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Delay tolerant networks can exhibit long phases of complete unconnectedness, where several nodes will not be able to communicate effectively. In those circumstances controlling the movement of helper nodes, that facilitate message exchanges instead of relying on direct communication, can improve network performance. We investigate when controlled movement is beneficial and how helper nodes need to move or position themselves to achieve maximum network performance. Three communication modes on a single link and in a hub model are analyzed: direct communication, communication through one or more relays, and communication via data ferries. Hereby we extend the traditional in-range, out-of-range communication model to include a distance dependence which gives a differentiated rate-distance profile. Further, we characterize controlled mobility of mobile data ferries to form static relay paths or dynamically ferry data between nodes. Achievable throughput, delay, and distance performance of the link modes change with channel and ferry characteristics. The union of these performance regions forms the achievable communication space for a given DTN scenario. Novel phase plots show when it is feasible to use one of the mobility schemes versus direct communication. The implications of these modes for practical link-layer designs are discussed using a real-world DTN example of sensor data collection with unmanned aircraft.