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What is a NTP Server and Why Do I Need One?

By David Evans

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Published: 26Jan2009
Word count: 696
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NTP is an acronym for Network Time Protocol. NTP is used throughout the computing industry to maintain synchronisation of computers and network infrastructure to an accurate time reference, or NTP Time Server. This article attempts to clearly and simply describe what a NTP server is and why it is an essential part of a modern computer network infrastructure.

A NTP time server is a network device that obtains accurate time information from an external timing reference. Accurate time is maintained internally by the device and provided to any network time client that requires it. Any network device can contact a NTP server to request time and to synchronise its own internal system clock. In this manner whole networks of computer systems can maintain synchronisation with an accurate timing reference.

NTP is a hierarchical protocol. At the highest level is a stratum1 primary reference NTP time server. A stratum 1 server obtains accurate time from an external hardware clock. A stratum 1 server is the most accurate time reference, since it synchronises directly to a precise external source of time. Time servers lower down the hierarchy synchronise to the stratum above and provide consistently less accuracy. However, using a hierarchical system allows multiple time servers to take workload away from the most accurate references and prevents them from becoming swamped with requests for time.

A number of external timing references, or hardware clocks, are available that provide accurate time. The Global Positioning System, GPS, is by far the most popular source of time for external reference clocks. Each GPS satellite has an onboard atomic clock that can provide extremely accurate timing information to ground based GPS receivers. GPS timing information can be received worldwide, provided an antenna can be shown a good view of the sky.

National radio time and frequency broadcasts are also a good source of accurate time. These local broadcasts can generally be received nationally with an indoor radio antenna and receiver. A number of broadcasts are available around the world including; MSF (UK), DCF-77 (Germany), WWVB (USA) and JJY (Japan).

Many NTP time servers also contain precision backup reference clocks in order to maintain accurate time during signal outages. A backup reference clock provides extended holdover while external reference clock time is unavailable. A number of backup timing solutions are available, the most accurate, and expensive, being a Rubidium Oscillator. These highly stable oscillators maintain very accurate holdover for extended periods of time.

Good price / performance alternatives are OCXO and TCXO oscillators. An OCXO, Oven-Controlled Crystal Oscillator, is a crystal oscillator that is housed in a very small oven to maintain constant temperature. Since the greatest cause of drift in crystals is temperature variation, OCXO's attempt to minimise drift by maintaining a constant temperature.

TCXO, Temperature-Controlled Crystal Oscillators, work on a similar principle. However, rather than maintain a constant temperature, they speed up or slow down slightly to compensate for any temperature variation. These various oscillators are disciplined by the GPS time signal to provide a very stable time output to maintain holdover during signal outages. This allows networks to remain synchronised when external reference clock synchronisation is lost.

Most modern computer operating systems provide integrated NTP client software, which allow synchronisation to a time server. LINUX and UNIX operating systems have a freely available GNU public licence NTP client available. Microsoft incorporates the 'Windows Time' service in the latest Windows operating systems, which has NTP functionality. Additionally, Novell also provides loadable modules for synchronisation of Netware servers.

Many modern computing applications are distributed systems operating across a large number of individual machines. Without synchronisation of cooperating devices many problems can arise. Event logging may not provide a true representation of when a specified event occurred. Transaction time stamps may not be accurate and may be out of sync if generated by more than one machine. Many other applications are time critical and require precise synchronisation of network devices. NTP is used throughout the Internet to synchronise the network infrastructure. It is also adopted by most organisations to provide an accurate, traceable, source of time for networked computing equipment. NTP is the 'de facto' standard protocol for network time distribution.

This article was written by Dave Evans. Dave has written a number of articles about computer time synchronisation and network timing. Click here, if you would like more information on the subject of NTP server solutions.

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