NTP or Network Time Protocol servers are network devices that are designed to distribute accurate time to network time clients and other network infrastructure. This article describes how to configure and install NTP server systems on a network and discusses the advantages and disadvantages of various reference clock options. NTP servers are generally supplied as 1U high rack mountable network devices.
They obtain an accurate time from an external time reference, such as GPS or radio, and provide an accurate timing resource for a computer network. NTP or Network Time Protocol is a protocol designed for distributing time to client computers over an IP network. The protocol is UDP based and as such requires the TCP/IP network infrastructure to be installed. Hardware Installation Stratum 1 NTP time servers rely on an external timing reference to obtain accurate time.
Various external timing references are available. Options may vary with the installations regional location. GPS (Global Positioning System) is a popular timing reference. The advantages of a GPS reference are that it is highly accurate and can be utilised anywhere in the world. A typical GPS NTP server installation can synchronise to within a few microseconds of UTC time. The disadvantage of GPS is that ideally a roof-mounted external antenna is required with a good view of the sky.
The maximum cabling distance between an NTP server and GPS antenna is governed by the quality of coax utilised. Relatively low-quality coax, such as RG58 can be used to around 50m. Higher quality coax, such as LMR200 can be utilised to around 80m.
Very high quality coax, such as LMR400 can be utilised to around 200m. Additionally, GPS amplifiers can be used to amplify the GPS signal and extend cable runs. It is also good practise to install a surge suppressor to externally mounted GPS antennas, to protect against the possibility of damage caused by lightning strikes. Local radio time references are available in many countries. Radio time references tend to be local to the country of origin and maybe neighbouring countries. The advantage of radio is that generally a good signal can be obtained indoors, close to the NTP server installation.
However, radio time services are less accurate than GPS and reception areas are regional. A typical radio NTP server installation can synchronise to within a few milliseconds of UTC time. A number of factors can affect radio reception, including: locating the radio antenna underground or in a basement; locating the antenna inside a metal cage (including metal cladding) and locating the antenna close to electrically noisy equipment. A number of regional radio time references are available including: WWVB, Colorado, US; DCF-77, Frankfurt, Germany and MSF-60, Rugby, UK.
The DCF-77 time transmission is available throughout Central and Western Europe. The MSF-60 time signal is available throughout the British Isles and much of North-West Europe. NTP Server Configuration For a minimal installation, NTP servers are extremely easy to install. They ideally need to be provided with a static IP address. DHCP is not a good option since the IP address is leased and can change periodically.
The device then needs to synchronise its internal reference time with the selected external timing reference. Synchronisation can take as long as 30 minutes depending on the drift of the local battery-backed clock when the device was powered down. Once synchronised, the NTP server can provide precise time to network time clients and other network devices and infrastructure.
Many other configuration options are available on an NTP server: authentication options are available for secure installations; IP address restrictions can be specified; status reporting functions such as 'syslog' as well as precision and status information. NTP Client Configuration Many operating systems such as Microsoft Windows 2000/XP/2003 and Vista have built in SNTP client functionality. The standard NTP distribution is available for LINUX, Free BSD and UNIX operating systems; Novell also has a NTP compliant NLM available. Many network infrastructure devices, such as Cisco routers and switches can also be synchronised using NTP. Essentially, a network time client only needs to be pointed to the IP address or DNS name of a NTP server in order to synchronise time. NTP and SNTP SNTP or Simple Network Time Protocol is a simplified version of NTP that is generally used by small computers and micro-controllers.
SNTP provides a subset of NTP functionality for computers that do not require the precise synchronisation ability of NTP. SNTP and NTP are however completely interchangeable. SNTP clients can synchronise to NTP servers and vice-versa. The client software supplied by Microsoft with Windows 2000 and XP is a SNTP rather than an NTP implementation. Summary NTP servers are extremely easy to install and configure.
However, a little thought needs to be given to selecting the correct external timing reference for your region and to antenna installation.
The author, D. Evans, develops GPS clock and NTP server synchronization solutions that ensure accurate time on PC's and computer networks. David has been involved in the development of dedicated time servers, NTP synchronized digital wall clock systems and atomic clock synchronization products. Click here to find out more about GPS Clock, SNTP and NTP Server solutions.