Replication is the process of copying Active Directory data between domain controllers to ensure that they all have the same information. The Windows Server 2003 multiple-master replication capabilities make the entire replication process more complex than it was in Microsoft Windows NT. On a Windows NT network, servers write all domain directory changes to the primary domain controller first, which then propagates the information to the backup domain controllers. This process is single-master replication. In Windows Server 2003, administrators can modify Active Directory by writing to any domain controller. All the domain controllers execute periodic replication events that copy their modifications to all the other domain controllers. The schedule and topology for these replication events differ depending on whether the domain controllers are at the same or different sites.