## 4 Sink Mobility Challenges  
Although exploiting sink mobility results in several advantages, at the same time it generates more network overhead that need to be addressed by the routing protocols. In this regard, any routing protocol dealing with sink mobility needs to carry out the following additional operations [13]:  
* Inform the neighbors upon link breakage with the mobile sink.
* Propagate the sink's topological updates for ensuring connectivity.
* Reduce the chances of packet loss while the sink moves from one point to another.  
However, these operations cannot be taken in a holistic manner as that would greatly compromise the sensor node's energy consumption for every single move of the mobile sink. The following are the various challenges that arise due to sink mobility [14] which cannot be otherwise seen in static sink scenarios:  
_Sink Contact Detection_--For communication with a mobile sink, first its presence needs to be detected in their communication range by the sensor nodes. Sink contact detection is greatly affected by the speed of the mobile sink and the nodes' duty-cycles, as nodes in sleep mode will not be able to detect the presence of a moving sink. Similarly high sink mobility results in a short contact time with the sensor nodes which consequently leads to a high packet loss ratios.  
_Mobility-aware Duty Cycle Management_--To propagate a sink's topological updates, sensor nodes need to be in listening mode. If the mobility of the sink can be predicted or computed by exploiting the knowledge about the sink mobility pattern, it could help the nodes optimize the sink detection. In situations where the visiting times are known _a priori_ or computable with certain accuracy, nodes' duty-cycles can be adjusted accordingly to bring them into active mode at the expected arrival time of the mobile sink. However, this is only applicable in situations where the sink always follows a certain trajectory while maintaining a constant speed.  
_Increased Message Delivery Latency_--There is always a trade-off between energy consumption and message delivery latency in a mWSN. The message delivery latency directly depends on the sink mobility pattern (speed, direction, and pause interval). If the latest location information of a mobile sink together with the sink mobility information is promptly propagated in the entire network, the end-to-end latency can be reduced by enabling the nodes to adjust their routes to the sink accordingly. However, doing so would result into enormous energy consumption and would decrease the network lifetime. On the other hand, if the latest location information is propagated infrequently or only a limited number of nodes are being informed, the path adopted for data delivery may not be the optimal one in terms of hop-count. In addition, the message delivery latency is greatly influenced by the temporary pause period of the mobile sink. In some situations, the sink knocks at every single node to collect the sensed data. In such scenarios, if the size of the event data is big enough that it cannot be uploaded to the sink within the contact time, the nodes wait for the next trip of the mobile sink. However, doing so results in increased message delivery latency or overriding of buffered message by fresh sensory data.  
_Increased Packet Loss Ratio_--Due to unavailability of fixed contacts with the mobile sink, data is forwarded by each sensor node towards the last known location of the mobile sink. However, if the latest mobility information is not propagated in the entire network, successful delivery of the forwarded message would be greatly compromised. This problem gets further complicated when a mobile sink significantly moves from the last known sojourn point, ultimately resulting in dropping of the message due to a long traversal time.