Components of SAP 3-tier Architecture

• The Internet Communication Manager (ICM) sets up the connection to the Internet. It can process both server and client Web requests.It supports the protocols HTTP, HTTPS, and SMTP. The SAP Web AS may be a Web server or a client. 
• The dispatcher distributes the requests to the work processes. If all the processes are occupied the requests are stored in the dispatcher queue.
• The Work Processes execute ABAP or Java programs.
• The SAP Gateway makes the RFC interface between the SAP instances available (within an SAP System and beyond system boundaries).
• The Message Server exchanges messages and balances the load in the SAP system.