The digital diagnosis function is a cost-effective performance monitoring method, which can monitor important performance parameters such as optical transceiver transmit optical power, receive optical power, temperature, supply voltage, laser bias current and warning information. With the digital diagnosis function, the network management unit can access the optical transceiver module through the two-wire serial bus for parameter monitoring, and finally can quickly find the fault location in the optical fiber link, simplify maintenance work, and improve system reliability.