A transparent optical overlay network (1) for providing end-to-end optical spectrum services over multiple transparent optical network domains (2) is described. The transparent optical overlay network (1) includes network domain interface devices, NDIDs, (3) provided at domain boundaries between adjacent transparent optical network domains (2). The network domain interface device, NDID (3), monitors and adjusts incoming optical signals received by the NDID (3) from a first transparent optical network domain (2-1) and monitors and adjusts outgoing optical signals output by the NDID (3) to an adjacent second transparent optical network domain (2-2). An overlay network controller (5) manages and controls the end-to-end optical spectrum services by controlling the NDIDs (3). The overlay network controller collects telemetry data (TDATA) for optical spectrum service characterization and SLA policing of the optical spectrum services.

The present disclosure provides a data transmission method and apparatus, a network device, a system, and a storage medium. The data transmission method includes: continuously sending downlink data frames of different rates at a first designated wavelength in a time-division multiplexing manner, with the downlink data frames carrying clock recovery information and time slot identification information of the downlink data frames of the different rates; and the clock recovery information is used for an optical network unit to perform clock recovery on the downlink data frames of the different rates, and the time slot identification information is configured to indicate time slot information of the downlink data frames of the different rates at the first designated wavelength.

A system for activating multi-rate optical network units (ONUs) includes an optical network having an optical line terminal (OLT) optically coupled to a plurality of ONUs, including one or more multi-rate ONUs configured to support multiple upstream rates. During discovery when other ONUs are permitted to transmit in the same quiet window, a multi-rate ONU that supports a higher rate is instead configured to send a discovery response at a sufficiently low rate so as not to require training of an equalizer, thereby allowing for a short preamble for the discovery response. Thereafter, during a quiet window for ranging when only the multi-rate ONU will be transmitting to prevent data collisions, the multi-rate ONU is transitioned to a higher rate and transmits a message with a sufficiently long preamble to train an equalizer at the OLT.