This Recommendation proposes a computational model that can be used for assessing the combined effects of network, media stream and terminal device related parameters on perceived quality. This model can be used for the supervision of videotelephony services and can also be used by videotelephony service providers to self-test, so as to improve the QoE and QoS. This model provides estimates of multimedia quality, interaction experience and comprehensive videotelephony quality perceived by users. The application of this Recommendation is limited to QoE/QoS monitoring. Other applications such as quality benchmarking and QoE/QoS planning are outside the scope of this Recommendation.