# Improving Bundle Routing in a Space DTN by Approximating the Transmission Time of the Reliable LTP
## 5 Evaluation with DTN Protocols  
To verify the proposed method with specific DTN protocols and more realistic assumptions than the ones used in the prior section, consider the satellite transmission of data bundles between two locations from node g0, located near latitude 29.7604 and longitude \(-\)95.3698, and the destination node g1, located near latitude 41.4993 and longitude \(-\)81.6944, via two possible paths that become available periodically. This scenario is highly representative of a typical satellite communications scenario, where satellites are used as relays with multiple satellites within view that are able to provide the same service but only during certain time intervals. In this case, it is assumed that the operations center fixes the schedule that constrains the availability of each satellite for the intended test flow. The network simulator defines an event-driven model of a queuing network that follows similar assumptions to the one used in the analysis: infinite queues for each outbound link and a first-come first-serve buffer serving policy. Figure 7 depicts the evaluation scenario. Both ground stations g0 and g1 and the satellites s0 and s1 are DTN nodes and therefore each capable of implementing store-carry-and-forward scheme. The satellites were assumed to be part of the tracking and data relay satellite (TDRS) constellation. Each satellite location was determined by the geosynchronous equatorial orbit (GEO) of TDRS-3 (s0) or TDRS-9 (s1) as given by the _Skyfield_ library ([[https://rhodesmill.org/skyfield](https://rhodesmill.org/skyfield)]([https://rhodesmill.org/skyfield](https://rhodesmill.org/skyfield)), accessed on 1 October 2022). The two-line element (TLE) set of the satellites was obtained from _CelesTrak_ ([[https://celestrak.org/](https://celestrak.org/)]([https://celestrak.org/](https://celestrak.org/)), accessed on 1 October 2022). The differences in the distance between the nodes produced slight variations in the propagation delay of up to approximately 7 ms, which made the path (g0, s1, g1) slightly faster than (g0, s2, g1). The propagation delay was calculated independently for each bundle at the start time of the transmission, as the wobbling effect of GEO satellites affected the propagation delay. Figure 6: Average response times on a heterogeneous network for four representative cases as a function of the per flow sending rate. In (**a**,**c**), the one-way propagation delay was \(D=0.01\), whereas in (**b**,**d**), \(D=0.1\). In (**a**,**b**) the packet loss ratio was \(p=0.01\) whereas in (**c**,**d**), \(p=0.1\). The test flow consisted of 200 bundles sent at a designated rate \(\lambda\). Two cases were considered to better observe the influence of the service time. In the first case, the size of each bundle was 1 MB, and the transmission rate of all of the links was set to 1 Mbps. The links were scheduled to provide unaligned contacts of 100 s each every 1000 s; that is, the link disruptions did not occur in this case due to node mobility but because of link scheduling by mission control. The second case considered smaller bundles of 50 kB each and faster links of 10 Mbps, which cut the nominal radiation period by 200 times. To accelerate the simulations, the contact durations were shortened to 10 s every 100 s for each link. This reduced the realism but still allowed observing the relative performance improvement of the proposed method in a shorter time. In both cases, the downlink (s0, g1) was affected by controllable signal degradation, yielding a different channel BER than the other links. In addition, different tests were run for the flow sent from g0 to g1 and in the reverse direction. The convergence layer adapter is given by the LTP, configured with a maximum block size equal to the bundle size so that a single LTP block is always needed for every bundle. The LTP implements retransmissions for reliability but is only allowed to retry a given block transmission for a certain number of times, given as a function of the expected channel BER.