## 7 Classification of Mobile Sink based Data Collection Schemes
### Path Unconstrained Sink Mobility-Based Schemes  
In this section, we discuss several data collection schemes where the sink moves autonomously and does not have any constraints on its trajectory. The sink can visit nodes anywhere in the sensor field for data collection. In the rest of this section, an overview of the state-of-the-art schemes is given, including their aims, methodologies, strengths and weaknesses. [PERSON] _et al._ proposed in [PERSON] _et al._ (2014) an efficient Data-Driven Routing Protocol (DDRP) for mWSNs which aims to reduce network control overhead in route discovery/maintenance and improve data delivery performance. In DDRP, the mobile sink exploits the broadcast feature of the wireless medium and periodically broadcasts beacon messages (containing sink ID, timestamp, and an optional variable beacon-interval field) to its one-hop neighbors as it moves around. In this way, the neighbor nodes come to know about the existence of the sink in their vicinity. These one-hop neighbors of the sink do not further propagate this beacon message, but rather each data packet carries an additional field, called _Dist2 Sink_ (Distance to Sink), which stores the shortest known distance (number of hops) to the sink using that node. For neighbor nodes of the sink, _Dist2 Sink = 1_ and for others, it might be _{2, 3,...., Kj_, depending on their depth inside the sensing field. \(K\) corresponds to some maximum value to restrict the maximum hop-counts, beyond which _Dist2 Sink = Infinity_, which means nodes with no routes to the sink. Nodes whose _Dist2 Sink_ is set to _Infinity_, wait for a certain amount of time to overhear about a valid route to the sink. However, if no overhearing is received within a time bound, nodes adopt a random-walk strategy till the packet finds a valid route to the sink or it is timed out and dropped. In addition to _Dist2 Sink_, each data packet also carries _Time-Stamp_ information (the time-instance when a route was created to a mobile sink) which is used to avoid routing loops. The main drawback of DDRP is that with each movement of the mobile sink, subsequent topological changes will occur that are propagated in the entire network in the form of the overhearing mechanism. The overhearing mechanism greatly compromises nodes' energy reserves [PERSON] _et al._ (2014) as nodes need to be in idle listening mode in order to overhear the sink's latest location. [PERSON] _et al._ proposed a hybrid (proactive and reactive) routing protocol in [PERSON] _et al._ (2014) that basically enhances the IPV6 Routing Protocol for Low power and lossy networks (RPL) to efficiently support sink mobility. In the basic RPL, the mobile sink frequently advertises its presence through a broadcast mechanism propagated in the entire sensor field. In this way, the nodes, upon receiving these messages form a Directed Acyclic Graph (DAG) to the sink. Thus, a single move of the sink results in extensive network traffic and high energy consumption. In the hybrid approach [PERSON] _et al._ (2014), the sink's topological update is not propagated in the entire network, but rather it targets only a confined zone (consisting of a few hops) around the sink. Thus nodes within the confined zone can immediately send data to the sink using the known DAGs (proactive routing). For low sink mobility, zones with bigger sizes are created to facilitate prompt data delivery to the sink and for high sink mobility, zone sizes are kept smaller. Nodes that do not belong to any of the confined zones around any mobile sink exercise on-demand sink discovery (reactive mechanism). For that purpose, the nodes only broadcast route requests bounded by a few hops, thereby avoiding broadcasts to the entire network. Consequently, if this route request is received by any sink within that zone or any of the nodes that has an active DAG to the sink, it responds to the source node. The source node thus records that path for subsequent communications.