Adaptive bitrate streaming

Design the client-side adaptive-bitrate (ABR) system for a global VOD service streaming HLS to 200M devices spanning fiber to congested 3G. The goal is to maximize quality-of-experience: high average bitrate, near-zero rebuffering, few quality switches, and fast startup. Design the rendition ladder, the segment/manifest format, and the ABR decision logic, and explain how the player decides what to fetch next under changing throughput.