Besides its original focus on space communications, the Delay-Tolerant Network (DTN) has been introduced into terrestrial mobile wireless networks. DTN is fundamentally an opportunistic communication system, where communication links exist temporarily, making it impossible to establish end-to-end connections. Therefore, most of conventional communication protocols developed for well connected networks simply fail here. Various new approaches such as SWIM [1], DFT-MSN [3], and ProPhet [2] have been investigated, where routing is largely based on nodal contact probabilities. They can be classified as more or less "flat", where every node plays a similar role in routing. The flat architecture is simple and effective in small networks, but not scalable to large size DTNs.
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