The physical structure of Active Directory helps to manage the communication between servers with respect to the directory. The two physical elements of Active Directory are domain controllers and sites. Each is described below.
Domain Controllers – domain controllers are Windows 2000 Server-based systems that store the Active Directory database. Every Windows 2000 domain controller has a writable copy of the directory. This is different that in NT 4, where only the PDC had this capability. Domain controllers in the same domain contain replicas of the directory that must be synchronized periodically.
Site – a site is a concept that did not exist in an NT directory service structure. In Active Directory, sites are groups of IP subnets that are connected at high speed. Although the definition of ‘high speed’ is open, it is generally considered to be subnets that are connected at LAN speeds (say 10 Mb) or higher. The purpose of defining sites in Active Directory is to control network traffic relating to directory synchronization, as well as to help ensure that users connect to local resources. For example, domain controllers located in the same site replicate with one another on a 5-minute change notification interval similar to in NT 4. However, replication between domain controllers in different sites can be scheduled according to your needs. This allows a much greater degree of flexibility that in NT 4. For example, you could set things up such that replication between sites could only happen between midnight and 6am – thus ensuring that replication traffic would not interfere with normal data transfer during business hours. Sites also help ensure that users avoid accessing resources over the WAN by having client systems access servers (such as domain controllers) that are in the same physical site first.