 control (MAC) addresses in Ethernet frames (at the UNI) when they enter the PBB network to transfer from one provider's network to 3.4 QoS Architectures for NGN another provider's network via a backbone network. A PBB uses an additional tag, called an I-tag (it has a length of 24 bits), to identify the services (i.e. Q-in-Q net- works), thus separating the PBB network (as a backbone Ethernet network) and ser- vice providers' networks. Besides the EVC-based services, the typical approach for providing Ethernet end-to-end connectivity is by using VPN. The VPN provides multipoint-to-multipoint Ethernet service that can span multiple metro Ethernet areas by providing connectivity between such multiple sites as if they were attached to the same Ethernet LAN. Simply, VPN encapsulates Ethernet frames (i.e. MAC frames) into IP packets and tunnels them from one Ethernet LAN or metro Ethernet network on one site to all other sites (and other VPN tunnels are established for the traffic in the opposite direction). With this approach, when a telecom operator uses MPLS in its transport network, VPN goes over MPLS [19]. The reference model for using VPNs for the end-to-end Ethernet service is shown in Figure 3.9. 3.4.2.1 QoS Services in Ethernet-based NGN Ethernet QoS services for NGN include premium service, gold service, and best-effort service. They are created according to the traffic parameters, provider edge rules, trans- fer capability (regarding the bandwidth for the service), and ITU QoS class for IP-based services. Providing QoS for Ethernet-based NGN requires connection admission control (CAC); however, this is operator specific [15]. Once a given end-to-end connection (over Ethernet-based NGN) has been accepted, the network operator's policies VLAN QoS MPLS CoS VLAN QoS Site 3 E-LAN/ E-LAN/ Site 4 E-Tree E-Tree tunnel Site 2 Site 5 E-Line E-Line Site 1 tunnel Site 6 CoS - Class of service QoS - Quality of service Provider backbone MEN - Metro ethernet network bridges (PBB) MPLS - Multi protocol label switching netw