What is a PDC emulator?

PDC Emulator - This role is the most heavily used of all FSMO roles and has the widest range of functions. The domain controller that holds the PDC Emulator role is crucial in a mixed environment where Windows NT 4.0 BDCs are still present. This is because the PDC Emulator role emulates the functions of a Windows NT 4.0 PDC. But even if you've migrated all your Windows NT 4.0 domain controllers to Windows 2000 or Windows Server 2003, the domain controller that holds the PDC Emulator role still has a lot to do. For example, the PDC Emulator is the root time server for synchronizing the clocks of all Windows computers in your forest. It's critically important that computer clocks are synchronized across your forest because if they're out by too much then Kerberos authentication can fail and users won't be able to log on to the network. Another function of the PDC Emulator is that it is the domain controller to which all changes to Group Policy are initially made. For example, if you create a new Group Policy Object (GPO) then this is first created in the directory database and within the SYSVOL share on the PDC Emulator, and from there the GPO is replicated to all other domain controllers in the domain. Finally, all password changes and account lockout issues are handled by the PDC Emulator to ensure that password changes are replicated properly and account lockout policy is effective. So even though the PDC Emulator emulates an NT PDC (which is why this role is called PDC Emulator), it also does a whole lot of other stuff. In fact, the PDC Emulator role is the most heavily utilized FSMO role so you should make sure that the domain controller that holds this role has sufficiently beefy hardware to handle the load. Similarly, if the PDC Emulator role fails then it can potentially cause the most problems, so the hardware it runs on should be fault tolerant and reliable. Finally, every domain has its own PDC Emulator role, so if you have N domains in your forest then you will have N domain controllers with the PDC Emulator role as well.