An ONU comprises: a receiver configured to receive a discovery gate message from an OLT, the discovery gate message comprises a channel assignment field, and a discovery information field, the channel assignment field indicates transmission characteristics for a plurality of channels, and the discovery information field indicates a 25G upstream capability and a 25G discovery window; and a processor coupled to the receiver and configured to process the discovery gate message. A method implemented in an ONU, the method comprises: generating a register request message, the register request message comprises a destination address field, a source address field, a length/type field, an opcode field, and a discovery information field, and the discovery information field indicates a 25G upstream capability and a 25G registration attempt; and transmitting the register request message to an OLT.