemes. Sink mobility has also been exploited to address coverage issues and interested readers may refer to [8�C11] for more details. The rest of this paper is organized as follows: first, the network architecture of mWSNs is described in Section 2. Next in Section 3, the potential advantages that are obtained by exploiting the sink mobility are discussed. Then some challenges for data dissemination that are caused by sink mobility Drug_discovery are identified in Section 4.

Different mobility patterns exhibited by sinks are discussed in Section 5, as they have a direct impact on the design of a strategy for data delivery towards a mobile sink. A procedure of data delivery to a mobile sink is described in Section 6 to gain more insight into the complexity and the different phases involved when delivering sensed data towards a mobile sink.