We saw earlier that end systems (user PCs, PDA's, Web servers, mail servers, and so on) connect into the Internet via an access network. Recall that the access network may be a wired or wireless local area network (for example, in a company, school, or library), a residential cable modem or DSL network, or a residential ISP (for example. AOL or MSN) that is reached via dial-up modem. But connecting end users and content providers into access networks -is only a small piece of solving the puzzle of connecting the hundreds of millions of end-systems and hundred of thousands of networks that make up the Internet. The Internet is a network of networks- understanding this phrase is the key to solving this puzzle. In the public Internet, access networks situated at the edge of the Internet are connected to the rest of the Internet through a tiered hierarchy of ISPs. Access ISPs (for example, residential cable and DSL networks, dial-up access networks such as AOL, wireless access networks, and company and university ISPs using LANs) are at the bottom of this hierarchy. At the very top of the hierarchy is a relatively small number of so-called tier-1 ISPs. In many ways, a tier-1 ISP is the same as any network-it has links and routers and is connected to other networks. In other ways, however, tier-I ISPs are special. Their link speeds are often 622 Mbps or higher, with the larger tier-I ISPs having links in the 2.5 to 10Gbps range; their routers must consequently be able to forward packets at extremely high rates. Tier-I ISPs are also characterized by being: * Directly connected to each of the other tier-1 ISPs * Connected to a large number of tier-2 lSPs and other customer networks * International in coverage Tier-l ISPs are also known as Internet backbone networks. These include Sprint, Verizon, (previously UUNet/WorldCom), AT&T, NT]', Level3, Qwest, and Cable & Wireless. Interestingly, no group officially sanctions tier-I status; as the saying goes-if you have to ask if you are a member of a group, you're-probably not. A tier-2 ISP typically has regional or national coverage, and (importantly) connects to only a few of the tier-I ISPs thus, in order to reach a large portion of the global Internet, a tier-2 ISP needs to route traffic through one of the tier-I ISPs to which it is connected. A tier-2 ISP is said to be a customer of the tier-I ISP to which it is connected, and the tier-1 ISP is said to be a provider to its customer. Many large companies and institutions connect their enterprise's network directly into a tier-I or tier-2 ISP, thus becoming a customer of that ISP. A provider ISP charges its customer ISP a fee, which typically depends on the transmission rate of the link connecting the two. A tier-2 network may also choose to connect directly to other tier-2 networks, in which case traffic can flow between the two tier-2 networks without having to pass through a tier-I network. Below the tier-2 ISPs are the lower-tier ISPs, which connect to the larger Internet via one or more tier-2 ISPs. At the bottom of the hierarchy are the access151's. Further complicating matters, some tier-I providers are also tier-2 providers (that is, vertically integrated), selling Internet access directly to end users and content providers, as well as to lower-tier ISPs. When two ISPs are directly connected to each other, they are said to peer with each other. An interesting study [Subramanian 2002] seeks to define the Internet's tiered structure more precisely by studying the Internet's topology in terms of customer- provider and peer-peer relationships.

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