# A Credit-Based Congestion-Aware Incentive Scheme for DTNs
## 5 Performance Evaluation
### Performance Comparison
#### 5.2.3 Influence of Buffer Size  
As shown in Figure 9, the delivery ratio increases as the buffer size increases for all the routing schemes. This is because nodes can carry more messages with larger buffer, and consequently deliver more messages successfully. The delivery ratio of the direct scheme is the lowest, because each node only buffers its own messages and cannot forward the messages to others with higher delivery probability. As a result, some messages are discarded due to the overflow of buffer or expiration of TTL. The delivery ratio of CBCAIS is much higher than SMART, Multicenter, and GTA when the  
Figure 8: Number of failed transmissions. Figure 7: Cumulative utility comparison. buffer is small. This is because the SMART, Multicenter, and GTA schemes only encourage nodes to deliver messages to other nodes with higher delivery probability, without considering the node congestion problem. Many messages will be discarded when a receiving node does not have free buffer. In addition, some messages fail in transmission due to a shortage of credits, as in Section 5.2.2. Since nodes are prevented from discarding messages deliberately, and have a good credit balance, the CBCAIS scheme has a higher delivery ratio. The impact of buffer size on delay is shown in Figure 10. It can be seen that delivery delay increases as buffer size increases for all schemes. This is because, when buffer size increases, nodes can deliver more messages with longer delay, leading to an increase in the average delivery delay. In the direct scheme, each node only stores its own messages and cannot forward the messages to others with smaller delivery delay, so the average delivery delay is largest when the buffer is larger than 20 M. When the buffer is small, the direct scheme has the smallest average delivery delay because only very few messages are delivered successfully, as shown in Figure 9, and many messages with longer delivery delay are discarded due to the overflow of buffer or expiration of TTL. The average delivery delay of CBCAIS scheme is slightly larger than SMART, Multicenter, and GTA. One reason is that the delivery ratio is higher, as in Figure 9, and more messages with longer delay are delivered. Another reason is that a receiving node with small delivery delay will decline to forward messages when the buffer is full, and then these messages will be forwarded to other nodes with more free buffers but slightly larger delivery delay. Figure 10: Delivery delay for varying buffer size. Figure 9: Delivery ratio for varying buffer size. The impact of buffer size on overhead ratio is shown in Figure 11. In the direct scheme, each node only stores its own messages, which are never forwarded to others, so the overhead ratio is always 1. For other schemes, the impact of buffer size has two aspects. On the one hand, as buffer size increases, nodes get more forwarding chances, so the number of forwards will increase, leading to an increase in the overhead ratio. On the other hand, more messages are successfully delivered, leading to a decrease in the overhead ratio. When the first impact outweighs the second one, the overhead ratio increases; otherwise the overhead ratio decreases. The overhead ratios of SMART, Multicenter, and GTA are higher than CBCAIS when the buffer is small. This is because many messages are forwarded to the nodes with high connection degree through many hops, but finally discarded due to node congestion. The invalid forwarding will increase the overhead ratio. As shown in Figure 12, the energy increases as the buffer size increases for all the routing schemes. Nodes can store more messages with larger buffer, so the number of message forwards will increase, leading to more energy consumption. The energy consumption of the direct scheme is the lowest because each node only buffers its own messages and cannot forward the messages to others. In CBCAIS, more messages are successfully delivered, so the energy consumption of CBCAIS is highest. Figure 11: Overhead ratio for varying buffer size.