## 7 Classification of Mobile Sink based Data Collection Schemes
### Path Unconstrained Sink Mobility-Based Schemes
However, this scheme is only applicable for delay tolerant applications because of the long convergence time for route adjustment in the case of sink mobility. Inspired by the foraging behavior of termites, the Termite-hill routing scheme for WSN [39] aims to achieve better performance in terms of reducing control traffic overhead, fast route discovery and overall energy consumption. It assumes no prior knowledge about network configuration and adjacencies. The routes are selected based on energy utilization. Routes are discovered on demand, _i.e._, when the nodes have to report some event. For route discovery to the sink, the node generates a _forward-soldier_ packet which is broadcast to all neighbors. Upon receiving this packet, the intermediate node looks for a valid route to the sink in its routing table and if finds one, it generates and sends back a _backward-soldier_ packet to the source node. However, if it does not have a valid route to the sink, it saves a reverse path to the source node (for future use) and continues broadcasting the _forward-soldier_ packet. This process goes on until the _forward-soldier_ packet gets to its destination (mobile sink). Accordingly, the sink, upon receiving this packet, sets up a _backward-soldier_ packet and unicasts it to the source node. All the intermediate nodes upon receiving the _backward-soldier_ packet modify their routing tables thereby setting up a forward pointer to the sink and forward the _backward-soldier_ packet along the reverse path hop-by-hop. This process goes on until the original source node receives the _backward-soldier_ packet. Although this scheme is simple enough, it does not take into account the sink mobility pattern and the associated costs in terms of route discovery, latency and packet losses. It only investigates the potential benefits that are obtained by adopting a mobile sink for the sake of improving network lifetime. To minimize the network control traffic due to sink mobility, Elastic routing was proposed by [PERSON] _et al._[7] that exploits the broadcast transmission nature of sensor nodes. To deal with sink mobility, the new sink's location is propagated along the reverse geographic routing path to the source node via the overhearing mechanism. It is claimed that the proposed solution results in reduced network control overhead, data delivery latency and energy consumption compared to other existing solutions dealing with sink mobility. It assumes that all nodes are location aware and hence share location information with neighbors via beacon messages. Furthermore, it assumes bidirectional channels between two neighbor nodes. In the initial phase, the source determines the sink location by some sink location service. For data forwarding, a source node first looks for a valid path to the sink in its neighborhood list, otherwise greedy forwarding approach introduced in [40] is adopted for route determination to the sink. However, before the forwarded data reaches the sink, the sink might have changed its position. To cope with this situation, Elastic routing proposes that along with periodic beacon messages to inform about new neighbor nodes, those nodes are also updated with the sink's latest location from which the sink has received the last packet. This phenomenon is illustrated in Figure 9, where the sink is unreachable by last hop \(A\), so it informs its latest location to node \(A\) by greedy forwarding. Accordingly, node \(A\) updates the sink location information in the data packet and sets \(B\) as the next hop towards the sink. Elastic routing traces the sink mobility in this way, and thus ensures uninterrupted data delivery to mobile sinks. For further propagation of the latest sink location information in the network, elastic routing employs the overhearing feature of wireless sensor nodes. Since it assumes a bidirectional channel between two neighbor nodes, therefore whenever any neighbor node of the sink sends some data to the sink, it is overheard by other neighbor nodes in the vicinity.