networks exist which are also standardized, including the SS7 signaling (as usual, every new technology enters first into the signaling segment of the telecom networks) as well as the already men- tioned ATM. Introduction to QoS, QoE, and Network Performance With the convergence of the telecom and Internet worlds, the QoS functions and requirements apply not only to traditional telecommunication and broadcast services but also to broadband Internet-based services. Overall, telecommunications/ICT services in the twenty-first century are increasingly being delivered using IP based networks, including: IP-based networks (access networks, core/backbone networks, and transit networks), IP-based services, which include two main types from the QoS viewpoint: QoS-enabled, i.e. managed services, such as voice, TV, and any other service provided by telecom operators with QoS guarantees end-to-end based on a signed agreement between the telecom operator and the customer (in such case the end-user becomes a customer for the operator); OTT services, which are provided in a best-effort manner over Internet access (either fixed or mobile), without end-to-end QoS, and based on the network neu- trality principle being implemented in the Internet (we will refer to network neu- trality in more detail in Chapter 8). 1 Introduction So, QoS is clearly moving from its initial definitions targeted at traditional telecommunication networks (e.g. PSTN/ISDN, broadcast networks) to QoS in IP networks and services. Networks and systems are gradually being designed in consideration of the end-to-end performance required by user applications. In the following subsection we define the QoS, QoE, and network performance (NP). 1.3.1 Quality of Service (QoS) Definition Traditionally, QoS was mainly addressed from the perspective of the end-user being a person (e.g. telephony), with the ability to hear and see and be tolerant of some degra- dations of services (e.g. low packet loss ratio is acceptable for voice, while end-to-end delay for voice shou