# Mars to Earth Data Downloading: A Directory Synchronization Approach
## 3 Mars to Earth Scenario
### Contact Plan  
The contact plan used in the experiments is a subset of that used in [15] (it lacks the contacts to and from a second orbiter, not considered here), with only few adjustments on the Tx rates to compensate the lack of the second orbiter. Original contacts were calculated using an orbital simulator based on EOTOOL by GMV [21]. To keep the length of the experiment manageable, these contacts have been time-scaled down in experiments by a factor of 60, as propagation delays (i.e., we have seconds instead of minutes and minutes instead of hours), as done in [14; 15]. This way the 24-h interval of the original contact plan corresponds to an actual experiment length of 24 min (1440 s). Note that link speeds (Table 1) have not scaled down, as this would not have helped in reducing the original length of the experiment.  
A few technical considerations about the correctness of scaling down contact lengths and propagation delays are most likely useful, to reassure the reader that by them we have not altered the behavior of the system. First, it is true that by reducing the contact times we have also reduced the contact volumes, i.e., the amount of data (in bytes) that can actually be transferred during a contact. However, as the length of the experiment is automatically reduced by the same factor, it is also correspondingly reduced the total amount of data generated, and thus preserved the balance between traffic load and network capacity, which is the essential aspect. Second, concerning the reduction of propagation delays, this is uninfluential if there are no losses on the links, as assumed here.  
Let us now examine the characteristics of scaled-down space contacts, plotted in Figure 6 as horizontal segments. Between the Lander (143) and the Orbiter (141) connectivity is very scarce, due to the fast orbital speed of the Orbiter. We have only two short pairs of contacts (first pair after 400 s, second at about 1200 s). Their very short duration limits their contact volumes. The Lander is also directly connected to the three GSs (201, 202, 203), with one long contact each, starting towards the end of the experiment (from 1000 s). Contacts between Orbiter (141) and GSs are much more frequent, and even more frequent for the link to EDRS (105), whose contacts are also uniformly distributed over time, with a significant gap only at 200 s.  
Considering that contacts from Lander to Orbiter are few and short, and that there is only one contact between Lander and each GS with a very limited bandwidth (only 32 kbit/s), it is clear that the connectivity of the Lander, in terms of total contact volumes available is critical. However, it is still compatible with the amount of traffic generated on board of the Lander. This makes the scenario particularly interesting to compare the DTNbox-based approach to the traditional one.  
Figure 6: Excerpt of the contact plan used in the experiments: intermittent contacts from Lander (143) and Orbiter (141) in top-down order. All contacts have been scaled down by a factor of 60 (seconds instead of minutes) with respect to the original contact plan.