ed techniques such as Fibre Channel over IP or Small Computer System Interface (SCSI) over IP. This trend may ultimately replace the reigning Fibre Channel. Backbone WAN edge Access network Provider core router Enterprise network Provider edge PE or Access edge router Customer edge CE router e.g. Gigabit Ethernet servers, etc. e.g. Ethernet workstations, PC's, etc. FIGURE 3.1 The conceptual hierarchy of networks. 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 PACKET PROCESSING 45 Highly specialized and dedicated gateways are also usually provided on the landscape to bridge the locally connected systems to the rest of the world, depending on the applications and policies. For instance, telephony gateways translate the evolving voice over IP (VoIP) or video over IP realm back and forth and to and from Public Switched Telephone Network (PSTN) signaling and traffic. Firewalls and routers handle the normal traffic that enters and exits a site. At some point, some of the LAN- based systems require legitimate access to the rest of the world. This could require connectivity with other companies, sites of the same enterprise, suppliers, customers, partners, or classical web access for an organization's members. Routers usually handle this access. Because these routers are situated at the periphery of the enter- prise network, they are called edge routers (probably inappropriately as we will explain later). Edge routers are different from the other routers that operate in the heart of the enterprise network, which have different requirements for protocol support, port speeds, and SO on. These routers are often called core routers. Edge and core routers should not be confused with one another and should be used appropriately in different contexts. The next layer in the global network hierarchy is the access network, which is also kno