A network time server is a device that provides a computer network with an accurate source of time. Accurate time is generally obtained from an external hardware clock reference, such as GPS or radio time and frequency broadcasts. Precise time is then maintained internally by a calibrated system clock. Servers, workstations and other network devices can then obtain timing information from the time server in order to synchronize their system clocks.
Synchronized time is an important aspect of modern computer systems design. Distributed applications rely on synchronization in order to ensure events are ordered correctly.
Network Time Protocol (NTP)
Network Time Protocol is the standard means of disseminating time throughout a network. A time server utilises the NTP protocol to synchronize client computers. The protocol has built-in security features to prevent attack from malicious users. It also has the ability to utilise multiple time references and implements complex statistical and filtering algorithms to provide a high degree of accuracy and reliability.
Simple Network Time Protocol (SNTP)
For many smaller, less-powerful, computers, the NTP protocol with its complex filtering algorithms has a larger footprint than can be easily accommodated. For this reason, the Simple Network Time Protocol was developed. SNTP does without the complex statistical analysis of time-stamps. It also generally only has the capability to obtain time from a single reference. SNTP is not as accurate or as reliable as NTP. However, for many micro-controller or small computer based systems, it offers a good overhead-performance compromise. Devices such as IP cameras, phone systems and Microsoft Windows operating systems utilise the SNTP protocol.
Hierarchical Operation – Stratum
NTP operates in a hierarchical manner. Each level in the hierarchy is represented by a stratum. A server that obtains time from a hardware clock reference sits at the top of the hierarchy and is designated stratum 1. Every device that obtains time from the stratum above is designated a stratum higher than the level it obtains time from. Often hardware time references, such as GPS, DCF77 and MSF are referred to a stratum 0 time sources.
Common Enclosure Formats
Time Servers are generally available in two types of enclosure. Most common is the standard 1U high 19-inch rack-mountable enclosure. However, compact desktop-type enclosures are also available where rack-mounting is not a requirement or if space is at a premium.
Power Supply Options
Generally appliances are provided with a universal mains power supply unit (PSU), which allows mains operation in most countries. A 9-36 VDC-in power-supply can also be provided for applications where operation with a low-voltage DC supply is required.
Reference Clock Options
The Global Positioning System is a satellite based positioning and navigation system. It consists of a network of 24 orbiting satellites, each with an atomic clock. Very precise timing information is continuously transmitted to the surface of the Earth, which can be received and decoded by a GPS synchronized time receiver. GPS provides a very accurate source of time, which is continuously available anywhere on or close to the surface of the Earth. A NTP server with an outdoor located GPS antenna can use GPS as a source of accurate time.
There are a number of terrestrial national radio broadcasts of time and frequency, which can also be used as a source of time. The DCF77 radio transmission, broadcast from Mainflingen, Germany is one such signal. It is broadcast at 77.5 KHz and can be received throughout much of Central and Western Europe.
The MSF radio broadcast is similar to the DCF77 broadcast, but is transmitted at 60KHz from Anthorne in the UK. The signal covers the whole of the British Isles and can also be received in much of North-Western Europe.
The WWVB signal is transmitted from Colorado in the USA. The broadcast provides accurate time and date information to much of the USA and Canada.