Recommendation ITU-T P.311 provides audio performance requirements for wideband (8000 Hz) handset and headset telephones. Requirements and test methods are specified for the major audio transmission parameters affecting wideband audio, including sending and receiving levels, frequency response, noise, distortion, sidetone, stability, echo path and delay. Wideband audio represents a considerable departure from traditional narrowband telephony, offering significantly improved quality. Major changes over the previous version of this Recommendation (2005) are as follows: - The structure of the Recommendation is modified, the objective measurement methods originally in Annex A have been transferred into the main body. - In addition to sinusoidal and noise signals, speech-like stimulus signals as described in ITU-T P.50 and ITU-T P.501 are also recommended for the measurements. - HATS method is used for better simulation of real telephone communication. - Specific headset requirements are incorporated. - Some technical requirements and related test methods have been modified based on the latest research, e.g. frequency response, distortion, STMR, D-factor, and delay.