Tier 1 - TAC - First line of customer contact, first call resolution would be great but it does not always happen. They essentially take as much info as possible and escalate
Tier 2 - NOC - Have access to systems and networks. They resolve 90% of the cases that come up from the TAC. They also are first level to priority customers or complex network or system issues.
Tier 3 - Engineering - Whether its Unix, Windows, or Networks the escalation stops here with resolution.