A communication method, a device, and a system, the method including sending, by an optical line terminal (OLT) of a passive optical network that comprises the OLT and a plurality of optical network units (ONUs), a first downstream frame to the activated ONU using at least one downstream wavelength path, where the OLT and the at least one ONU of the plurality of ONUs communicate with each other using at the least one downstream wavelength path and a plurality of upstream wavelength paths, the plurality of ONUs comprises an activated ONU, wherein the first downstream frame comprises a first path identifier used to identify a first upstream wavelength path, and the first upstream wavelength path is one of the plurality of upstream wavelength paths, and receiving, by the OLT, upstream service data from the activated ONU using the first upstream wavelength path.