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.
The Network Time Protocol (NTP) is a protocol designed purely for the transfer of accurate time around a computer network. The protocol allows network time clients to accurately synchronise system time with a precise reference clock. This article describes what a NTP time server is, how it operates, and why modern organisations need synchronised network time.
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 device on a network can synchronise it's internal system time to a NTP time server. 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. The GPS constellation of satellites each has super-accurate synchronised atomic clocks on board to provide precise timing information. 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. Radio time signals can generally be received by an indoor antenna within the broadcast range of the transmission. A number of broadcasts are available around the world including; JJY (Japan), WWVB (Colorado, USA), MSF-60 (Anthorn, UK) and DCF-77 (Frankfurt,Germany).
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. The Microsoft Windows operating system utilises the 'Windows Time' service to provide NTP synchronisation. Additionally, Novell also provides loadable modules for synchronisation of Netware servers.
Many of the latest data processing systems operate on distributed systems. Without synchronisation of cooperating devices many problems can arise. Event logging may not provide a true representation of when a specified event occurred. Synchronised time is required to ensure time-stamped transactions are generated in the correct order. Many modern computer applications require accurate timestamps for correct operation. The Network Time Protocol is widely used throughout the Internet to synchronise time critical processes and 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.
Network Time Synchronisation using GPS
This article describes the components required to utilise GPS for computer timing applications. It also discusses how the GPS system can be used for computer network timing by using the Network Time Protocol (NTP) and NTP time servers.
Installing a Radio Referenced NTP Server
Many networks in organisations suffer from a lack of time synchronisation between servers, workstations and other network components. This can lead to problems processing time critical information and transactions. However, a solution has been available for many years in the form of NTP.
Using the GPS System for Accurate Computer Time
This article discusses how the GPS system can be utilised by NTP server systems for precise network timing. It also describes the hardware required to utilise the GPS system for timing applications.
