or operation and maintenance of the network equipment) and hence puts predefined routes in the network and traffic is aggregated (e.g. in VPNs) and after that is routed (or switched, in the case of MPLS or Ethernet-based NGN) through the network. So, in practice we have pre-established tun- nels (over IP networks) or paths (over MPLS-based transport networks) between pairs of nodes in each core and transport network of the telecom operators. 3.4.3 Multi-Protocol Label Switching (MPLS) The main transport network technology from the beginning of the twenty-first century has been IP/MPLS in different variants (or shortly MPLS). NGN is the definitive stan- dardized approach for QoS provision which is based on the RACF between the service and transport stratums (to transfer service requests from the service stratum into con- crete traffic management policies and triggered mechanisms in networks nodes in the transport stratum). A combination of MPLS and NGN (and with it the RACF) results in two main architectures: 3 QoS in NGN and Future Networks Centralized RACF architecture [20]: In this case there is a single centralized RACF in the core network, which typically consists of PD-FE and TRC-FE. TRC-FE mon- itors all traffic in the MPLS core network, while admission control is carried by the PD-FE. The flows are mapped into LSPs at the edge of the MPLS network. Bandwidth reservation in LSPs or tunnels is performed in two modes: Static mode: reservation of predetermined bandwidth associated with a given LSP or tunnel. Dynamic mode: aggregate bandwidth in a given LSP is adjusted with each new admitted or released media flow. Distributed RACF architecture [21]: In this case the two RACF functional entities are separated. There is a single centralized PD-FE and many distributed TRC-FEs at the edges of the MPLS core network. Each flow is mapped into an LSP or a tunnel in the same manner as in centralized RACF architecture. Overall, one may note that the NGN puts the MPLS in a standardized QoS framework within the transport s