## 2 Message Replication in OppNets for Sensed Data Collection
### Current State of Message Replication in OppNets
#### 2.4.2 Performance Analysis of Existing Replication Techniques  
Ideally, with respect to sensed data collection with OppNets, message loss would be minimal if only one copy of each message existed in the network at a time. Due to fewer messages in node buffers, more successful message transmissions would be realized per encounter. Results in Figure 4a show that the throughput achieved when PRoPHET utilizes single-copy replication is comparable with multiple-copy replication. In fact, generating excess copies in an ideal state is likely to result in lower throughput due to buffer overflows. However, real-world implementation scenarios are far from ideal. The wireless communication interfaces on user devices may not always be turned on. Hence, a message may never be delivered if only one copy exists in the network, as its custodian may miss important encounter opportunities. Also, the wireless communication interface on handheld user devices could be shared among different applications, including other OppNet applications such as content dissemination [64] and location-targeted services [65]. With single-copy replication, the only existing message copy would thereby compete with these other applications and messages whenever a data transmission opportunity arises. Hence, there is a higher chance of missing a suitable relay node  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value(s)** \\ \hline Total simulation time (days) & 5 \\ Warm-up period (days) & 1 \\ Cool-down period (days) & 1 \\ Wireless communication interface & Bluetooth \\ Transmission range (m) & 10 \\ Transmission rate (MBps) & 2 \\ Buffer size (MB) & 10 \\ Message size (KB) & 10 to 15 \\ Message TTL (days) & 1 \\ Number of scenarios & 3 \\ Simulation area (Km\({}^{2}\)) & 4.5 \(\times\) 3.4, 9 \(\times\) 6.8 \& 18 \(\times\) 13.6 \\ Number of nodes & 19, 76 \& 304 \\ Ave. message generation rate/node & 1 message every 10 to 15 min \\ Battery capacity (Joules) & 4800 \\ Receive/transmit energy (mW/s) & 0.08 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Simulation parameters for the small, medium and large scenarios.  
that may never be encountered during the lifetime of the message. Accounting for these factors in the simulation results in significantly less delivery guarantees, as shown in Figure 4b.  
Our results in Figure 4b support the notion that only one copy of a message may not be sufficient to guarantee its delivery [66]. The chances that the message may be lost due to buffer overflows or TTL exhaustion is heightened under higher network activity coupled with network characteristics such as high node population and degrees of spatial locality. In this regard, multiple-copy replication is a more preferable strategy. This section provides qualitative and quantitative analysis of the multiple-copy replication techniques overviewed in Section 2.3 under the network characteristics presented in Section 2.2. We start by addressing high node population and its impact on message transmission overhead, metadata overhead as well as the impact when coupled with limited bandwidth and high network dynamicity. Next, we investigate the impact of spatial locality on encounter opportunities and on the overhead-throughput trade-off for different sets of messages.