Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

Distributed network systems require time synchronization to ensure accurate timestamping and event executions. Time synchronization helps establish and maintain the order of events. A time service keeps track of time in networks and determines the accuracy associated with each clock used to synchronize time. The service provides fault-tolerant clock synchronization for systems in both local and wide area networks. The NTP (Network Time Protocol) may be used to obtain time values from outside sources. The Tymserve 2000 Network Time Server from Datum, Inc. provides NTP time-service functions.

NDS (Novell Directory Services) provides an example of how time-synchronization services help to keep the activities of servers synchronized. NDS holds information about user accounts and network resources. Changes must be copied (replicated) to all servers where the NDS database is stored. Time synchronization helps to ensure that everything is kept synchronized.

There are two time-synchronization schemes. The first uses a single-reference time server for relatively small, geographically close networks. The time server is the sole source of time on the network. Any time changes are set on this server, and then others synchronize with it. The other method is used for geographically distant networks and includes the following time servers:

• Primary server    A primary server synchronizes time with other primary time servers or a reference time server, and provides the correct time to secondary time servers.


• Reference server    A reference server gets its time from an external source (such as a radio clock) and is a contact to what the outside world says the time should be.

All other servers on the network can be secondary time servers, which get their time from single reference, primary, or reference time servers and do not participate in the establishment of a common time over the network. Note that the master time signal is obtained from public time servers.

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