e IP traffic add more delay (each AS adds its own delay to the transferred IP packets) and with that a higher probability of bottlenecks appearing somewhere on the path between the two endpoints of a given call/session. However, the number of ASs that the IP traffic passes end-to-end remains almost constant, although the total number of active ASs increases either exponentially or linearly over time. Of course, this positively influences QoS (as an end-to-end feature) and shows that the Internet has good scalability on a global scale. So, regarding QoS from the end-to-end point of view, the extreme case will be a scenario where IP networks have different traffic management mechanisms, different levels of performance control, and different QoS provision (e.g. telecom operator 1 uses IntServ, backbone provider 1 uses DiffServ, backbone provider 2 uses MPLS traffic engi- neering, telecom operator 2 uses bandwidth over-provisioning, as given in Figure 4.10) [34]. In such a case it is hard to provide services with guaranteed end-to-end QoS level when each of the networks (and network sections) on the end-to-end path uses a differ- ent QoS mechanism. The basic network model introduced in Chapter 1 of this book (which consists of access, backbone, and transit networks) should be applied commonly by network providers, but the technologies that compose such a network model would be different depending on the providers and their geographic regions or countries (Figure 4.10). Additionally, differences exist between national regulatory frameworks regarding addressing SLAs between service providers (typically national telecom operators) and customers (typically residential and business customers) and accompanied QoS and QoE frameworks. Overall, end-to-end network design is essential in order to provide end-to-end QoS for both fixed and mobile access networks, which however requires QoS to be part of the peering or transit agreements between the telecom operators and the transit network providers. Certain QoS issues at interco