o a service on an internal network. Proxy servers are also becoming critical in support of edge caching of content. People are con- stantly becoming less tolerant of lengthy downloads, and information streams (such as video, audio, and multimedia) are becoming more demanding of timely delivery. Chapter 8 The Internet and IP Infrastructures Say you want to minimize the number of hops that a user has to go through. You could do this by using a tracing product to see how many hops it takes to reach a Web site. You'll see that sometimes you need to go through 17 or 18 hops to get to a site. Because the delay at each hop can be more than 2,000 milliseconds, if you have to make 18 hops when you're trying to use a streaming media tutorial, you will not be satisfied. ISPs can also use proxy servers to cache content locally, which means the informa- tion is distributed over 1 hop rather than over multiple hops, and that greatly improves the streaming media experience. Not all proxy servers support caching, however. Access router-An access router filters local traffic. If a user is simply checking e-mail, working on a Web site, or looking up newsgroups, there's no reason for the user to be sent out over the Internet and then brought back to this particular POP. An access router keeps traffic contained locally in such situations. Distribution router-A distribution router determines the optimum path to get to the next hop that will bring you one step closer to the destination URL, if it is outside the POP from which you are being served. Typically, in a higher-level ISP, this distribution router will connect to an ATM switch. ATM switch-An ATM switch enables the ISP to guarantee QoS, which is especially necessary for supporting larger customers on high-speed inter- faces and links and for supporting VPNs, VoIP, or streaming media applica- tions. The ATM switch, by virtue of its QoS characteristics, enables us to map the packets into the appropriate cells, which guarantee that the proper QoS is administered and delivered. Typ