A method of communication, comprising: sending, by an optical line terminal, OLT, to an optical network unit, ONU, a discovery window message for registration through a specific downstream wavelength; receiving, by the OLT, a registration request from the ONU through a specific upstream wavelength; assigning, by the OLT, out of a plurality of normal service upstream wavelengths and a plurality of normal service downstream wavelengths, a normal service upstream wavelength and a normal service downstream wavelength to the ONU; sending, by the OLT, through the specific downstream wavelength to the ONU, a confirmation that the OLT has received the registration request, wherein the specific downstream and upstream wavelengths are reserved for a registration process that includes receiving, through the specific upstream wavelength, the registration request and sending, through the specific downstream wavelength, the discovery window message and information regarding the normal service upstream wavelength and the normal service downstream wavelength; initially reserving the specific downstream wavelength and the specific upstream wavelength for the registration process; and assigning, after assigning all of the plurality of normal service upstream wavelengths to ONUs, the specific upstream wavelength as a normal service upstream wavelength, or assigning, after assigning all of the plurality of normal service downstream wavelengths to ONUs, the specific downstream wavelength as a normal service downstream wavelength, or assigning, after assigning all of the plurality of normal service upstream wavelengths and normal service downstream wavelengths to ONUs, the specific upstream wavelength and downstream wavelength as normal service upstream wavelength and normal service downstream wavelength. sending, by an optical line terminal, OLT, to an optical network unit, ONU, a discovery window message for registration through a specific downstream wavelength; receiving, by the OLT, a registration request from the ONU through a specific upstream wavelength; assigning, by the OLT, out of a plurality of normal service upstream wavelengths and a plurality of normal service downstream wavelengths, a normal service upstream wavelength and a normal service downstream wavelength to the ONU; sending, by the OLT, through the specific downstream wavelength to the ONU, a confirmation that the OLT has received the registration request, wherein the specific downstream and upstream wavelengths are reserved for a registration process that includes receiving, through the specific upstream wavelength, the registration request and sending, through the specific downstream wavelength, the discovery window message and information regarding the normal service upstream wavelength and the normal service downstream wavelength; initially reserving the specific downstream wavelength and the specific upstream wavelength for the registration process; and assigning, after assigning all of the plurality of normal service upstream wavelengths to ONUs, the specific upstream wavelength as a normal service upstream wavelength, or assigning, after assigning all of the plurality of normal service downstream wavelengths to ONUs, the specific downstream wavelength as a normal service downstream wavelength, or assigning, after assigning all of the plurality of normal service upstream wavelengths and normal service downstream wavelengths to ONUs, the specific upstream wavelength and downstream wavelength as normal service upstream wavelength and normal service downstream wavelength.