Revealing patterns of opportunistic contact durations and intervals for large scale urban vehicular mobility

Opportunistic contact between moving vehicles is one of the key features in vehicular delay tolerant networks (VDTNs) that critically influences the design of routing schemes and the network throughput. Due to prohibitive costs to collect enough realistic contact recodes, to the best of our knowledge, little experiment work has been conducted to study the opportunistic contact patterns in large scale urban vehicular mobility environment. In this work, we carry out an extensive experiment involving tens of thousands of operational taxis in Beijing city. Based on studying this newly collected Beijing trace and the existing Shanghai trace, we find some invariant characteristics of the opportunistic contacts for large scale urban VDTN. Specifically, in terms of contact duration, we find that there exists a characteristic time point, up to which and including at least 80% of the distribution, the contact duration obeys an exponential distribution, while beyond which it decays as a power law one. This property is in sharp contrast to the recent empirical data studies based on human mobility, where the contact duration exhibits a power law distribution. In terms of contact interval, we find that its distribution can be modelled by a three-segmented distribution, and there exists a characteristic time point, up to which the contact interval obeys a power law distribution, while beyond which it decays as an exponential one. Our observations thus reveal fundamental patterns for large scale vehicular mobility, and further provide useful guidelines for the design of new urban VDTN' routing protocols and their performance evaluation.