Recommendation ITU-T K.131 describes the principles and design methods for soft error measures for the equipment that composes carrier telecommunications networks. It also describes basic configurations of telecommunication equipment, definitions and methods to determine reliability requirements and procedures for the design of equipment from the perspective of mitigation of failures caused by soft errors. Also included are the methods to determine the areas, e.g., circuit blocks or circuit packs, requiring soft error measures in telecommunication equipment in order to conform to the reliability requirements. The main design issues to be considered for soft error measures are described as well as the actual design methods for the application of measures against soft errors and their effects. Finally, the reliability evaluation methods using theoretical calculations and tests of actual equipment are described to confirm the effect of the applied measures and conformity to the reliability requirements.