## 2 Problem Background
### Limitations of Existing Fair Routing Solutions for Sensed Data Collection
#### 2.4.1 Neglecting the Impact of Spatial Locality  
Most solutions assume throughput is not compromised after burden is distributed from popular nodes to less popular nodes. However, less popular nodes have less chances of delivering messages through encounter-based or social-based utilities alone [19]. This is due to less popular nodes being less socially-connected in the network, which is worse when coupled with the effects of spatial locality inherent to node movement in sensed data collection scenarios [55]. Concepts of spatial locality include high likelihood of regularly visiting only one or few regions [62; 63; 64; 65] and traveling relatively shorter distances [66; 67]. Therefore, unlike popular nodes (which Mtibaa and Harras [68] term as \"MultiHomed\" nodes, e.g., devices carried by postmen and campus bus drivers that are more suitable to disseminate messages across sub communities), the movement of less popular nodes is more confined to a local region, thereby limiting encounter opportunities with nodes from other regions. Hence, throughput is degraded since opportunities to forward messages to more suitable relay nodes are missed for the sake of maintaining a fair distribution of burden.  
FairRoute is based on the idea that a diversity of routes can be found through contact and social networks. Therefore, when popular nodes have high queue lengths, less popular nodes rely on the forwarding utility (i.e., nodes that have long-term interactions with the destination) to find alternative routes. Under higher degrees of spatial locality, however, less popular nodes have less encounter opportunities as their movement is often restricted to smaller geographical areas. Since less popular nodes form the majority of the node population, most nodes may not be able to encounter more suitable relays on time with this forwarding utility, and may still rely on popular nodes to carry messages across multiple regions.  
The metrics from which CAFE's forwarding utility is derived (i.e., smoothed locally computed degree centrality, a filtered common neighbour metric, and interaction strength between a node pair) are biased towards popular nodes, especially under higher degrees of spatial locality. In such scenarios, the diversity of routes that may exist for less popular nodes are unlikely to extend across multiple regions through centrality and social ties alone. CCAF modifies the existing forwarding utility to allow messages to be forwarded only when the relay presents a much higher forwarding utility. For similar reasons as FairRoute's approach, less popular nodes may not be able to make much progress under high degrees of spatial locality.  
Unlike the typical rule that forwards messages when a relay presents a higher forwarding utility, FOG permits messages to be forwarded to slightly lower-ranked relays. It acts as a one-way filter that only allows messages towards slightly lower-ranked nodes. Hence, when the burden on popular nodes is high, messages are directed towards nodes with decreasing forwarding utility. This may not significantly degrade throughput under low degrees of spatial locality. Under higher degrees of spatial locality, however, achievable throughput is compromised since lower-ranked nodes cannot make enough routing contributions due to less encounter opportunities.