## 6 Performance Evaluation
### Performance Comparison of Three Mechanisms  
We use the epidemic routing protocol to compare the performance of the _ASMSS_ with the random working mechanism and the periodical working mechanism in _Infocom6_ trace. In opportunistic routing, if the contacts between nodes can be efficiently discovered, the opportunity for message forwarding will increase, which can improve the network performance to some extent. Under each one of the above three working mechanisms, the nodes with the same value of duty cycle will have the same energy consumption in contact probing. With various values of duty cycle, the number of contacts discovered under the three working mechanisms is shown in Figure 6, and data transmission performance with epidemic routing is shown in Figure 7.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Trace** & _Infocom6_ \\ \hline Device & iMote \\ Network type & Bluetooth \\ Duration (days) & 4 \\ Granularity (seconds) & 120 \\ No. of internal contacts & 170,601 \\ No. of devices & 98 \\ Contact frequency/pair/day & 6.7\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Trace properties.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Simulation duration (seconds) & 343,000 \\ Bluetooth interface & SimpleBroadcastInterface \\ Bluetooth transmitSpeed (bytes per second) & 250 k \\ Bluetooth transmitRange (meters) & 10 \\ Node bufferSize & 5 M \\ TTL of the messages-msgTtl (minutes) & 300 \\ MessageEventGenerator interval (seconds) & 1000, 2000 \\ Message size & 1 k \\ \hline \hline \end{tabular}
\end{table}
Table 2: Simulation parameters.  
From Figure 6, we can find that the number of the effective contacts increases as the duty cycle increases in the three working mechanisms. The number of the effective contacts of the _ASMSS_ is more than the other two working mechanisms when the duty cycle is the same. Compared with random and periodical working mechanisms, for _ASMSS_ the number of effective contacts increased by 7.2% and 12.1% respectively, in average. The main reason behind is that the _ASMSS_, which is proposed based on the self-similarity of the inter-contact time, uses the past contact history to predict the future contact time. Then, the state of a node can be set adaptively according to predicted result of future contacts with other nodes. Hence, many missed contacts in the random working mechanism and the periodical working mechanism turn out to be the effective ones.  
Figure 7 shows the performance comparison of the _ASMSS_, the random working mechanism and the periodical working mechanism in terms of delivery ratio, delivery delay, and delivery cost. In the figure, when the duty cycle increases from 10% to 100%, the delivery ratio and the delivery cost rise correspondingly. In contrast, the delivery delay decreases in all working mechanisms. That is because when the duty cycle increases, there are more contacts to be effectively discovered, and there are more chances to exchange data. When the duty cycle is less than 70%, the _ASMSS_ has better performance than the other two, which means the proposed mechanism gives the nodes more effective contacts for data delivery than the other two mechanisms. When the duty cycle exceeds 70%, we find that these three working mechanisms are almost the same shown in Figure 7, that is because that there are few contacts missed in this situation for all mechanisms. Under epidemic routing, compared with random and periodical working mechanisms, the delivery ratio of _ASMSS_ increased by 27.1% and 87.8%, the delivery cost decreased by 2.9% and 3.8%, and the delivery delay decreased by 4.1% and 8.8%, respectively.  
Figure 6: Effective Contacts _ASMSS_.