## 5 Conclusions and Future Work  
OppNets for sensed data collection are characterised by high node population, dynamicity and degrees of spatial locality inherent to user movement. Under these characteristics, forwarding messages without controlling the number of generated copies may result in excessive transmission overhead and resource consumption on portable handheld user devices. Since only one copy of each message may be insufficient, existing techniques for controlling message replication generate multiple copies, which may either be fixed or variable. However, limiting the number of message copies to a particular value is not flexible enough to adapt to the requirements of each message. Also, varying the replication of each message by allowing network properties (e.g., total number of nodes) or message parameters (e.g., TTL) to determine generated copies require careful selection, which may be impractical in the absence of global knowledge about the network. Another drawback of existing message replication techniques is their obliviousness to spatial locality inherent to user movement. High degrees of spatial locality in sensed data collection scenarios reduce encounter opportunities between nodes from different locations in the network, thereby degrading routing performance. Our contribution in this regard is twofold:  
1. We first established relevant characteristics of OppNets for sensed data collection introduced by the presence of portable handheld user devices, since existing message replication techniques are not specifically designed to cope with the characteristics. Then we investigated the impact of these characteristics on existing message replication techniques and suggested design guidelines that need to be observed in order to improve routing performance and feasibility in real-world implementation. 2. Next, we followed our design guidelines to propose a set of mechanisms that collectively form LARep, a message replication technique that can be incorporated into existing encounter-basedrouting protocols to reduce message transmission overhead without compromising throughput. LARep exploits the concept of spatial locality to replicate messages according to the proximity of nodes' preferred locations in the network. This allows different messages to be replicated at varying rates, thereby achieving a near optimal quota for each, without requiring global knowledge about the network. LARep also introduces a locality aware policy for dropping messages, which further improves throughput. Incorporating LARep into PRoPHET reduced message transmission overhead, reduced energy consumption and increased throughput. Experiments in the ONE simulator showed that LARep achieves better performance as compared with existing message replication techniques. We expect this work to motivate further research contributions. Future proposals should benefit from our design guidelines and lessons learned from this work, such as the best practice of including external messages when evaluating the performance of routing protocols and message replication techniques. Future work includes investigating the impact of LARep on different encounter-based routing protocols. Future work also includes investigating how LARep can be improved with learning mechanisms on sink nodes and feedbacks from gateways. Also, LARep requires GPS for obtaining location-based information. In reality, users may not be willing to always have the GPS on their devices turned on due to energy consumption. Furthermore, real-world implementation also raises the issue of unreliable GPS signal in geographically restricted locations. Future work could reduce the need for consistent GPS readings by incorporating prediction mechanisms that exploit the regularity embedded in human movement to infer node location. Also, it would be interesting to observe the behaviour of our proposed LARep and compare its performance with existing solutions in a real-world trace.