## 2 Message Replication in OppNets for Sensed Data Collection
### Impact of High Degrees of Spatial Locality  
Uneven distribution of encounter opportunities  
As observed in Section 2.2, spatial locality plays an important role in the encounter opportunities experienced between different sets of nodes. Users often visit only few places and mostly move within a local region, thereby reducing the likelihood of seeing people on a regular basis the farther away their homes are located. In terms of OppNets (depending on the geographic location of source and destination nodes), the delivery of some messages through encounter-based utilities may be less likely than others. Hence, in the design of OppNet routing solutions for collecting sensed data, entirely neglecting spatial locality inherent to user movement may lead to poor throughput. Figure 7: The number of messages replicated per hour with n-Epidemic (\(n=3\)) under the (**a**) small (**b**) medium and (**c**) large scenario. From Section 2.3, we observe that existing fixed-quota replication techniques overlook the impact of spatial locality on network performance. First, in the spray phase, some techniques tend to allocate the remaining quota according to a ratio determined from encounter-based or social-based node relationships. For example: SAS, a function of the forwarding utilities of the encountered nodes; [PERSON] et al. [51], connection strength with the destination; ISW, encounter duration with the destination; degree centrality [48]; average waiting time between successive encounters [49]; probability of encountering the destination in the near future [50]; frequency of encounters [51]; or one derived from two or more encounter-based features--frequency of encounters and encounters with the destination [52], for instance. Second, most of them utilize only encounter-based properties in the second phase of replication after the spraying quota is exhausted. For example, SnF utilizes single-copy replication, entirely relying on the encounter-based forwarding utility for the number of transmissions. Unfortunately, since the chances of determining more suitable relay nodes through encounter-based or social-based forwarding utilities alone reduce under higher degrees of spatial locality, allocating message quota accordingly may not be effective. Hence, messages may make little progress towards the destination despite the quota allocated to relay nodes during the spray phase. In the second phase, less encounter opportunities between nodes from different local regions reduces the chances of finding relay nodes with better encounter-based forwarding utilities. While a transitive property (i.e., a property that infers the forwarding utility of a node for a destination it has never encountered from neighbouring nodes that encounter the destination, e.g., PRoPHET and SnF) is able to perceive encounter-based knowledge over multiple hops, the resulting forwarding utilities are fine-grained. For messages traversing multiple regions, the quality of more suitable relay nodes becomes less, i.e., the difference between the sender's and receiver's forwarding utility becomes less (cf. Figure 8). Consequently, such messages are subjected to more number of transmissions. Figure 8: _Cont._  
Some variable-quota replication techniques may also suffer from the obliviousness of spatial locality. For instance, [PERSON] et al. 's [59] proposal generates more replicas when relay nodes have a low chance of delivering the message. Without considering the properties of spatial locality, this replication may result to excessive transmission overhead in high degrees of spatial locality, since most neighbouring nodes may have low chances of delivering messages to distant destinations through encounter-based or social-based forwarding utilities alone. Similarly, the proposal by [PERSON] et al. [51] may result in excessive replication, since more message copies are generated when the connection strength between the source and the destination is low.