# Geo-DMP: A DTN-Based Mobile Prototype for Geospatial Data Retrieval
## 4 Experimental Validation and Evaluation
### Experiments in Comprehensive Scenarios
#### 4.3.2 With Intermediate Cached Contents  
In this part, we study the effect of intermediate caches in the content acquisition process. We select six sets of trajectories by which the contents can be delivered successfully in the previous experiments. For each set of trajectories, we initiate the new experiment based on the results of the former one. To put it more clearly, the experiment without intermediate caches is executed prior to the new experiment to bring contents to some nodes in preparation. Except for Node 1, Node 10, and nodes that possess the cached contents, the other nodes will play the role of the requester in turn, in separate executions.  
Because in former experiments, contents are mostly forwarded back to the requester in two or three hops, not too many nodes will have cached contents. However, we think that, in reality it is quite common that demanded contents are scattered over surrounding areas. Many factors could be the cause of this: the lower popularity of the contents, the unwillingness of some users to expose their cache to the public, or limited support of BPQ mechanism among DTN nodes.  
We categorize the results by where the contents are retrieved from in Figure 25. As it can be seen, when the communication range is 300 m, due to cached contents only existing on a few intermediate nodes, other nodes have rather limited chances to discover them, so a large portion of the requests is still fulfilled by RSU. Despite this, cached nodes can contribute nearly a half of successful retrievals, which implies that Geo-DMP is capable of effectively utilizing a moderate amount of cached contents to markedly reduce the unsuccessful retrievals and promote retrieving contents from nearer locations compared to experiments without caches in Figure 22. With the communication range up to 600 m, we observe that the number of requests fulfilled by caches decreases a little instead. The reason for this is that, as stated before, larger communication range will create more contact opportunities and hence intermediate nodes can more easily contact with better next hops, even the RSU. So there is a tendency that forwardings are preferred to the direction of the RSU and the role of intermediate caches will inevitably be diminished.  
To close this section, we sum up the extensive experiments presented above in brief. Firstly, we prove the framework of Geo-DMP works properly and correctly on delay-tolerant content retrieval in a simplistic scenario with different cache distributions. Secondly, with a more realistic scenario and trajectories of nodes, we show that the delivery ability of Geo-DMP is acceptable for most cases, and the movement history information can help to select proper next hops as well as reduce average hops and delays. Thirdly, we discuss the reason behind some delivery failures for potential amelioration of our region-based routing strategy. Finally, we validate that Geo-DMP can leverage a small amount of intermediate caches to have a remarkable positive effect on the overall efficiency.  
Figure 25: The sources of content retrieval with cached nodes.