This Recommendation provides guidance regarding transmission impairments introduced by digital speech processing systems. The information provided is for use in conjunction with the transmission planning approach described in ITU-T G.107, G.108 and G.109. The Impairment Factor method, used by the E-model of ITU-T G.107, is now recommended. The earlier method that used Quantization Distortion Units is no longer recommended. Updated Impairment Factor values for various digital processing systems are provided in Appendix I. Appendix II contains guidance on how an Advantage Factor can be used to reflect the variation in user expectation of quality for different communications systems (e.g. mobile).