## 4 Evaluation
### Evaluation Methodology  
While utilizing no replication technique and forwarding messages solely based on forwarding utilities of relay nodes may guarantee acceptable throughput, there is the risk of excessive transmission overhead and resource consumption. Although this can be controlled with fixed-quota replication, the number of copies cannot be adjusted to suit the requirements of each message. Simply put, some messages may be over-replicated and others may be under-replicated. This lack of flexibility makes it difficult to achieve an optimal trade-off between resource consumption and delivery guarantees. To show this, we evaluate the performance of routing without using a replication technique as well as routing with fixed-quota replication. In order to investigate the impact of our proposed message dropping policy and replication technique, we evaluate the performance of our proposed LAMeD (without incorporating any replication mechanism) and LARep (which includes LAMeD and our proposed message replication mechanisms). We also evaluate the performance of the variable-quota replication technique proposed by [PERSON] and [PERSON] [57], namely the Adaptive Spraying Scheme--ADS for short.  
ADS starts by spraying message copies and defines a switch value that permits messages to enter the focus phase without waiting for their replication quota to reach one. The switch value, \(S\), of a message is given by Equation (9), where \(L\) and \(l\) are the initial replication quota generated at the source and remaining quota on the message, respectively, \(R\) is the reference value, \(T\) is the initial TTL of the message, \(H\) is the number of hops travelled by the message, and \(A\) is the area of the network. A message remains in the spray phase if an only if \((l>1)\bigwedge(t>S)\), where \(t\) is the remaining TTL of the message. Hence, a message enters the wait phase once the remaining TTL on it is below the switch value.  
\[S=\Big{[}T^{2}H^{2}+R(L-l)\Big{]}/\sqrt{A} \tag{9}\]  
We incorporate our proposed LAMeD, LARep and each of the message replication techniques selected for comparison into a base routing protocol and analyse their results based on selected performance evaluation metrics. PRoPHET is chosen as the base routing protocol with its configuration parameters according to the values given by [PERSON] et al. [34]. For configuration settings of the replication techniques, fixed-quota replication uses binary spraying, and we select \(L\), the replication quota, as 2, 4 and 8, based on best overall performance from several trials. For ADS, the value of \(L\) and the reference value are taken as 8 and 10, respectively, which yield best overall performance from several trials. For LARep, \(r\) is taken as 10 m, and location-based information is maintained at hourly intervals in a location table of 24 time slots.  
Since our performance evaluation is done through simulations, node mobility needs to portray realistic human movement properties. Although real-world traces represent actual movement scenarios, they are less suitable for evaluating message replication for OppNets in Smart City scenarios due to: (i) the lack of realistic node density and encounter frequencies portrayed in city-wide scenarios; and (ii) the inability to change network properties such as node population and geographical area without interfering with the encounter opportunities between nodes. Hence, we resort to using WDM, a synthetic mobility model that provides the desired level of flexibility and is also able to reproduce realistic properties of human movement.  
For the remainder of this section, Section 4.1.1 presents the simulation set-up while the performance evaluation metrics for analysing the replication techniques under comparison are defined in Section 4.1.2.