ayering Model IP/MPLS Internet Access/Serving Gateways Fiber Access/Serving Gateways PDN-GW (IP IP-Transit Backbone) Access/Serving Gateways Access/Serving Gateways 2G/3G Metro Networks BACKBONE NETWORK Figure 7.24 Backbone core transport network 7.5 Backbone Transport Network The backbone or core transport network considered in this book comprises of connecting intercity MSCs and regional SGSNs to each other. The MSCs are then connected to PSTN while SGSNs are connected to GGSN which connects to the outside world via IP-Transit providers. With respective to EPS (and WiMAX) it would simply mean connecting intercity Serving Gateways (Access Gateways) to a common PDN-GW (IP backbone) which then connects to an IP-Transit provider respectively or to the carrier's own IMS (Figure 7.24). The common transmission techniques that are used in the core transport network are IP/MPLS and DWDM over Optical Fiber. The other method includes the use of leased lines that provide STM1 or higher connectivity for carrying inter-city traffic. 7.5.1 MPLS Multi Protocol Label Switching (MPLS) defined in RFC 3031 [38] is a data-carrying mechanism that belongs to the family of packet-switched networks. It is a protocol-agnostic data-carrying mechanism; data packets are assigned labels and packet forwarding is based on the label without the need to examine the packet content. MPLS operates at an OSI Model layer that is generally considered to lie between traditional definitions of Layer 2 (Data Link Layer) and Layer 3 (Network Layer), and thus is often referred to as a "Layer 2.5" protocol. It was designed to provide a unified data-carrying service for both circuit-based clients and packet-switching clients. It can be used to carry many different kinds of traffic, including IP packets, as well as native ATM, SONET, and Ethernet frames. IP/MPLS is a consolidated and mature carrier-grade transport technology that offers quality of service, service management, scalability, and rapid recovery from failure and used extensively in the backbone t