# Geo-DMP: A DTN-Based Mobile Prototype for Geospatial Data Retrieval
Furthermore, as in decentralized wireless scenarios, by virtue of the intrinsic mobility and confined communication range of mobile devices, the absence of contemporaneous end-to-end path is commonplace, which contradicts classical network protocols. Delay/disruption tolerant networking (DTN) [27] enhances the resilience and reliability of transmissions in these challenging environments with the \"store-carry-forward\" paradigm. By taking advantage of the ephemeral pairwise encounter chances, heterogeneous underlying physical networks and local storage resources, data can asynchronously traverse a network via multi-hop relays. Consequently, it can be envisaged that the fusion of D2D, DTN and spatial-related information would be particularly advantageous in two respects: One is being able to fetch contents that are infrequently changed and not in urgent need across long delay from distant places with classical DTN capabilities, such as maps or geotagged photos [28]. The other is, thanks to such decentralized communication mechanism, some real-time or emergent information could be announced or exchanged among nearby neighbors before it is reported to the supervisor, so it can be made known and dealt with as soon as possible. Some examples are evacuation guidance [29], warning for accident prevention [30], and updates of real-time traffic states [31], among others. At the same time, it should be admitted that research on DTN is obviously biased. A voluminous literature focuses on strategies (routing, caching, congestion control, incentive encouragement, etc. ), while only a small fraction of research work is devoted to actual deployment and application. As a result, there is still a long way to go to fully exploit the potential of DTN in a real-world setting. Filling this gap between theoretical and empirical sides of DTN is one of the chief motivations of our research. In our former work [32], we have implemented a wireless prototype system, named GeoTile, for geospatial data acquisition through connected multi-hop relay nodes. We thoroughly studied to what extent in-network caches could benefit the end-user and overall performance. In this paper, through the integration with DTN, persistent connections between nodes can be extended to intermittent ones. We also proceed to explore the supportive mechanisms for providing better adaptability to mobile contexts. The principal objective of this work is to present a complementary method for mobile users to obtain or exchange geospatial data under exceptional cases, e.g., when other wireless networks are unavailable at the moment or events of great urgency have just happened. This would similarly be profitable for crowd-sourcing applications, as participants do not have to  
Figure 2: The average price of \(1\) GB of mobile traffic and the proportion it constitutes in the daily earnings per capita by country. constantly keep contact with a central server and it is more friendly and convenient to interchange gathered data amongst themselves. Briefly speaking, the highlights of the work in this paper are as follows. Firstly, we propose a distributed system aimed at geospatial content sharing in opportunistic environments. Secondly, to characterize the mobility of users and embody it in a more concise representation, we introduce a multi-layer map partition scheme that combines road network and administrative information. Thirdly, we design an exploratory hierarchical DTN routing protocol to associate the bundle forwarding process with the history of movement knowledge based on realistic geospatial regions. Finally, to validate whether the system works properly and evaluate system performance, we run experiments on a virtual machine (VM)-based emulation platform. The outline of the remainder of the paper is as follows. In Section 2 we begin with an overview of relevant academic studies. Next, we will sequentially delineate each point of the main work stated above in Section 3.