Internet service provider
An Internet service provider (ISP), also known as an Internet access provider (IAP), is a company that provides other companies or subscribers with access to the Internet.
- Subscriber networks: these are networks that belong to individuals, corporations, or organizations that are connecting to the Internet to access services available from, or provide services to, other individuals, corporations, or organizations connected to the Internet.
- ISP backbones: these are networks built by companies for the purpose of providing access to their customers or subscribers. In a real sense, it is the collection of all interconnected ISP backbones that forms the fabric of the Internet to which all of us (the subscribers) connect.
ISPs come in various shapes and sizes. They differ in geographic scope, access options, access rates, and cost of services, among other things. ISPs are often described in terms of tiers. This language used to have a very precise definition that was based on the size and number of connections the ISP had to the network access points (NAP). Today that language has largely disappeared. In general, the tiers can be described as follows:
- Tier 1 ISP: These are the largest ISPs in the Internet. They have backbones that are usually national (i.e., domestic U.S.) or international in scope. More important than scale, Tier 1 ISPs have backbones that function as transit networks. That is to say, they do not pay other ISPs for connection to the Internet, and they carry packets across their backbone that are "just passing through." Some amount of the traffic on these backbones originate from, and are destined to, customers that are not paying the ISP for the service. Tier 1 ISPs peer directly with one another without charging one another. To become a Tier 1 ISP, you have to successfully get the other Tier-1 ISPS to perceive you as a peer and to be willing to peer directly with you. Tier 1 providers include AT&T, Verizon Business, and
- Tier 2 ISP: These ISPs can also be very large (i.e., national or international in scope), but they do not connect to the rest of the Internet purely via peering links. They pay the Tier-1 ISPs (or possibly other Tier 2 ISPs) for at least some of their access to the rest of the Internet, although they will do some amount of peering. When they need to purchase access, they usually purchase Internet access at wholesale prices. They also are typically connected to the rest of the Internet at several points.
- Tier 3 ISP: These are the smaller ISPs that typically have metropolitan or regional backbones. They also purchase all of their Internet access wholesale from other ISPs. They typically are not as richly connected as Tier 2 providers, and they do not carry any transit traffic.
ISP Access Options
ISPs can provide any of a host of connection options to their backbone, including (but not limited to):
- Cable modem
- Dial-up (POTS, ISDN)
- Private line
- Frame Relay
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