The process whereby the domain directory databases of backup domain controllers (BDCs) in a Microsoft Windows NT domain are synchronized with the master directory database on the primary domain controller (PDC). Accurate and reliable directory synchronization is the foundation for effective operation of Windows NT Directory Services (NTDS).
If directory synchronization must be performed over slow WAN links, you can adjust some registry parameters to make directory synchronization more efficient and prevent it from consuming excessive bandwidth. For more information, see the Microsoft Official Curriculum (MOC) course MS 689: Microsoft Windows NT Server 4.0 Enterprise Technologies.