# Geo-DMP: A DTN-Based Mobile Prototype for Geospatial Data Retrieval
## 4 Experimental Validation and Evaluation
### Experiments in Comprehensive Scenarios
#### 4.3.1 Without Intermediate Cached Contents  
As seen from Figure 22, except for Case B, in most situations (over 80% on average) we can accomplish the two-way deliveries. As there is no contents cached on intermediate nodes, queries could only be satisfied by RSU after they are brought to their smallest target region. Likewise, only when the responses are routed back to the requester can the deliveries be successful. Hence, the high delivery ratio reveals that the multi-layer segmented regions can appropriately profile the movement of nodes. During the process of bundles approaching their target regions, they can serve as a valuable reference for selecting proper next hops under realistic scenarios. Moreover, in agreement with intuition, the enlargement of communication range contributes to more successful deliveries as many new contact opportunities emerge. All in all, we confirmed that we achieve our goal to design and implement a prototype that supports cooperatively retrieving named geospatial data units across intermittent links and a long time span.  
Next, we present an analysis of the decline of delivery ratio when nodes have knowledge of movements in advance. We deem that this decline can be ascribed to the underutilization of the movement history so far. According to our investigation of some delivery failures, if both nodes report that they have visited the smallest target region, the current routing protocol is unable to judge which one is superior. The consequence is, bundles will be forwarded to the one showing up first. But sometimes that intermediate node fails to get in touch with the destination node after it enters the smallest target region. Two reasons may account for this phenomenon. The first one is, due to the  
Figure 21: The movement range of comprehensive scenario and one set of traces as an example.  
Figure 22: The delivery ratio of experiments without intermediate caches.  
limitation of the experimental condition, the amount of nodes and executed experiment sets did not suffice to dilute the impact of the characteristics of the selected trajectories. The second one is that, as the communication range is a variable, it is very likely that there is a mismatch between region size and communication range. When the communication range becomes smaller, this problem will get more severe. That is why the problem looks more evident for case B than case D. We recognize that two nodes coming to the same finest target region and two nodes being able to contact with each other are not absolutely equivalent. Similar to the classical \"last mile\" problem, although target regions can indicate correct forwarding directions, additional strategies should be conceived for the \"last hop\" problem.  
With respect to hop counts and average delay, as we can see from Figures 23 and 24, case A with smaller communication range and without initial movement knowledge takes the longest latency and the highest forwarding hops to retrieve contents from the RSU. In case C, with a larger communication range, nodes can more easily find an eligible next hop, leading to a decrease of both delay and hop counts. The movement knowledge shows its advantage in case B and D. The average delay of case B reduces to about 50% of that in case A, while the average delay of case