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There are no requirements on the viewer side to enable ABR streaming. As long as you have multi-bitrate on the publisher, the viewers will automatically connect to the highest quality layer that is sustainable for the network connection. The viewers will also switch between quality layers as needed throughout the duration of their streams.
Given that Phenix real-time streaming is not chunk based, but rather a packet-based frame-by-frame approach, Phenix is able to dynamically generate keyframe requests to switch quality levels for each viewer.