The limits of theory: Pragmatic challenges in mobile ad hoc systems

The development of mobile ad hoc systems have considerably emphasized the need for a better understanding of the factors that influence the systemspsila performance, i.e., mobility patterns, radio propagation, traffic characteristics and their interrelations. Depending on the context where the system is being used-whether at work, at home, or in some means of transportation-there will be various applications that provide benefit of using the system. These different applications in turn generate traffic with very heterogeneous characteristics. In addition, the radio propagation depends on the surrounding environment and the density of communicating nodes in the system and in other systems that compete for radio resources; hence the available data rates in the system are hard to ascertain. The uncertainty regarding both the traffic and the available data rates make it hard to evaluate the performance of the system. This uncertainty is due to the lack of deployed systems. We hence believe that research in mobile ad hoc networks has to couple theory and practice in a continuous feedback loop with experimentations running full-fledged applications. We show how this methodology has proved to be beneficial in order to overcome challenges and to determine main issues for future research in the context of our PodNet project. It has also allowed us to revisit assumptions and scepticism surrounding the feasibility of mobile ad hoc networks. PodNet is an architecture, instantiated in a prototype, that is dedicated to cooperative content distribution. It enables exchange of podcasts (for any kind of multimedia content) amongst mobile devices in a peer-to-peer fashion using IEEE 802.11 in ad hoc mode. PodNet has clear advantages over traditional content distribution approaches (e.g., newspapers, 3G) in terms of public availability, practicability, capacity, resilience to failures, and jamming.