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Network Time Protocol and Network Timing Glossary

Atomic Clock

An atomic clock is the most accurate timekeeping device available and measures time by the oscillation frequency of atoms energised by an electromagnetic field or optical pumping.

There are various types of atomic clock including Caesium, Rubidium and Hydrogen. Today, the most accurate atomic clocks use caesium atoms and the clocks themselves are typically operated by research facilities and National Standards Agencies.

The basis of an atomic clock was first proposed in 1945 by Columbia University Professor Isidor Rabi, who theorised the development of a clock using atomic beam magnetic resonance. The very first atomic clock became operational in 1949, and was maintained by the National Bureau of Standards, known today as the National Institute of Standards and Technology, (NIST). This first atomic clock used ammonia molecules, it was in 1955 that the first atomic clock using Caesium atoms, designed by Louis Essen, was built at the National Physics Laboratory in the UK.

Coordinated Universal Time (UTC)

Coordinated Universal Time, (UTC), is the primary standard adopted throughout the world to regulate time, referenced by highly accurate atomic clocks.

UTC utilises a 24 hour clock and is based on the local standard time on the 0° longitude meridian running through Greenwich, UK. Being based on atomic measurements as opposed to the earth’s rotation, UTC divides the world into 15-degree wide longitudinal bands which create the 24 basic time zones around the world. Time zones are then organised as positive or negative offsets of Coordinated Universal Time.


DCF-77 is a German long-wave time signal. The transmission is based at Mainflingen, close to Frankfurt, Germany and is controlled by Germany’s National Physics Laboratory, Physikalisch-Technische Bundesanstalt (PTB), while being operated by Media Broadcast GmbH.

Operating at a frequency of 77.5kHz, the signal is generated by caesium atomic clocks linked with master clocks at the PTB. The broadcast provides continual 24 hour operation and features 2 transmitters, a primary transmitter and a back-up transmitter. Coverage of the DCF-77 broadcast includes most of central and western Europe.

External Reference Clock

External reference clocks are a source of precise time information. There are numerous external reference clocks available which include the Global Positioning System and Radio time and frequency broadcasts.

Fibre Optic

Fibre optic is a method of information transference using pulses of light through an optical fibre, made of transparent fibres of glass or plastic. An optical signal is created by a transmitter, and sent along a length of fibre optic cable, on receipt the signal is converted to an electrical signal.

Fibre optics is widely used in the telecommunications industry. It is recognised as having a very high data transmission rate and is ideal for long distance applications.


Galileo is a global navigational satellite system commissioned by the European Union and which is currently under construction.

It is intended that Galileo will work as an independent system to the United States GPS system and the Russian Glonass system, giving EU countries a guaranteed service. Named after the Italian astronomer Galileo Galilei, the system will encompass 30 satellites and is estimated to be fully operational by 2019.

Global Positioning System – GPS

The GPS system consists of 24 satellites which orbit the Earth. Each of the satellites has on board a highly accurate atomic clock, ground based GPS receivers can obtain the precise reliable time information GPS offers for synchronization of a devices internal system time.

The GPS signal is available from anywhere in the world and the format of the signal is also universal. NTP time servers reference the GPS system as a reliable external time source and GPS is recognised as being the favoured option for time synchronization across computer networks worldwide.

Greenwich Mean Time – GMT

GMT was established in 1884 at the International Meridian Conference with the purpose of agreeing on a universal time standard. It was the original term used for the mean solar time at the prime meridian, longitude (0°) where east meets west, and Greenwich, London, was proclaimed as this location, hence the name Greenwich Mean Time. International time zones were then acknowledged as offsets to GMT

GMT was also the same as Universal Time (UT), a standard astronomical concept referenced in various scientific and technical arenas. In 1972 the introduction of Coordinated Universal Time (UTC) overtook GMT as the primary international time standard. UTC references highly accurate atomic clocks which are the most accurate devices that exist for time measurement. Time zones worldwide are now recognised as offsets of UTC.


GMT is the abbreviation for Greenwich Mean Time. Once used as the primary standard for regulating time, GMT was superseded by Coordinated Universal Time, (UTC), for international time measurement.

See Greenwich Mean Time for more information.


GPS is an acronym for Global Positioning System. Originally developed by the US military to provide accurate global positioning information for navigational purposes, the satellite based navigational system also provides precise time information.

Available globally, the GPS system is utilised by time synchronization devices worldwide as a reliable and highly accurate source of time information.

For more information see Global Positioning System.

Leap Seconds

A leap second is an adjustment of one second to the universal time standard, Coordinated Universal Time (UTC). Introduced in 1972 its aim is to bring UTC time in line with the Earth’s rotation which is slowing down and irregular due to climatic and geological events.

UTC is based on highly accurate atomic clocks without the leap second being added atomic time and time determined by the rotation of the Earth would drift apart. Since 1972 there have been 25 leap seconds added.

The decision to insert a leap second is made by the International Earth Rotation and Reference Systems Service (IERS). Adjustments are carried out on the last day of either June or December.
The last leap second was added on June 30 2012, and is added at the end of the day UTC is shown as 23:59:60 before turning to 00:00:00.

Master Clock

A master clock is a highly accurate clock which obtains and maintains time information from an external time source, such as GPS or radio time broadcasts. A master clock then shares this accurate time information with time clients which are often given the term ‘slave clocks’.

Adopting a master clock system provides a synchronized time/clock network with time clients operating and applying the same synchronized time stamp. A master clock system is utilised for numerous time critical applications throughout the financial and industrial sector.


MSF is the call sign to a UK-based radio time and date transmission. The MSF signal was initiated in 1950, and was originally transmitted from Rugby, Warwickshire. Due to the location of the transmission it was commonly referred to as ‘the Rugby Clock’, however in 2007 the transmission was moved to Anthorn in Cumbria.

The MSF signal is transmitted at 60kHz and covers the UK and some parts of northern and western Europe. The time information is traceable to caesium atomic clocks located at the transmitter site and is monitored by the UK’s National Physics Laboratory (NPL) in Teddington. The service is provided by Babcock International under licence from the NPL and generally provides a continuous service. The signal is however taken out of service for scheduled and unscheduled maintenance and being the sole UK transmitter no reserve transmission is available during this period.

Network Time Protocol

Network Time Protocol, (NTP), is a protocol used for computer time synchronization throughout the Internet and local computer networks.

Developed by Dr David Mills of the University of Delaware in the mid 1980’s NTP aims to synchronize computer time to within a few milliseconds of Coordinated Universal Time, (UTC). NTP utilises a hierarchy of time sources as a means of distributing time information. Each level of the hierarchy is a termed a ‘stratum’ and is followed by a number.

At the top of the hierarchy is stratum 0 which are highly accurate time sources, for example atomic clocks, the Global Positioning System (GPS), or radio time and frequency broadcasts, which NTP references to determine true time. Stratum 1 is the level below, here computer devices synchronize directly to the nominated Stratum 0 time source. Below this are Stratum 2 devices which synchronize to the Stratum 1 devices on the level above. In turn below this is Stratum 3, devices here synchronize to Stratum 2 servers and so on.

Network Time Server

A network time server is a network device which synchronizes its internal system time to a precise external time source, such as GPS or radio time and frequency broadcasts. The network time server then distributes this accurate time information to time clients across a computer network using the Network Time Protocol, (NTP), which is a widely used standard for distributing time over the Internet.


NTP is the abbreviation for Network Time Protocol, which is a standard used for synchronizing computers and computer network equipment to an accurate time source.

For further information see Network Time Protocol.

NTP Server

A NTP server utilises the Network Time Protocol, (NTP), to circulate accurate time information to sub-devices for synchronization.

Also see Network Time Server.

Pulse Per Second

A pulse that is created once every second, often abbreviated to PPS.

Radio Reference Clock

A radio reference clock is a clock which obtains time information from a radio time and frequency broadcast.
Various countries transmit radio time broadcasts. MSF is a UK-based radio time transmission from Anthorn in Cumbria and transmitted at a frequency of 60kHz. DCF-77 is German transmission from Mainflingen near Frankfurt and broadcast at 77.5kHz. The USA, Canada, France and Japan all have their own radio time transmissions which vary in broadcast frequency.


Recommended Standard 232, or RS-232 in its abbreviation, is a telecommunications standard for binary serial communications between devices. It provides a means of communication between devices using serial ports. Typically the devices are referred to as data terminal equipment, (DTE), and data communications equipment, (DCE).

A RS-232 port can be found in computers and other computer network equipment for connections to other devices.

Slave Clock

A slave clock is a clock which is connected to a master clock for its time information. As such the slave clock is not operating independently but is being controlled at a central point. Utilising a master clock system can provide a synchronized time network, with slave clocks all operating the same time stamp which is given to them by the master clock.


SNMP is the abbreviation for Simple Network Management Protocol, and is an Internet standard for managing devices on IP networks. As a reporting and monitoring facility of networked attached devices including routers, workstations, servers etc in networked management systems, it detects instances which may require attention by IT administrators.


SNTP is Simple Network Time Protocol and as it suggests is a simplified version of the NTP protocol. NTP can be overly complicated for some systems, SNTP features less internal algorithms that are not required for all servers. SNTP does not however, feature the high level of accuracy characterised in NTP.


Stratum refers to the levels in the NTP hierarchy and denote the distance a network device is from a reference clock.

Stratum 0 is the top of the NTP hierarchy and these are devices that act as time sources, atomic clocks, the Global Positioning System or radio time transmissions.

Stratum 1 is the next level down and these devices synchronize directly to the accurate time references in Stratum 0.

Stratum 2 devices are synchronized to stratum 1 servers on the level above.

Stratum 3 devices are synchronized to stratum 2 devices on the level above, and so on.


TCXO is the abbreviated term for Temperature Controlled Crystal Oscillator. A TCXO is a precision oscillator utilised in a wide number of applications where a precision frequency source is required. It gives a higher degree of temperature stability than a standard crystal oscillator.

Incorporated in TimeTools Ltd SR9750 and SR9860D models, the TCXO oscillator provides an oscillation frequency to keep track of time, operating as an accurate reference clock for extended holdover.

Time Server

A time server is a device which obtains accurate time from an external reference clock, such as GPS or radio time broadcasts and shares this precise time information with time clients across a computer network for synchronization. Typically using the NTP protocol for distribution of time information, a time server can be either a ‘local’ time server or an ‘internet’ based time server. Internet-based time servers do require a port in your firewall being left open, which can introduce a risk to network security. A local time server resides inside your firewall so your computer network remains protected.

Time Synchronization

Time synchronization provides a solution to multiple computer network devices operating with different internal system times.

A synchronized time system means devices across a computer network reference a master clock for time information and to synchronize internal system clocks. Network devices can then operate running the same accurate time stamp.


UTC is an acronym for Coordinated Universal Time.

Introduced on 1 January 1972, UTC is the global standard for time measurement. Referencing a caesium atomic clock, UTC demonstrates a much higher degree of accuracy than the previous world time standard, Greenwich Mean Time, GMT, which derived time on an astronomical basis.

Operating on a 24 hour clock, time zones worldwide are referenced as a positive or negative offset of UTC time.

Also see Coordinated Universal Time.