1.3.3 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 provide either wired or
wireless connectivity, using an array access technologies including DSL, cable,
FTTH, Wi-Fi, cellular, and WiMAX. Note the the local ISP does not have to be a
teleo or a cable company: it can be, for example, a university (providing Internet
acces~ 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 situated at the edge of the Internet are connected
to the rest of the Internet through a tiered hierarchy of ISPs, as shown in
Figure 1.15. Access ISPs are at the bottom of this hierarchy. At the very top of the
hierarchy is a relatively small number of so-called tier-lISPs. In many ways, a tier- I
ISP is the same as any network-it has links and routers and is connected to other
networks. In other ways, however, tier-liSPs are special. Their link speeds are often
622 Mbps or higher, with the larger tier-lISPs having links in the 2.5 to 10 Gbps
range; their routers must consequently be able to forward packets at extremely high
rates. Tier-lISPs are also characterized by being:

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

Tier-lISPs are also known as Internet backbone networks. These include
Sprint, Verizon, MCI (previously UUNetlWoridCom), AT&T, NIT, 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, amI (importantly) connects
to only a few of the tier-lISPs (see Figure I. I 5). 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-lISPs to which it is connected. A tier-2 ISP is said to be a customer of the
tier-lISP to which it is connected, and the tier-lISP is said to be a provider to its
customer. Many large companies and institutions connect their enterprise's network
directly into a tier-lor 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-l 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-l 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 2008].

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-l provider typically has many POPs scattered across
different geographicaf 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 typically 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. Furthermore, two ISPs may have multiple 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-I and tier-2 ISPs and thousands of lower-tier ISPs. The ISPs are diverse in their
coverage, with some spanning multiple continents and oceans, and others 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 lSPs are customers of
higher-tier ISPs.