## 7 Classification of Mobile Sink based Data Collection Schemes
### Controlled Sink Mobility-Based Schemes
This problem becomes severe if the nodes density in an area is high or if the nodes have a significant amount of recorded data [12]. To alleviate the short stay problem of the mobile sink in a densely deployed area, this scheme introduces an adaptive pause time proportional to local data traffic. To do so, it requires knowledge about global network resources such as the initial energy reserves of the network and thus information about the expected network lifetime. Moreover, it assumes that in certain areas more sensors are deployed compared to other areas for fine grain monitoring purposes. Thus, it assumes that few pockets (areas with high node density) exist in the network. Accordingly, during sink movement, it makes relatively large pauses at regions with high node density. For network traversal, it assumes that a graph is formed during the network initialization phase which results in a lattice graph G\({}_{0}\) = G(V,E) overlayed in the network area as shown in Figure 12.  
To support different application scenarios, it employs both deterministic-walk and biased-random-walk mobility patterns for network traversal. In the deterministic-walk, the sink traverses every single cell thus covering the whole network area. In certain scenarios, where network topology is not known to  
Figure 12: Example of overlay graph G\({}_{0}\) for network traversal [12]. the sink, a random walk mobility pattern is adopted. Furthermore, to provide fair coverage to different areas of the network, biased-random-walk mobility is thereby adapted at the end of every stop period, less frequently visited regions are favored for the next move of the sink by keeping a record of visits of every vertex. The scheme proposed in [12] achieves relatively better performance in terms of reducing latency by introducing the adaptive pause time mechanism. Although the adaptive pause time mechanism is beneficial for the current network segment, it may incur large latencies in data delivery from other segments of network due to the increased pause time of the sink. To address the hot-spot or energy-hole problem, [PERSON] and [PERSON] proposed a Mobile Sink based Routing Protocol (MSRP) in clustered WSNs [48] aiming at prolonging the network lifetime. Within a cluster, a CH performs data collection from its member nodes and waits for the arrival of the mobile sink. Whenever a mobile sink moves, it broadcasts a TDMA schedule to all the nearby CHs in its coverage area. Each CH will follow the schedule to forward its deposited data to the mobile sink. The decision about the next CH to be visited is taken based on the residual energy of the CH and for that purpose, the mobile sink keeps a record of the residual energy of all the CHs. Thus, it favors movement towards energy rich zones and consequently, moves towards the CH having more residual energy. A mobile sink collects data not only from CHs, but also from other neighbor nodes while moving. In this way, a balanced use of the node's energy results and this avoids the hot-spot problem thereby prolonging the overall network lifetime. This approach manipulates the next move of the sink towards energy rich zones in the network for improving network lifetime. However, it does not take into account the data delivery latency and the associated packet losses due to sink mobility. Using this approach, the sink will always move towards energy rich zones, thereby causing very huge data delivery latency from those parts of the network where some events are occurring most frequently and thus their residual energy will be relatively less. To prolong network lifetime while ensuring the delay requirements of real-time applications, [PERSON] _et al._ employ multiple energy-rich mobile CHs in [49], where the mobile CHs work in a collaborative manner to collect data from different segments of a network and deliver it to a base-station. The base-station, being static, is situated at the centre of the sensing field and receives data from the CHs within its radio range.