  We present PCach, a smartphone-based approach for relieving the congestion in
cellular network resulting from the exponential growth in mobile data traffic.
The basic idea underlying PCach is simple: use WiFi to proactively cache
content on the smartphone's memory, which otherwise would have been delivered
through the cellular network. However, it leads to several challenging
questions, including how much mobile data actually flows through cellular
networks, how much data can be pre-cached, and when and what to pre-cache. We
address these questions progressively using a thorough analysis of user data
collected from our purpose-built crowdsensing Android application, actively
utilized by 45 users for periods dating back to July 2014. Our analysis shows
that the median smartphone user transfers 15% of their data via the cellular
network and that 80\% of it can be pre-cached via WiFi. To capitalize on these
results, we draw on a careful analysis of the measurement data to introduce an
algorithm that can run stand-alone on off-the-shelf smartphones and predict
with good accuracy when and what to pre-cache.
