Tier 1 network
A Tier 1 ISP is a telco or internet service provider IP network which connects to the rest of the Internet only via a practice known as peering. They are commonly called backbone networks because they handle massive amounts of data.
The name Tier 1 refers to their position at the top level of the hierarchy of network providers. The Tier 1 providers make settlement-free interconnection arrangements with other Tier 1 providers, in which the two networks agree to carry each other's traffic (so-called "peering" with one another) at no cost.
In general, Tier 1 providers own the physical medium over which information is carried, as well as the network equipment which manages that information, and are either telcos who pre-dated the Internet or early movers in the Internet market who managed to build up critical mass in the days prior to the introduction of paid transit agreements.
Tier 1 providers, in turn, provide IP transit access to their networks, called to other companies for a fee. Many of these other companies are still very large Internet providers, but since they purchase transit they are not considered Tier 1.
These lower-tier companies are in general divided into two other categories:
- Tier 2 - A network who peers with other networks, but still pays for transit to reach some portion of the Internet.
- Tier 3 - A network who solely purchases transit from other networks to reach the Internet.
Although there is reasonable consensus about the meaning of the term "Tier 1", the term is today more significant in marketing terms than in terms of network engineering or performance, with many of the larger Tier 2 carriers having almost identical network performance to the Tier 1 carriers. In the same way, a sufficiently multihomed Tier 3 provider may also provide a high quality service, even though it owns very little physical network itself.
Marketing issues
The definition of a Tier 1 is a hotly-debated topic, due to its frequent use as a marketing term. Many networks incorrectly claim to be a Tier 1 simply due to ignorance of the term, leading to a corruption of meaning in which the vast majority of those claiming to be a Tier 1 network are not. The issue is further complicated by the non-disclosure agreements which surround almost all Tier 1 level peering; this makes it difficult for carriers to officially confirm or deny the existence of peering, the physical connection of two backbone networks to allow data to flow between them. Without peering, customers using computers at home that are connected to one Tier 1 network would not be able to reliably connect to users on another Tier 1 network.
Some of the incorrect measurements which are commonly cited include numbers of routers (devices that direct data traffic between networks like a very advanced switchboard), route miles of fiber optic cable, or number of customers using a particular network. These are all valid ways to measure a network, but have no direct relationship to the status of "Tier 1". Another common area of debate is whether it is possible to become a Tier 1 through the purchase of "paid peering", or settlement-based interconnections which would allow a network to buy this status. The current opinion of most Peering Coordinators is that a true "Tier 1" is one which is entirely settlement-free and gained on the network's own merits. Tier 1 networks' peering points are typically done freely between networks of equal sizes because it is mutually beneficial. When one company is doing more work than the other, often quoted as twice the data, the larger will sometimes demand payment. This payment agreement is called transit instead of peering.
Peering issues
Tier 1 networks are often noted for their extremely restrictive peering policies, which are generally regarded as being intended to set the bar for peering so high that no new networks can meet them. In addition to the obvious competitive advantage for marketing, Tier 1's gain a significant portion of their IP Transit revenue from traffic which stays "on-net", by being delivered between two customers without ever leaving its network.
Disputes over peering arrangements between Tier 1 providers, and often between Tier 1 and 2 providers, have frequently been the cause of disruptions of Internet traffic. These disruptions primarily affect direct customers of each provider, who will no longer be able to contact direct customers of the other provider.
List of Tier 1 IPv4 ISPs
The following are believed to be the only Tier 1 ISPs worldwide:
- AOL Transit Data Network (ATDN) AS 1668
- AT&T AS 7018 (not 2685, 2686, 2687, 2688 or 7132)
- Global Crossing (GX) AS 3549
- Level 3 AS 3356
- Verizon Business AS 701 (not 702, 703 or 19262)
- Nippon Telephone and Telegraph (NTT) AS 2914
- Qwest AS 209
- SAVVIS AS 3561
- Sprint Nextel Corporation AS 1239
This list follows the definition of a tier 1, that is an ISP that is settlement free and buys no transit or peering from anyone.