 a replacement for PSTN/PLMN telephony, and data service which is in fact Internet access service). One may add IPTV as a service; however, this 2 Internet QoS is typically packetized (this refers to transformation of digital TV into an IPTV stream based on Internet technologies) inside the telecom operator's own network. Also, there are services offered to business customers via VPNs as a replacement for leased lines from the PSTN era. However, the number of QoS classes is not a constant value for every network (QoS classes will be discussed in more detail in the following chapters). In different networks the traffic can be grouped in different numbers of QoS classes, depending upon operators' service offerings. That creates a slightly different picture: consider QoS for a network of a single network provider and for the interconnection between providers. 2.10.1 Basic DiffServ Model for a Single Provider In the case of a single network provider for the whole path of a connection (session/call), customer sites connect to the provider network via CE (customer edge) devices [38]. The provider's routers which connect to customer sites are referred to as PE (provider edge) devices (or nodes). The CE router can therefore prioritize and police the traffic based on the DiffServ concept (classifying and marking the traffic at the CE toward the PE, and vice versa from PE to CE), SO different classes will be treated differently on the link (or network segment) between CE and PE on both ends of the connection. This basic DiffServ model for a single network provider is shown in Figure 2.17. 2.10.2 Basic DiffServ Inter-Provider Model In this section the DiffServ model is extended to a simple inter-provider scenario, as shown in Figure 2.18. In general, there can be multiple network providers on the path of the user information transfer between two endpoints (two CEs). However, the simple inter-provider scenario is to have just two providers in the path between the two sites of each customer, which are interconnected via (as a