# A Combinational Buffer Management Scheme in Mobile Opportunistic Network
## 4 Simulation
### Simulation Results and Discussion  
#### 4.2.1 Overall Performance  
During the experiment, we calculated the utility value of the message and the node according to the characteristics of the six management methods. For the CBM strategy, we calculate the utility value of the message and its negative correlation according to Equations (1) and (2), and calculate the utility value of the node according to Equation (3). When the nodes randomly contact with other nodes and exchange messages, the nodes update the message information table, and the utility value of the node changes.  
Figure 3 shows the results of the six buffer management strategies in terms of the delivery ratio, where Figure 3a presents the results of the delivery ratio of the KAIST. Obviously, the CBM strategy improves the delivery ratio of the message, and it has the highest delivery ratio after 2000 s. The most obvious is that, compared with the RED, the CBM enhances the delivery ratio almost by 15%, from 0.82 of RED to 0.97 of CBM. Figure 3b presents the results of the delivery ratio of the SLAW. Similarly, the CBM strategy has the highest message delivery ratio leading other strategies about 4-28% when the emulation time exceeds 1500 s. One of the advantages of the CBM strategy is that the message arrives at the destination node with the maximum delivery ratio.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Simulation field size & 600 \(\times\) 600 m\({}^{2}\) \\ Simulation time (KAIST/SLAW) & 15 \(\times\) 10\({}^{3}\) s/18 \(\times\) 10\({}^{3}\) s \\ Number of nodes (KAIST/SLAW) & 90/500 \\ Transmission range & 25 m \\ Node storage size & 20 MB \\ Message storage size & [0.5,1] MB \\ The TTL of the message & 300 s \\ \hline \hline \end{tabular}
\end{table}
Table 1: Simulation parameters.  
Figure 4 presents the number of hops of the six methods. The CBM achieves the least number of hops compared with the other five strategies both KAIST and SLAW. In general, a message arriving at the destination node experiences 11 hops in CBM in Figure 4a, while the other five strategies (RED, DH, DR, DT and DOA) require 33, 29, 32, 26 and 35, respectively. Furthermore, the average number of hops for the messages arriving at the destination node only needs 25 hops in CBM strategy in Figure 4b, which is less than half of the DR strategy. Meanwhile, the number of hops for the other five strategies (RED, DH, DR, DT and DOA) is 130, 74, 156, 116 and 61, respectively. Therefore, another advantage of the CBM strategy is that the message arrives at the destination node with a minimum number of relay nodes.  
The high delivery ratio and low hop also validate the effectiveness of our CBM strategy, which not only combines the attributes of the message with the properties of the node, but also introduces the concept of message migration to avoid node buffer congestion.  
Table 2 illustrates the network overhead ratio and the average delay of the message of the six buffer management policies, where the overhead ratio is the number of message copies of the original messages. We can see that the CBM policies with the least network overhead ratio are both KAIST and SLAW. In this way, we can effectively avoid the network congestion caused by the excessive number of copy messages. Similarly, the average delay of CBM strategy is superior to the other strategies because the node updates the queue based on the real-time information of the neighbor node and continuously receives the message with the higher utility value in CBM. We use the migration method and transfer the messages with the lower utility value to neighbors instead of directly deleting them when the node buffer is congested and the neighbor nodes have larger available buffer size. Message migration leads to the survival time of the message growing longer in the network, but it ensures the message delivery ratio and does not affect the CBM strategy overall performance.  
Figure 4: Comparison of hops.  
Figure 3: Comparison of delivery ratio.