A method for operating a passive optical network transmitting sub-frames of data arranged in a common transmission frame in at least two signalling modes from an optical line termination to a plurality of optical network units comprises the step of arranging first sub-frames of data to be transmitted in a first signalling mode which requires lower specifications of the optical network units ahead of second sub-frames of data to be transmitted in a second signalling mode which requires higher specifications of the optical network units. The invention is also realized in an optical line termination which comprises means for performing the method, a passive optical network equipped with such an optical line termination, and a corresponding transmission frame structure.