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We present the design and evaluation of a lightweight mobile S.O.S. system that facilitates ad-hoc communication between first responders and victims in emergency situations. Our approach leverages established protocols and standards in unforeseen ways to provide a platform supporting the creation of short-lived communication links. The system comprises two mobile applications: one victim application that allows the broadcasting of distress signals by a novel use of Wi-Fi SSIDs; and a responder application that allows first responders to discover and trace the people broadcasting the signals. The main difference of our system with other platforms enabling communication in crisis situations is that our system is independent from existing network infrastructure and runs on off-the-shelf, commercially available smartphones. We describe the results of our evaluation process in the context of both a design …