  A mobile wireless delay-tolerant network (DTN) model is proposed and
analyzed, in which infinitely many nodes are initially placed on R^2 according
to a uniform Poisson point process (PPP) and subsequently travel, independently
of each other, along trajectories comprised of line segments, changing travel
direction at time instances that form a Poisson process, each time selecting a
new travel direction from an arbitrary distribution; all nodes maintain
constant speed. A single information packet is traveling towards a given
direction using both wireless transmissions and sojourns on node buffers,
according to a member of a broad class of possible routing rules. For this
model, we compute the long-term averages of the speed with which the packet
travels towards its destination and the rate with which the wireless
transmission cost accumulates. Because of the complexity of the problem, we
employ two intuitive, simplifying approximations; simulations verify that the
approximation error is typically small. Our results quantify the fundamental
trade-off that exists in mobile wireless DTNs between the packet speed and the
packet delivery cost. The framework developed here is both general and
versatile, and can be used as a starting point for further investigation.
