Windows 2000 Server comprises layers of software. Each layer is independent and shields lower layers from applications and users. The top-level layers are:
· Hardware Abstraction Layer (HAL) - lowest layer in the architecture. HAL responsibility is to hide hardware details from the upper layers of the operating system and at the same time let the Windows 2000 to run on different hardware platforms.
· Kernel Layer - is located between the HAL and System services layer. The kernel cooperates with hardware through the HAL layer, and performs low-level o.s. functions (thread scheduling, exception, and interrupt handling).
· System services layer - the next in order layer has a number of software components. This layer provides services to applications through well-defined and formal interfaces.
· Subsystems layer - this is the highest layer of the operating system. This layer includes three environment subsystems that deliver to application software. The environment subsystems reveal well-defined applications programming interfaces (APIs) that application use. Any applications that run on Windows 2000 must be configured with these APIs.
The logical structure of Active Directory is created to perform a variety of tasks such as installing, configuring, administering, and troubleshooting Active Directory. To be able perform these tasks the logical structure of Active Directory is made of the following components:
· Domains - a domain is a logical group of objects, users, and computers that forms an administrative and replication boundary. A domain can be defined in Active Directory as an administrative unit with unique name. In Windows 2000, domains are named according to DNS naming conventions and not according to NetBIOS. In Active Directory, a domain can contain millions of objects and creation of multiple domains would not be required, unless for a specific purpose.
· Trees - in Active Directory, a group of domains can be created such that they form a hierarchical structure referred to as a tree. In this structure, domains have a parent/child relationship. The domains in a single tree share a contiguous namespace. The child domain takes the domain name of its parent. Each domain in the tree is an independent administrative unit.
· Forests - a forest can be described as a collection of Active Directory trees. Each tree in a forest has it own different namespace. The first domain created in a forest is considered the forest root. The name of the forest root domain is used to refer to the whole forest. A forest can also be made up of a single tree, wit no connection to any other trees.
· Organizational units - this is a container object within Active Directory. OUs can be created to organize various objects, such as users, groups, computers, and printers, according to their functions, locations, resources, and so on. In another words an OU is created to delegate administrative control of a se of objects by assigning specific rights to users and groups. An OU can contain only objects from the domain in which it exist.
· Global catalog - a global catalog is a searchable master index with information about all objects in Active Directory. It contains information that is required to determine the location of an object using queries. This information can be a user name, a division, or an e-mail address. The global catalog enables users to find Active Directory information regardless of the physical location of the data.
