  Study on human mobility is gaining increasing attention from the research
community with its multiple applications to use in mobile networks,
particularly for the purpose of message delivery in the Delay Tolerant
Networks. To better understand the potential of mobile nodes as message relays,
our study investigates the encounter pattern of mobile devices. Specifically,
we examine the extensive network traces that reflect mobility of communication
devices. We analyze the periodicity in encounter pattern by using power
spectral analysis. Strong periodicity was observed among rarely encountering
mobile nodes while the periodicity was weaker among frequently encountering
nodes. Further, we present a method to search regularly encountering pairs and
discuss the findings. To our knowledge, we are the first to analyze the
periodicity of encounter pattern with large network traces, which is a critical
basis for designing an efficient delivery scheme using mobile nodes.
