## 7 Classification of Mobile Sink based Data Collection Schemes
### Path Constrained Sink Mobility-Based Schemes  
In this category, we discuss those data collection schemes where the sink follows a constrained path such as a straight line, periphery, circular path, or visits only a selected subset of nodes in the sensor field for data collection. In the rest of this section, an overview of the state-of-the-art is given, including their aims, methodologies, strengths and weaknesses. In [23] a scheme called Multiple Enhanced Specified-deployed Sub-sinks (MESS) for WSNs with path-limited sink trajectories is proposed. A similar approach has also been adopted in [24]. MESS employs multiple sub-sinks for sensory data collection from more remote nodes. The aim is to reduce the long-haul communication between the source nodes and mobile sink and thus the delay as well. The sub-sinks are considered as enhanced wireless nodes having more storage capacity and are deployed at equal distances along the accessible path after the deployment of the rest of the network nodes. The placement of sub-sinks in this fashion creates a strip in the sensing field and provides a set of meeting points with the mobile sink as shown in Figure 4. Each sub-sink working as an access-point to the mobile sink, notifies the underlined network segment about the service that it is offering. For that purpose, each sub-sink sends a broadcast message (containing its ID) so all the recipients would set the sub-sink as the next-hop. In case if a node receives broadcasts from more than one sub-sinks, it selects its next-hop to the sink based on received signal strength. Consequently, the one-hop neighbors of sub-sinks would broadcast this route alert further in the network till all sensor nodes are informed. Figure 4: Strip-based structure in MESS. Figure 3: Classification of sink mobility based data collection/dissemination schemes. In the data collection phase, MESS assumes that the speed of mobile sink is quite slow such that it collects all the harvested data from at least two of the sub-sinks along its trajectory while moving continuously. However, in large scale networks each sub-sink might be depositing data for a large population of sensor nodes and consequently that cannot be delivered to the mobile sink in a single trip. As a result, huge latency would be caused in the data delivery to the sink. Another implication is the use of specialized nodes working as sub-sinks, which not only need to be rich in storage capacity, but also in energy. These implications together with the fixed speed of mobile sink limit its applicability to certain application environments only. Similarly, nodes in the vicinity of sub-sinks would suffer from early energy-depletion thereby reducing the overall network lifetime. [PERSON] _et al._ proposed a converge-cast algorithm called Virtual Circle Combined Straight Routing (VCCSR) [25] for efficient data collection in mWSNs. VCCSR aims to reduce the path reconstruction cost upon sink mobility and for this purpose, it constructs an adjustable routing tree. In the proposed algorithm, it forms a virtual backbone structure which is comprised of several virtual circles and straight lines where Cluster-Head (CH) nodes are placed along the virtual circles and straight lines. An example virtual backbone structure is shown in Figure 5.  
The CH points are computed as the midpoints of virtual paths and accordingly nodes which are closer to CH points are appointed as CH nodes. Furthermore, all nodes located at the centre of circles, the rendezvous points (intersecting points of the circles and straight lines), and the boundary points (midpoints of the routes to rendezvous points) are considered as CH nodes. Together all the CH nodes form the virtual backbone network. Similarly, the boundary points in the virtual structure are also used in deciding which nodes need to be adjusted in readjusting the routing tree upon sink mobility. CH nodes assume the responsibility of collecting sensed data from its cluster members and delivering it to the sink via a tree-based structure.