or delivered by the service provider, and for purposes of comparison it should be expressed through the same QoS parameters as the QoS offered to the cus- tomer (e.g. specified in the SLA). This QoS viewpoint can be used by the regulator for the purposes of QoS regulation, including publication of the results from QoS audit in the telecom operators' networks and then QoS encouragement or enforcement (when and where needed). Customer QoS perception. This is the QoS level experienced by the customer, typi- cally obtained from user ratings of the QoS provided by the service operator. This is also a customer viewpoint, SO it is not expressed in technical terms but in terms of degrees of satisfaction (e.g. from "not satisfied" up to "very satisfied"). For example, a customer may rate the service on a 5-point scale, with grade 1 being the worst and grade 5 being the best experience with the service. The perceived QoS can be used by service providers or regulators to determine the customer's satisfaction, which may further lead to corrective actions by providers or regulators (e.g. in situations when there is a significant mismatch between the perceived QoS and the QoS offered by the provider). 1.4 ITU's QoS Framework 1.4.3 Four-Market Model The four-market model is created for multimedia services [13], such as web browsing or video streaming. It defines the chain of actions going from content creation on one side, to service provision (e.g. by media servers of the service provider), service transport (via interconnected telecommunication networks, including access, core, and transit net- works) and finally customers' equipment (personal computer, smartphone, etc.). These four elements can be provided independently of each other, and in such case differ- ent parties may be in charge for installation, operation, and maintenance of each of them. For example, a movie studio may produce the content, a video streaming ser- vice (e.g. YouTube, Netflix) may provide the service, the networks of telecommunication operators will 