Measure of underlying technical aspects or characteristics of media streaming (ITU.T P.10 item 6.212), including: performance, availability. Poor QoS can impact PVQ and/or QoE. In contrast to QoE, QoS is an expression of the system's various technical characteristics including: network performance (e.g. packet loss, e.g. network jitter, e.g. bitrate, e.g. throughput); system availabillity (e.g. server uptime); playout characteristics (e.g. Time To First Byte, e.g. Time To First Frame, e.g. Startup Time, e.g. Video Stall); etc. QoS can impact PVQ, e.g. packet loss causes video macroblocking (see Perceptual Video Quality), and more generally, can impact QoE, e.g. loss of bitrate due to a failed network link which causes end-viewers to not be able to log in and authenticate to an OTT service (see Quality of Experience).