An Empirical Study of Vertical Handover Behavior on Android During Mobility and a Kernel-Aware Proactive Connectivity Manager

Abstract

Mobile users spend much of their time in environments where Wi-Fi and cellular networks coexist, such as campuses and homes, and they frequently move across coverage boundaries. Intelligent vertical handover – the switch between Wi-Fi and cellular – is critical in these environments to maintain seamless connectivity, especially for latency-sensitive applications such as video calls and live-streaming. Today's mobile operating systems rely on simple physical-layer thresholds – primarily the Wi-Fi Received Signal Strength Indicator (RSSI) – to trigger vertical handovers, which can lead to poor decisions on real devices. Whether these decisions actually serve applications well is an empirical question that can only be answered through direct measurement. In this thesis, we conduct a large-scale measurement study of a campus wireless environment, collecting 43 hours of concurrent Wi-Fi and cellular network performance from a commodity Android phone. We find that the chosen default network matches the better-performing interface only about half the time under an application-quality oracle, and that the majority of vertical handovers are short round-trips back to Wi-Fi within 30 seconds – a "ping-pong" pattern that often returns to a different Wi-Fi access point than the one the device just left. RSSI, the dominant input to the current selection logic, is a weak discriminator for these decisions. Instead, we identify a kernel-maintained MAC-layer counter – the rate of unacknowledged transmissions on the Wi-Fi link – that reliably anticipates impending Wi-Fi degradation roughly ten seconds in advance. We propose a kernel-aware proactive handover strategy built on this signal. It pre-warms the cellular radio before Wi-Fi fails, scans for an alternative Wi-Fi access point before falling back to cellular, and re-checks the underlying link a few seconds later before committing to the switch. The strategy reduces handover-related packet loss by 23% and substantially reduces the number of unnecessary handovers. Since this signal is already collected by every Android device, deployment requires only minimal changes to the Android Open Source Project.