 lower-tier providers focus on rock-bottom pricing. There are therefore large variations in terms of the capacity available, the performance you can expect, the topology of the network, the levels of redundancy, the numbers of connections with other operators, and the level of cus- tomer service and the extent of its availability (i.e., whether it is a 24/7 or a Monday- through-Friday, 9-to-5 type of operation). Ultimately, of course, ISPs vary greatly in terms of price. Figure 8.21 shows an idealized model of the service provider hierarchy. At the top of the heap are research backbones. For example, Internet 2 replaces what the original Internet was for-the academic network. Some 85% of traffic within the academic domain stays within the academic domain, SO there's good reason to have a separate backbone for the universities and educational institutions involved in research and learning. Internet 2 will, over time, contribute to the next commercial- ized platform. It acts as a testbed for many of the latest and greatest technologies, SO the universities stress-test Internet 2 to determine how applications perform and which technologies suit which applications or management purposes best. Other very sophisticated technology platforms exist, such as the Abilene Project and the Interplanetary Internet (IPN), which are discussed later in this chapter. Research Backbones Internet 2, Abilene Project, Interplanetary Internet GigaPops California NAP Chicago NAP Wash., DC NAP New York NAP PacBell Ameritech MCI Worldcom Sprint Network Service Cable SprintLink UUNet Providers (NSPs) & Wireless WorldNet NAPs, MAE East IXPs, EPs Worldcom London Hong Kong Sydney Korea Internet Service Top-Tier Top-Tier Top-Tier Top-Tier Top-Tier Providers (ISPs) Lower-Tier ISPs Figure 8.21 Service provider hierarchy The Organization of the Internet In the commercial realm, the highest tier is the NSP. NSPs are very large back- bones, global carriers that own their infrastructures. The NSPs can be broken down into three major subsets: National