  Design of clock synchronization for networked nodes faces a fundamental
trade-off between synchronization accuracy and universality for heterogeneous
platforms, because a high synchronization accuracy generally requires
platform-dependent hardware-level network packet timestamping. This paper
presents TouchSync, a new indoor clock synchronization approach for wearables
that achieves millisecond accuracy while preserving universality in that it
uses standard system calls only, such as reading system clock, sampling
sensors, and sending/receiving network messages. The design of TouchSync is
driven by a key finding from our extensive measurements that the skin electric
potentials (SEPs) induced by powerline radiation are salient, periodic, and
synchronous on a same wearer and even across different wearers. TouchSync
integrates the SEP signal into the universal principle of Network Time Protocol
and solves an integer ambiguity problem by fusing the ambiguous results in
multiple synchronization rounds to conclude an accurate clock offset between
two synchronizing wearables. With our shared code, TouchSync can be readily
integrated into any wearable applications. Extensive evaluation based on our
Arduino and TinyOS implementations shows that TouchSync's synchronization
errors are below 3 and 7 milliseconds on the same wearer and between two
wearers 10 kilometers apart, respectively.
