Modulation formats in optical networks

A method for communicating between an optical line terminal and optical network units in a passive optical network, wherein downstream transmission from the optical line terminal to the optical network units is organized into frames including segments, wherein a respective segment is modulated according to at least a first modulation format or a second modulation format, and wherein the method comprises:

• sending, by the optical line terminal to at least one optical network unit, a request to estimate a channel quality based on at least one reference segment modulated according to the second modulation format, wherein the at least one optical network unit operates according to the first modulation format, wherein the at least one reference segment is located at one or more selected positions within downstream transmission frames, and wherein the downstream transmission frames further include segments modulated according to the first modulation format;
• sending, by the at least one optical network unit to the optical line terminal, a response including an indication of the estimated channel quality to the optical line terminal;
• and
• deciding, by the optical line terminal, to switch the segments in the downstream transmission assigned to the at least one optical network unit to the second modulation format based on the response.