Haoran Wan will present his General Exam "Facilitating the Co-Design of Applications and Cellular Network both Internally and Externally" on May 12, 2025 at 12pm in CS 302. Committee: Kyle Jamieson (adviser), Maria Apostolaki, Wyatt Lloyd Title: Facilitating the Co-Design of Applications and Cellular Network both Internally and Externally Abstract: 5G has emerged as the dominant cellular Radio Access Network (RAN) technology, serving as a critical last-mile solution for modern connectivity. Building on significant innovations in the physical (PHY) and medium access control (MAC) layers over LTE, 5G promises substantially higher throughput and lower latency. However, wireless networks suffer from highly variable end-to-end throughput due to user movement, channel fading and interference. Compounding this issue, the base station keeps deep packet buffers for retransmissions to mitigate packet loss caused by the varying channels, which paradoxically causes the high end-to-end latency. The variable throughput and hidden buffer pose performance challenges for many applications, ranging from congestion control for bulk downloading to interactive videos. And the closed nature of the RAN inhibits co-design of applications and the cellular network. To bridge the gap and facilitate the co-design of applications and the cellular network, we worked from two perspectives: 1) Externally, we developed a non-intrusive telemetry tool called NR-Scope to decode the control information passively from the raw radio signals, providing the MAC/PHY throughput of the base station in sub-millisecond level granularity; 2) Internally, we designed a base station module called L4Span that predicts the egress rate of the hidden queue and exposes this information through explicit congestion notification (ECN) bits in the IP header. Our work paves the way for co-designing future cellular systems and applications to fully leverage 5G’s capabilities.