We saw earlier that end systems (user Pcs, Pda’s, Web servers, mail servers, and so on) associate into the Internet via an passage network. Recall that the passage 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 passage 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 social Internet, passage networks situated at the edge of the Internet are associated to the rest of the Internet straight through a tiered hierarchy of Isps. passage Isps (for example, residential cable and Dsl networks, dial-up passage networks such as Aol, wireless passage networks, and company and university Isps using Lans) are at the lowest of this hierarchy. At the very top of the hierarchy is a relatively small amount 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 associated 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 transmit packets at highly high rates. Tier-I Isps are also characterized by being:
Modem
* Directly associated to each of the other tier-1 Isps
* Connected to a large amount of tier-2 lSps and other buyer networks
* International in coverage Tier-l Isps are also known as Internet backbone networks.
These consist of 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 quantum of the global Internet, a tier-2 Isp needs to route traffic straight through one of the tier-I Isps to which it is connected. A tier-2 Isp is said to be a buyer of the tier-I Isp to which it is connected, and the tier-1 Isp is said to be a victualer to its customer. Many large companies and institutions associate their enterprise’s network directly into a tier-I or tier-2 Isp, thus becoming a buyer of that Isp. A victualer Isp charges its buyer Isp a fee, which typically depends on the transmission rate of the link connecting the two. A tier-2 network may also select to associate directly to other tier-2 networks, in which case traffic can flow between the two tier-2 networks without having to pass straight through a tier-I network. Below the tier-2 Isps are the lower-tier Isps, which associate to the larger Internet via one or more tier-2 Isps. At the lowest 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 passage directly to end users and content providers, as well as to lower-tier Isps. When two Isps are directly associated to each other, they are said to peer with each other. An tantalizing study [Subramanian 2002] seeks to define the Internet’s tiered buildings more surely by studying the Internet’s topology in terms of customer- victualer and peer-peer relationships.
Isp’s and Internet Backbones
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