capabilities of the host or router regarding the processing power of the machine, operational memory, and optimization of the applications and os as well as os and network interfaces in use for communication. The horizontal model reflects the network architecture, the positioning of network nodes (routers, gateways), the nodes' functionalities (e.g. in which nodes and where the signaling functions are implemented, for example in mobile networks), and server hosts (data centers, CDNs), and their connection (e.g. connection of the CDN to a certain network provider) and interconnection (each IP network must be connected to other neighboring IP networks, which may belong to different telecom operators and also to different countries and regions around the globe). 4.5.1 End-to-End Network Performance Parameters for IP-based Services The transition from PSTN/ISDN to all-IP networks and Internet technologies-based services which run on the application layer over TCP/IP or UDP/IP protocol stack requires definition of end-to-end network performance parameters. Such parameters are defined by the ITU (performance parameters are also being defined by other organizations, but we will focus on those from the ITU as the largest organization in the world for standardization of ICT/telecommunications), to be used in specifying and assessing IP network performance. In that regard, the most important performance parameters are the following [33]: IPTD (IP packet transfer delay). This is calculated over all successful and errored pack- ets and refers to time difference between the occurrences of two corresponding IP packet reference events (an IP packet reference event is a packet transmission via a given measurement point in the network). There are several possible definitions or uses of this parameter. Minimum IPTD is the smallest IP packet delay, median IPTD refers to 50th percentile of the frequency distribution of IPTDs, while average IPTD is calculated as an average of all measured IPTDs. IPDV (IP packet delay variation, also 