To protect optimal performance, Cognos services are mixed together to fulfill all the certain roles for better business intelligence applications. These roles can also define the architecture in IBM Cognos BI server uses.
Web tier:
Generally, the web tier provides session connectivity to IBM Cognos Bi applications. This Cognos component can fulfill the roles of IBM Cognos gateway. This gateway manages all the web communication for the IBM Cognos platform. IBM Cognos gateway server requires minimal processing resources. One can also deploy multiple redundant gateways along with external HTTP router.

Application tier:
Application tier for IBM Cognos platform has mainly three server components. They are IBM Cognos report server, IBM Cognos Dispatcher and Cognos content manager application tier and Cognos report server. Cognos training helps to understand these concepts quickly