Actually,, it was UUNET. I was a leading WAN engineer for Tier 2 providers in the 90s and witnessed the growth of UUNET and its transition to Worldcom->MCI->Verizon. ATT was a different pipe and was important but not dominant in the 90s growth. ATT was one of 5 major Tier 1 providers at the time: UUNET, Cerf, ATT, Sprint, and Exodus.
UUNET
Actually,, it was UUNET. I was a leading WAN engineer for Tier 2 providers in the 90s and witnessed the growth of UUNET and its transition to Worldcom->MCI->Verizon. ATT was a different pipe and was important but not dominant in the 90s growth. ATT was one of 5 major Tier 1 providers at the time: UUNET, Cerf, ATT, Sprint, and Exodus.