On the scalability of delay-tolerant routing protocols in urban environment

In these last years the demand for personal and ubiquitous communication has raised at a tremendous rate. Mobile services, which are nowadays typically offered based on location and context, suffer from various 3G shortcomings, and many data providers are looking for innovative solutions offering a carrier-independent, location-based, and cheaper than 3G data distribution. The deployment of a Delay Tolerant Network (DTN) in a urban scenario seems to be a viable solution to support city-wide information services exploiting public transportation systems as virtual backbones. The majority of DTN routing protocols make use of a multi-copy forwarding approach and are usually designed for campus-wide or rural area communication. The contribution of this paper is an assessment on the scalability limits of popular DTN routing protocols when applied to a real city using its actual public transportation system. We show that multi-copy algorithms do not scale well to a huge area and demonstrate that performance of a probability-based single-copy algorithm degrades more gracefully even with very high levels of traffic.