hy [SONET/SDH] and Plesiochronous Digital Hierarchy [PDH]). The major innovation was that equipment designed for metro networks needed to be able to handle data traffic. As such, the core of the WAN still contains several Asynchronous Transfer Mode (ATM) switches. Although a move is being made toward handling fast- switched IP traffic on the WAN, the core and edge switches must be able to handle multiple protocols at wire speed, often including time division multiplexing (TDM) traffic and frame relay. IP traffic is still being transmitted as IP over ATM during this transitional era, while backbone service providers are adopting newer technologies to use on the more modern optical networks, such as terabit routers offered by companies such as Avici, Cisco, and Juniper. To better depict the market situation of several companies competing from completely different angles of new data-driven business while supporting lucrative legacy business, it is more customary to use the four-layer approach shown in Figure 3.2. In this example, the WAN has been broken into the edge and core network, and the term metropolitan network denotes the combination of the access and edge networks. The terms edge and core are used correctly in Figure 3.2. Downloaded from Digital Engineering Library @ McGraw-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. PACKET PROCESSING 46 NETWORK PROCESSORS such as IXC carriers offering transport services such as ISPs or ILECs carrying out definition of services and aggregation of traffic Metro Network such as ILECs/CLECs/BLECs/DLECs Access offering points of convergence for voice and data traffic access, and aggregation of subscribers Access Service Termination occurs at such as campus, enterprise, SME, or residential customers FIGURE 3.2 Four-layer model of network reality. When examining the requirements of the switching equipment at these various levels of function- ality, one sho