  This paper shows that the deployment of an opportunistic network on any
public transportation system leads to obtain a scalable and efficient urban
communication platform. We use the term Bus Switched Networks (BSNs) to
indicate this urban backbone that complements the services of 3G networks and
enables to meet the application level requirements for a large class of
applications by ensuring high delivery ratio and acceptable delays under
different conditions of packet load. We sustain these arguments by providing
three contributions. The first contribution is a novel and lightweight
probabilistic routing protocol for BSN which we prove to be highly effective in
satisfying the loose QoS required by urban-wide delay-tolerant information
services. The second contribution is the proposal of URBeS, an analysis
platform that, given a specific city served by public transportation, produces
real bus mobility traces and traffic analysis for any given routing protocol.
The last contribution is an extensive benchmark analysis on three real cities
which have been selected to explore geo and structural diversity.
