This illustration shows a 3-tier application. The three tiers are:
The Microsoft Transaction Server application is running on a Windows NT or Windows 2000 host computer. This constitutes Tier 2 of the 3-tier application model.
The Microsoft Transaction Server application hosts transactional COM components that implement the business logic. These components can be invoked from smart clients directly through Distributed COM (DCOM). These components can also be invoked by internet browsers indirectly through Microsoft's Internet Information Server (IIS). These smart clients and browsers constitute Tier 1 of the application.
Tier 3 of the application consists of various backend databases. The Microsoft Transaction Server components in Tier 2 access these databases in Tier 3 using OCI, ODBC, Oracle Provider for OLE DB, or OO4O.