ISPs and Internet Backbones - 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. |
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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.
Author: Imran Rashid
Date: Dec 6, 2009 - 10:41:34 AM
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.
Imran Rashid has working knowledge about windows operating systems.For more information visit
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Free Windows Tutorials
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