# Improving Bundle Routing in a Space DTN by Approximating the Transmission Time of the Reliable LTP
## 6 Results  
The average performance metrics for the first evaluation case are shown in Figure 8 for sending rates of 0.1 and 10 bundles/s from g0 to g1. The vertical bars indicate the 95% confidence interval. The baseline performance shown in red corresponds to method 2, with the bundle service time calculated without regard for the retransmission time, and this  
Figure 7: Evaluation scenario. Two paths connect nodes g0 and g1 through TDRS-3 (s0) and TDRS-9 (s1). is the method implemented currently for CGR. The results for the improved service time estimation are shown for various values for parameter \(f\) (the multiplying factor for the service time estimation given by method 4). The channel BER, along with the LTP segment and block sizes, affects the bundle loss rate, but only the former was allowed to vary in this study. It can be observed that the bundle loss ratio exceeded 0.5 once the channel BER for the link (s0, g1) entered the \(10^{-7}\) zone and continued growing with the current service time estimation method. The reported channel BER in this case was the value after the coding gain and therefore the BER that produced the segment losses. With the proposed improvement, it is feasible to continue operating with a low bundle loss ratio by allowing CGR switching to the second path. The response time metric aggregates the effective bundle transmission time (i.e., including retransmissions, and the buffering time). The average bundle response time was observed to be higher with the improved method than with the standard one, and the reason for this behavior was that the heavy bundle losses experienced by the latter did not contribute to increasing the average response time. The fact that the average response time was higher with the proposed method shows that it allowed CGR to better decide how to switch paths. The results for the flow in the reverse direction (g1 to g0) are depicted in Figure 9. The performance was observed to be  
Figure 8: Results for test case 1 with the traffic flowing from g0 to g1. The bundle loss ratio for a range of values of BER after the coding gain is shown for a bundle sending rate of (**a**) 0.1 bundle/s and (**b**) 10 bundles/s. The average bundle response time with the proposed method tended to be higher for larger BER values, given the queuing delay of the second path. Note that this outcome is not a weakness but is expected given the lower bundle loss. Charts (**a**,**c**) depict the results for a low sending rate \(\lambda=0.1\). Results for a high sending rate \(\lambda=10\) are shown in (**b**,**d**). higher in this case, given that the higher-loss link was directly connected to the source for this test, allowing CGR to switch paths earlier in the path. In the prior case, after CGR decided to use s1, the next hop necessarily used the affected link, as it was not possible to return the bundle due to the split horizon mechanism for loop prevention. Similar trends were observed for the second test case, but they occurred at a higher zone of BER values, given that the bundle sizes were reduced to 50 kB (see Figure 10 for the results for the g0 to g1 direction). Figure 11 shows the results for the reverse direction. As in the first case, the bundle loss rate continued to grow with the standard service time estimation method but was better controlled with the proposed method. The use of actual DTN protocols in the evaluation and the resulting performance metrics from four test sets of different experimental conditions allowed making the following observations:  
1. The use of the proposed expression for the bundle service time helped CGR better identify the remaining volume of the contacts, yielding a better selection of the contact to be used for a bundle transmission for high BER values. No side effects were observed for low BER values, with the proposed service time expression achieving the same end-to-end performance as the standard. 2.