Which network tier forms the internet backbone?

Tier 1 Tier 1 providers have the largest and fastest network connections available. A reasonable definition of a tier 1 provider is a network that can reach every other network on the Internet without purchasing IP transit or paying for peering. You might consider a Tier 1 network to be an ISP for other ISPs. Because of their extremely good connectivity, other network providers pay Tier 1 networks to carry their data for them to other locations. As of this time, there are less than 20 Tier 1 network providers worldwide.