vi- sion). Such services have strict requirements regarding QoS (guaranteed bitrates, losses, delay, delay variation, jitter, etc.). So, NGN provides a standardized implementation of QoS (instead of proprietary case-by-case implementations) [9], which is mandatory for the transition from PSTN and public land mobile network (PLMN) to all-IP networks. So, the telecommunication world is transiting from circuit-switched networks (e.g. PSTN/ISDN) to all-IP networks, including fixed and mobile access networks, as well as core and transit networks. First, the transition was completed in transit and core networks, then in fixed access networks, and lastly in mobile access networks (due to mobility of users, which makes the continuous QoS provisioning more complex). When the transition to all-IP-based networks/services is completed in the telecommunica- tions/ICT world, the Internet technologies are also used for traffic management and for QoS standardization, monitoring, and enforcement. The transition from separate 1.3 Introduction to QoS, QoE, and Network Performance Applications (e.g. VolP, IPTV, www, P2P, OTT applications, etc.) Internet networking (TCP/IP, UDP/IP) Transport (fixed/mobile/wireless access networks, transport networks) IPTV - IP television P2P - Peer-to-peer OTT - Over-the-top VolP - Voice over IP www - World Wide Web Figure 1.3 Transition from separate networks for different services (the traditional telecom approach) to horizontal separation of services/applications provided via broadband IP networks (the new way). networks for different services (traditional telecom approach) to a horizontal layered approach in an all-IP environment with broadband access is illustrated in Figure 1.3. Once the path to packet-switching in the telecom world had been well established by the start of the 2000s, the ITU defined an architectural framework for the support of QoS in packet-switched networks. Nowadays, although there are unified packet-switched networks, the IP-based networks, other different packet-switched 