## 2 Message Replication in OppNets for Sensed Data Collection
### Impact of High Node Population  
Increased transmission overhead  
Considering the versatile nature of portable handheld devices, routing should take minimal toll on available energy. The versatility of portable handheld user devices requires them to perform a lot of other operations besides routing. Considering this, only a limited amount of node resources may be allocated to routing. The overhead incurred by some of the existing message replication techniques tends to rise at noticeable rates with node population. The effects of this may not be significant in networks that consist of a few hundred users and participating endpoints. However, in sensed data collection scenarios, where network coverage increases and extends to thousands of nodes, the number and frequency of encounters are likely to rise. In turn, the number of message transmissions may contribute to significant overhead and energy consumption. Under high node population, lack of a replication technique may generate excess message copies, hence, high message transmission overhead. Forwarding messages without considering the forwarding ability of nodes (e.g., Epidemic) rapidly exhausts node and network resources. The rate of resource consumption can be reduced by considering the forwarding ability of nodes. In that case, messages are forwarded only when the encountered node is a more suitable relay (i.e., presents a higher forwarding utility, e.g., PRoPHET). However, messages continue to be replicated in this manner unless their TTL  
Figure 4: Results when PRoPHET utilizes single-copy replication (SC) and fixed-quota replication (FQ) under the large scenario: (**a**) without external messages (**b**) in the presence of external messages, i.e., messages generated by other applications and routed using PRoPHET without a replication technique (for external messages, a randomly selected source node generates a message between 10 KB and 100 KB to a randomly selected destination node at every 1 to 5 min interval). is exhausted or dropped due to buffer overflow. Without mechanisms that notify nodes about the delivery status of messages they carry (which are usually unsuitable in OppNets due to increased resource consumption [67] and delays [68]), the resulting amount of message copies mostly depends on the forwarding utility of the source node and the total node population. The number of more suitable relay nodes is increased when messages are generated by nodes with relatively low forwarding utilities or when there is a large number of nodes in the network. This results in increased replication such as in Figure 5 (transmission overhead increases steadily with node population), most of which may be redundant, and eventually lead to higher energy consumption. Some variable replication techniques also result in excessive transmission overhead under high node population. EpiPRo may generate excess copies of a message before its hop count property reaches the threshold. In [PERSON] et al. 's [54] Gossiping and timer threshold proposals for instance, the optimal \(p\) and timer threshold values may cause excessive transmission overhead in higher node population. The proposal by [PERSON] and [PERSON] [57] also requires an optimal reference value in order to terminate message replication at the right time, which may still result in excess message copies in highly populated areas. n-Epidemic replicates messages according to \(n\), i.e., the number of nodes in contact. The choice of this parameter is crucial to the performance of the network since extreme values reduce network performance. Higher values of \(n\) reduce the probability of transmission while lower values tend towards Epidemic. Hence, there is the risk of excessive replication in high node population, as a large number of nodes may be in contact most of the time-in shopping malls, for instance.