Domain controller

The Domain Controller is a Windows server computer that stores user account information, authenticates users, and enforces security policy for a Windows domain. Domain controllers detect changes to user accounts and synchronize changes made in Directory Server user entries.

Distributed SCADA networks typically employ multiple Domain Controllers at strategic network locations.

The Domain Controller node includes the DNS service and Time Synchronization to manage network communication requests.

Domain name server (DNS)

An Internet service that translates domain names and host names into IP addresses.

Best practice

Implement Universal Time Synchronization (UTS) at each site. Doing so provides absolute certainty that data is properly time-stamped and forwarded to the historian under all circumstances. Current technology uses GPS or radio broadcast software along with dedicated hardware devices that may linked to one or more computers at the site using serial ports, Ethernet or USB. See Synchronize time across a galaxy for detailed information.

The following considerations apply when implementing UTS:

• If time master provider is not in same geographical area, there is a risk of losing a connection to the time server through time "drift." In other words, nodes in the same geographical area should have access to a master in the same area, or use a GPS.

• Synchronize all Domain Controllers with a common time provider.

  The Windows service uses time stamp as a part of the Kerberos security implementation. Kerberos is a system that provides a central authentication mechanism for a variety of client/server applications, using passwords, secret keys, and time-sensitivity.

  If the time between clients/Domain Controllers, or between Domain Controllers (in other geographical areas) drifts, the operating systems may fail.