Protocol Layers and Their Service Models in Java

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Protocol Layers and Their Service Models
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Figure 17-5: Hosts, routers and bridges - each contain a different set of layers, reflecting their differences in functionality
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References
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[Wakeman 1992] Ian Wakeman, Jon Crowcroft, Zheng Wang, and Dejan Sirovica, "Layering considered harmful," IEEE Network, January 1992, p 7 Return to Table Of Contents
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Copyright Keith W Ross and Jim Kurose 1996-2000
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Internet structure: Backbones, NAP's and ISP's
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18 Internet Backbones, NAPs and ISPs
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Our discussion of layering in the previous section has perhaps given the impression that the Internet is a carefully organized and highly intertwined structure This is certainly true in the sense that all of the network entities (end systems, routers and bridges) use a common set of protocols, enabling the entities to communicate with each other If one wanted to change, remove, or add a protocol, one would have to follow a long and arduous procedure to get approval from the IETF, which will (among other things) make sure that the changes are consistent with the highly intertwined structure However, from a topological perspective, to many people the Internet seems to be growing in a chaotic manner, with new sections, branches and wings popping up in random places on a daily basis Indeed, unlike the protocols, the Internet's topology can grow and evolve without approval from a central authority Let us now try to a grip on the seemingly nebulous Internet topology As we mentioned at the beginning of this chapter, the topology of the Internet is loosely hierarchical Roughly speaking, from bottom-to-top the hierarchy consists of end systems (PCs, workstations, etc) connected to local Internet Service Providers (ISPs) The local ISPs are in turn connected to regional ISPs, which are in turn connected to national and international ISPs The national and international ISPs are connected together at the highest tier in the hierarchy New tiers and branches can be added just as a new piece of Lego can be attached to an existing Lego construction In this section we describe the topology of the Internet in the United States as of 1999 Let's begin at the top of the hierarchy and work our way down Residing at the very top of the hierarchy are the national ISPs, which are called National Backbone Provider (NBPs) The NBPs form independent backbone networks that span North America (and typically abroad as well) Just as there are multiple long-distance telephone companies in the USA, there are multiple NBPs that compete with each other for traffic and customers The existing NBPs include internetMCI, SprintLink, PSINet, UUNet Technologies, and AGIS The NBPs typically have high-bandwidth transmission links, with bandwidths ranging from 15 Mbps to 622 Mbps and higher Each NBP also has numerous hubs which interconnect its links and at which regional ISPs can tap into the NBP The NBPs themselves must be interconnected to each other To see this, suppose one regional ISP, say MidWestnet, is connected to the MCI NBP and another regional ISP, say EastCoastnet, is connected to Sprint's NBP How can traffic be sent from MidWestnet to EastCoastnet The solution is to introduce switching centers, called Network Access Points (NAPs), which interconnect the NBPs, thereby allowing each regional ISP to pass traffic to any other regional ISP To keep us all confused, some of the NAPs are not referred to as NAPs but instead as MAEs (Metropolitan Area Exchanges) In the United States, many of the NAPs are run by RBOCs (Regional Bell Operating Companies); for example, PacBell has a NAP in San Francisco and Ameritech has a NAP in Chicago For a list of major NBP's (those connected into at least three MAPs/MAE's), see [Haynal 99]
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