Praise for Fundamentals of WiMAX

232 Chapter 7 • Networking and Services Aspects of Broadband WirelessMPLS DomainFIBLER: Label-Edge RouterFIB: Forward Informaton BaseLSR: Label Switching RouterFIBLERFIBLSRFIBLSRFIBLERMPLS edge ingressrouter adds label.LSRMPLS edge egressrouter removes label.Layer 2 Header "Shim" IP Header PayloadLabel Value Exp S TTL0 20 23 2432Figure 7.3 MPLS network and componentslabel switching routers (LSR), maintain a forward information base (FIB) and forward MPLSpackets by looking up the next hop in the FIB. It should be noted that the “label” has only localsignificance and that it is replaced by the LSR with a new label as packets are forwarded fromone node to another along the LSP. This is similar to the virtual path identifier/virtual circuitidentifier (VPI/VCI) concept used in ATM. In fact, an ATM switch could be used as an LSR,where the label is simply the VPI/VCI. The label swapping and forwarding continue until thepacket reaches the egress-edge router, where the label is deleted before forwarding to a non-MPLS node.By having predetermined paths, MPLS speeds up the forwarding process, albeit at the costof additional processing at the edge router that converts IP packets to MPLS packets. MPLS canalso be used to alleviate congestion through traffic engineering (TE). Unlike traditional IP networksthat route traffic automatically through the shortest path, MPLS can route traffic throughengineered paths that balance the load of various links, routers, and nodes in the network. Alongwith such signaling protocols as RSVP-TE or LDP-CR, 6 it is possible in an MPLS network tocompute paths with a variety of constraints and to reserve resources accordingly. Dynamic trafficmanagement allows the network to operate closer to its peak efficiency. It is also possible toengineer paths for specific applications—for example, to set up dedicated circuits by configuringpermanent LSPs for voice traffic or for virtual private network (VPN) applications.6. RSVP-TE is an extension of RSVP for use in traffic engineering applications. LDP-CR stands forLabel Distribution Protocol-constraint-based routing.