et sent through the network in Figure 12.19. The packet is forwarded through an MPLS network or domain. When the packet arrives at the first MPLS router, also called the ingress label edge router (ingress LER) of the MPLS domain, the source and destination IP addresses of the packet are analysed and the packet is classified in a forwarding equivalence class (FEC). All packets within the same FEC use the same virtual circuit, called the label switched path or LSP. There are LSPs for both individual and aggregated traffic streams. Separate LSPs are used to serve flows with different QoS requirements. Suppose a virtual circuit has already been established for the FEC of a packet. Then the ingress LER inserts or pushes an MPLS header (L1) on the packet. Subsequent routers of the MPLS domain update the MPLS header by swapping the label L1 against L2, L2 against L3 etc. Finally, the last router of the LSP, called egress LER, removes or pops the MPLS header SO that the packet can be handled by subsequent MPLS-unaware IP routers or hosts. The main benefit of using MPLS in an IP core is optimization of resources through dynamic bandwidth allocation and traffic engineering. Other benefits include scalable IP over ATM support, separation of routing and forwarding and support for VPNs. Understanding Changing Telecommunications - Building a Successful Telecom Business Switch according to label Conversational class, Remove incoming label, What LSP provides such QoS? Insert a new one according to calculated LSP Need for new LSP? (for aggregate traffic) Label switched path, LSP PSTN/ISDN Ex WCDMA RAN 140.20.10.50 Internet Intranets MPLS signalling 140.20.10.20 to set up LSP, Topology based e.g RSVP-TE, routing protocols LDP, CR-LDP e.g. OSPF, OSPF-TE (present on all links in the figure) Figure 12.19 MPLS principles 12.10.1 SIGNALLING AND ROUTING LSPs are set up between MGWs based on topology information provided by routing protocols, such as OSPF or OSPF-TE (TE = traffic engineering) for GMPLS. Each LSP carries traffic from only