The complexity and enormous costs of installing new long-haul fiber-optic infrastructure has led to a significant amount of infrastructure sharing in previously installed conduits. In this paper, we study the characteristics and implications of infrastructure sharing by analyzing the long-haul fiber-optic network in the US. We start by using fiber maps provided by tier-1 ISPs and major cable providers to construct a map of the long-haul US fiber-optic infrastructure. We also rely on previously under-utilized data sources in the form of public records from federal, state, and municipal agencies to improve the fidelity of our map. We quantify the resulting map's connectivity characteristics and confirm a clear correspondence between long-haul fiber-optic, roadway, and railway infrastructures. Next, we examine the prevalence of high-risk links by mapping end-to-end paths resulting from large-scale traceroute campaigns onto our fiber-optic infrastructure map. We show how both risk and latency (i.e., propagation delay) can be reduced by deploying new links along previously unused transportation corridors and rights-of-way. In particular, focusing on a subset of high-risk links is sufficient to improve the overall robustness of the network to failures. Finally, we discuss the implications of our findings on issues related to performance, net neutrality, and policy decision-making.
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