ISPs and Internet Backbones

ISPs and Internet Backbones

We saw earlier that end systems (user PCs, PDAs, Web servers, mail servers, and so on) connect into the Internet via a local ISP. The ISP can supply either wired or wireless connectivity, using an array access technologies including DSL, cable, FTTH, Wi-Fi, cellular, and WiMAX. Note that the local ISP does not have to be a telco or a cable company: it can be, for instance, a university (providing Internet access to students, staff, and faculty) or a company (providing access for its employees). But connecting end users and content providers into local ISPs is only a small piece of solving the puzzle of connecting the hundreds of millions of end systems and hundreds 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 ISPs located at the edge of the Internet are connected to the rest of the Internet through a tiered hierarchy of ISPs, as shown in the following figure. Access ISPs are at the bottom of this hierarchy. At the very top of the hierarchy is a comparatively small number of so-called tier-1 ISPs. In various ways, a tier-1 ISP is the same as any network - it has links and routers and is attached to other networks. In other ways, though, tier-1 ISPs are special. Their link speeds are often 622 Mbps or higher, with the larger tier-1 ISPs having links in the 2.5 to 10 Gbps range; their routers must consequently be able to forward packets at very high rates. Tier-1 ISPs are also characterized by being:

● Directly connected to each of the other tier-1 ISPs
● Connected to a large number of tier-2 ISPs and other customer networks
● International in coverage

Tier-1 ISPs are also known as Internet backbone networks. These include Sprint, Verizon, MCI (previously UUNet/WorldCom), AT&T, NTT, Level3, Qwest, and Cable & Wireless. Interestingly, no group officially sanctions tier-1 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 normally has regional or national coverage, and (importantly) connects to only a few of the tier-1 ISPs (see the following figure). Therefore, in order to reach a large portion of the global Internet, a tier-2 ISP needs to route traffic through one of the tier-1 ISPs to which it is connected.

Interconnection of lSPs

A tier-2 ISP is said to be a customer of the tier-1 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-1 or tier-2 ISP, hence becoming a customer of that ISP. A provider ISP charges its customer ISP a fee, which normally 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-1 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 access ISPs. Further complicating matters, some tier-1 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 at the same tier, 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. For a readable discussion of peering and customer-provided relationships, see [Van der Berg 2O08].

Within an ISP's network, the points at which the ISP connects to other ISPs (whether below, above, or at the same level in the hierarchy) are known as Points of Presence (POPs). A POP is simply a group of one or more routers in the ISP's network at which routers in other ISPs or in the networks belonging to the ISP's customers can connect. A tier-1 provider normally has many POPs spread out across different geographical locations in its network, with multiple customer networks and other ISPs connecting into each POP. For a customer network to connect to a provider's POP, the customer normally leases a high-speed link from a third-party telecommunications provider and directly connects one of its routers to a router at the provider's POP. In addition, two ISPs may have numerous peering points, connecting with each other at multiple pairs of POPs.

In summary, the topology of the Internet is complex, consisting of dozens of tier-1 and tier-2 ISPs and thousands of lower-tier ISPs. The ISPs are diverse in their coverage, with some spanning several continents and oceans, and other limited to narrow regions of the world. The lower-tier ISPs connect to the higher-tier ISPs, and the higher-tier ISPs interconnect with one another. Users and content providers are customers of lower-tier ISPs, and lower-tier ISPs are customers of higher-tier ISPs.


end systems, transmission rate, packets

Copy Right

The contents available on this website are copyrighted by TechPlus unless otherwise indicated. All rights are reserved by TechPlus, and content may not be reproduced, published, or transferred in any form or by any means, except with the prior written permission of TechPlus.