Time Synchronization

Packey Velleca

See Sidebar 1

See Sidebar 2

See Sidebar 3

When computers are networked together to share information, it is important they have a common time reference. Networked filesystems, centralized compilers, license servers, and realtime control systems all need to be synchronized to some degree, and what differentiates a realtime control system from a license server is the degree of accuracy needed. The license server could be synchronized within a few minutes, whereas the control system may need to be within a millisecond. This article discusses some of the options available for synchronizing machines to within a milllisecond for use in a realtime data processing system.

There are a few factors that drive the design of a time synchronization system, and they include: available time reference, price, reliability, and accuracy. This article discusses some of these choices, and then describes my choice, ntp, in detail.

Most time distribution schemes for computer systems deliver time-of-day, which is monotonic. This article does not cover the topic of non-realtime time distribution, such as time codes recorded and replayed on tape.

Time References

Every time distribution architecture requires a time reference (see sidebar for a glossary). Here are a few of the choices available, and a short description of their relative merits.

GPS

The Global Positioning System (GPS) is a network of satellites maintained by the U.S. government that is able to deliver very accurate UTC (coordinated universal time) to nearly any location on the globe.