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Computers utilise real-time clock chips and crystal oscillators to maintain internal system time. However, due to the need to keep costs low, relatively cheap components are utilised. Therefore, computer systems are notoriously poor at maintaining accurate system time and can drift by many seconds each day. This article discuses various methods of maintaining precise time on computer systems and describes how to use Internet NTP servers as accurate timing references.
There are a number of commercially available hardware time references that utilise GPS or National radio time and frequency broadcasts to maintain accurate time on computers. These hardware clocks generally have serial or USB interfaces that provide regular time updates. Software running on the host computer periodically reads time and date information from the clock and updates system time. More sophisticated software may be available that provides a software interface between the hardware clock and the NTP software distribution. This allows a PC to be configured as a precise stratum 1 NTP server.
GPS hardware clock systems have a number of advantages. They are generally highly accurate providing timing precision to within nanoseconds of UTC. The GPS system is also global, so that GPS hardware clocks can be utilised anywhere in the world. Radio based hardware clocks are generally less accurate and can only be received within the locality of the radio transmitter. However, they do have the advantage that generally a good signal can be received indoors close to the host computer. GPS clocks require an antenna with a good view of the sky, which can be impractical for many installations.
The Internet has developed precise timing algorithms to maintain accurate time on client computers. The Network Time Protocol (NTP) was developed over 25 years ago just to fulfil such a requirement and is now the dominant method of time synchronisation across a computer network. Many operating systems such as Microsoft Windows 2000, 2003, XP, LINUX, UNIX and Novell have adopted the NTP standard. There are a vast number of highly precise Internet NTP server systems that can be utilised free of charge to provide accurate computer time synchronisation.
Microsoft Windows 2000 and 2003 server operating systems provide a pre-installed SNTP (Simple Network Time Protocol) client that can synchronise to any NTP server. The Windows SNTP client is called 'Windows Time'; it runs in the service list and is configured by editing registry entries. Windows XP also has an integrated SNTP client that is configured from the 'Time Properties' applet in the control panel.
LINUX and UNIX operating systems have much more sophisticated timing applications. NTP was originally developed for the LINUX operating system and provides complex timing algorithms to provide a highly precise timing solution. The NTP software distribution is freely available and downloaded under the GNU public licence. Configuring NTP to utilise an external hardware reference clock provides a local stratum 1 NTP server for use on an intranet.
The SNTP protocol was developed to provide a means of time synchronisation without many of the complex algorithms utilised by NTP. SNTP provides less accuracy, but most of the functionality of the NTP protocol. Microsoft adopted the SNTP protocol for use with its Windows 2000 and 2003 server operating systems. SNTP allows a client to synchronise to a NTP server but with some loss of precision.
To summarise, NTP and SNTP protocols are the standard means of time dissemination around the Internet and computer networks. Client computers can maintain synchronisation to within milliseconds of the correct time. GPS and radio hardware clocks can provide accurate synchronisation of computer systems that have no access to the Internet or to provide a Stratum 1 NTP Server resource.
Dave Evans is a technical author in the field of computer timing and NTP Server systems. Dave has a number of years practical experience in the computer timing industry and has proved technical authoring services to a number of manufacturers. Please click here for more information on NTP Server solutions and systems.
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