The Web Doctor

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.
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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. For more information on NTP Server
solutions, please take time to visit:
http://www.timetools.co.uk/ntp-servers/ntp-server-s5000.htm