Adaptive bitrate (ABR) is the streaming approach where the player chooses a quality level for each short segment in real time, picking a rung of the bitrate ladder by what the network can sustain. It is the mechanism behind the central QoE trade-off: pick a bitrate higher than the network can carry and the buffer drains to zero and the stream stalls; pick one lower and playback is steady but softer than it needed to be. Two logic families split on what they watch, throughput-based estimation that fits startup with an empty buffer, and buffer-based control that reads buffer occupancy to cut stalls in steady state, and many players hand off from one to the other. The catch for measurement is that ABR trades picture quality against rebuffering every segment, so its effects surface across several metrics at once, average bitrate played, bitrate switching, and the rebuffering ratio, rather than in any single number.
