Cellulars Go Underground: Practical Obstacles and Effective Mitigation

摘要

Underground cellulars are arguably the most sophisticated public network utilities in modern metropolises due to constrained environments and mobility-driven use cases. To date, their QoS has been largely unsatisfactory to users, especially on subway trains where video apps encounter frequent and lasting video stream stalls (VSSes). With the goal of deeply understanding and practically boosting underground cellular QoS, we conduct the first large-scale study of underground cellular networks, spanning 207 subway lines, 82K BSes, and 1.8M Android phones. Our study reveals the unique multi-stage signal attenuation pattern and the defective infrastructure deployment in today's subway cellular environments, which collectively contribute as the major causes of VSSes. To address this, we enable user devices to actively adapt to the environmental changes, and develop a comprehensive radio/link state collection module to effectively inform video apps. Our optimizations on device-side Android cellular management have reduced 39% VSSes in subway networks; our collaborative optimizations with popular video apps have further reduced 23% VSSes.