proach. When the mobility in sparse networks increases, mo- bile nodes begin to meet others. This is the tradi- tional DTN scenario, where nodes forward one or more copies of a given message until it reaches its destina- tion. There are many strategies for optimizing the forwarding decision. The most straightforward ap- proaches, such as Epidemic or Spray-and-Wait [13] do not require nodes to acquire information on the oth- ers’ positions, movements or trajectories. More elabo- rate schemes involve a utility function where each node collects direct and indirect knowledge of other nodes’ meeting probabilities. They require a certain learning period to aggregate statistics before making good for- warding decisions. For example, Lindgren et al. [10] use past encounters to predict the probability of meet- ing a node again while Daly et al. [4] use local estimates of betweeness and similarity. hybrid approach positions itself compared to existing DTN and MANET approaches. In section 3, we de- scribe the HYMAD routing protocol principles. We explain how nodes can agree on forming disjoint groups and how such groups rather than individual nodes can be used as the basis for DTN routing. We then evalu- ate the scheme on a real data set, the Rollernet exper- iment, in section 4. Finally we conclude our work in section 5. In dense networks, conventional MANET protocols start to break down under high mobility down even if the network is almost always fully connected. Indeed the sheer instability of the links would result in a del- uge of topology updates in the proactive case and route error and new route requests messages in the reactive case. DTNs protocols on the other hand can handle high mobility regardless of the density of the network. However by narrowly focusing on per-encounter events, they ignore a lot of available information. For exam- ple, simply asking nodes to regularly broadcast a list of their neighbors would give all nodes a picture of its two-hop neighborhood even under high mobility. Repeat this once and everyone knows their three hop neighborhood. A node may therefore have a topology “knowledge horizon” which determines how far into the real topology a node can “see”. The more extreme the mobility, the shorter the “horizon”. Mobile wireless ad-hoc networks were first studied under the assumptions of moderate node mobility and sufficient density to ensure end-to-end connectivity. Both conditions are necessary for traditional MANET approaches, be they proactive or reactive. Let us characterize the various occurrences of mo- bile wireless networks along the two main parame- ters of node density and node mobility. In Fig. 1, which maps the different routing approaches on the bi- dimensional mobile wireless network space, traditional MANET routing appears in the top left corner. When the density of nodes diminishes end-to-end connectivity can disappear. In such sparse net- works nodes have very few, if any, neighbors within their transmission ranges. The topology eventually splits into several non-communicating connected com- ponents. This is typically the realm of Delay Tolerant Networking which one can further subdivide in two [2]: the Assisted DTNs (A-DTN), in case of low mobility of nodes, or Unassisted DTNs (U-DTN) where mobil- ity is high. The latter corresponds to traditional DTN scenarios. The Hybrid DTN-MANET approach that we ad- vocate in this paper aims at filling the gap for effi- cient routing in highly connected and highly mobile networks, which have so far, to the best of our knowl- edge, received little attention. Hybrid DTN-MANET routing, like the HYMAD protocol that we describe be- low, combines the resilience of DTNs with the greater knowledge of local network topology provided by a MANET protocol. It adapts naturally to the dynam- ics of the network and its applicability spans a large spectrum of the mobile wireless network space. Routing in A-DTNs typically involves special mobile nodes, known as message ferries or data mules, which relay the messages between the separate connected components [16, 12]. The packet-switching method of MANETs is replaced with a store-and-forward ap- 2 2 Routing in a mobile wireless network Figure 1: Mobility vs Density: when different paradigms apply Node Density Node Mobility HYMAD A-DTN MANET DTN