w-Hill (www.digitalengineeringlibrary.com Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Chapter 7 MPLS delivers QoS by establishing virtual circuits known as label switched paths (LSPs) which, in turn, are built around traffic-specific QoS requirements. An MPLS network, such as that shown in Figure 7-41, comprises label switch routers (LSRs), at the core of the network, and label edge routers (LERs), at the edge. It is the responsibility of the LERS to set QoS requirements and pass them on to the LSRs, which are responsible for ensuring that the required levels of QoS are achieved. Thus, a router can establish LSPs with explicit QoS capabilities and route packets to those LSPs as required, guaranteeing the delay that a particular flow encounters on an end-to-end basis. It's interesting to note that some industry analysts have compared MPLS LSPs to the trunks established in the voice environment. MPLS uses a two-part process for traffic differentiation and routing. First, it divides the packets into forwarding equivalence classes (FECs) based on their quality of service requirements, and then maps the FECs to their next hop point. This process is performed at the point of ingress at the edge of the network. Each FEC is given a fixed-length "label" that accompanies each packet from hop to hop; at each router, the FEC label is examined and used to route the packet to the next hop point, where it is assigned a new label. MPLS is a "shim" protocol that works closely with IP to help it deliver on QoS guarantees. Its implementation will allow for the eventual dis- missal of ATM as a required layer in the multimedia network protocol stack. And while it offers a promising solution, its widespread deploy- Figure 7-41 Schematic representation of an MPLS network showing LERS and LSRs: The LERS are at the periphery of the network; the LSRS are in the core. Downloaded from Digital Engineering Library @ McGraw-Hill (www.dig