Oracle Enterprise Manager architecture consists of a three-tier framework.

Console and Management Applications--First Tier
Enterprise Manager Console (GUI-based management application) Administrative tools for all management tasks
Oracle Management Pack applications

Oracle Management Server(s)--Second Tier
One or more Oracle Management Servers
A repository that stores system data, application data, information about the state of the managed nodes, and information about any system management packs

Managed nodes containing databases and other services--Third Tier
Services (such as the Oracle Intelligent Agent and data collection service)

Set up steps:

Second Tier:

1. Install Oracle Enterprise Manager
2. Creating a Repository (connect to a database)
3. Start Management Server

Make sure database for the repository is up. Each time database is down, Managerment server may stop. So you need to start the managerment server again.

Third Tier: (database you want to manage)

1. Start the intelligent Agent
2. Start the Agent Data Gatherer

First Tier:

1. Install Oracle Enterprise Manager
2. Start Oracle Enterprise Manager Console
If you are logging in to Oracle Enterprise Manager for the first time, type in the default credentials.
Administrator = sysman
Password = oem_temp.