Computer and Network Timing Technical Glossary


Atomic Clock

An atomic clock is a timekeeping device regulated by atomic oscillations, and is recognised to be the most accurate means of time measurement available today.

The first atomic clock, Caesium 1, was built in the UK at the National Physics Laboratory in 1955, having been designed by Louis Essen, a graduate from Nottingham University. Mechanical clocks reference the oscillation of pendulums or balance wheels, but the frequency of oscillation is not constant and can be compromised by environmental influences, such as temperature. The frequency of caesium atoms oscillations is extremely high at exactly 9,192,631,770 cycles per second, resulting in a higher degree of accuracy being attained.

Other types of atomic clock include rubidium and hydrogen, but all are extremely expensive devices. Most atomic clocks are found in National Standard Agencies and Educational Research facilities. Modern atomic clocks have attained an even greater level of accuracy and are referenced worldwide as a source of precise time.

Coordinated Universal Time (UTC)

Often abbreviated to UTC, Coordinated Universal Time has over-taken Greenwich Mean Time as the worlds principal system for time measurement. Time zones internationally are observed as offsets, either positive or negative, of UTC time.

Coordinated Universal Time is determined by atomic clocks in different locations around the world. Coordinated by the International Bureau of Weights and Measures in Paris, UTC is founded on International Atomic Time (TAI) and takes the format of a 24 hour clock. UTC is accepted as demonstrating a much higher level of accuracy than GMT which is based in astronomical measurements.


DCF-77 is the call sign of a radio time and frequency transmission, broadcast from Mainflingen in Germany . Broadcast at a frequency of 77.5 kHz, the transmission is available 24 hours a day.

The Mainflingen site operates 2 transmitters to maintain provision of service should downtime be required for essential maintenance to the primary transmitter. The DCF-77 transmission can be received in many parts of Europe, indeed signal reception can be as far as 2,000km from the broadcast site.

External Reference Clock

An external reference clock provides highly accurate timing information. There are different types of external reference clock, for example radio time and frequency broadcasts, and the Global Positioning System (GPS). Of these, GPS is considered to offer a higher level of timing accuracy.

Fibre Optic

Fibre optic technology is extensively used in the telecommunication industry and involves the transmission of light signals in relaying information over long distances. Optical fibre consists of extremely long, thin strands of glass, comparable to a human hair.

The optical signal is generated by a transmitter, which on receipt is converted to an electrical signal for communication with network devices. Ideal for long distance cable runs optical fibre, unlike electrical cable installations is immune to electro-magnetic interference.


Galileo is a new, EU authorised, global satellite navigational system, initiated as an alternative and independent system to the US owned Global Positioning System, and the Russian GLONASS system.

While it is highly unlikely, access to satellite navigational systems owned and governed by other countries do have the potential to be limited or even withdrawn in times of political conflict. The introduction of the Galileo system provides an assurance to provision of service to EU nations.

When fully commissioned the Galileo system will incorporate 30 satellites in total, 27 will be operational with 3 in reserve for spares. While presently still in development, the Galileo system is estimated to be in service by 2019.

Global Positioning System (GPS)

The Global Positioning System is an US owned satellite navigation system, which was originally developed as a military application to provide precise positioning information for global navigation.

A network of 24 satellites make up the GPS system and the highly accurate atomic clocks each satellite has on board provides precise time information necessary for the triangulation process utilised in attaining precise location data.

Many civilian applications for GPS clocks are now prevalent. The precise timing information available through GPS can be referenced by TimeTools GPS synchronised time server equipment to provide a global, reliable, and highly precise solution to synchronised computer time.

Greenwich Mean Time (GMT)

Greenwich Mean Time refers to the ‘mean solar time’ at the Royal Observatory, Greenwich, England, and was utilised historically as a global time standard. However, as the earth does not orbit the sun at an even pace, noon at GMT is the annual average/mean time the sun crosses the Greenwich meridian, also referenced as the prime meridian.

Coordinated Universal Time has since replaced GMT as the primary time standard as it can offer a higher level of timing accuracy due to being based on caesium atomic clocks.


Acronym for Greenwich Mean Time.


Acronym for Global Positioning System, a global satellite navigational system commissioned by the US military.

GPS is available worldwide as a civilian application and referencing the GPS system is completely free of charge. Offering highly accurate time information, the GPS system is an ideal reference for computer network time synchronisation solutions.
Also see Global Positioning System

Internet Time Server

An internet time server is a server that is available to the public over the internet, which can provide computers and computer network equipment with time information from an external reference clock using the Network Time Protocol.

Internet time servers are typically provided by research facilities and government standards agencies and are completely free of charge. However referencing an internet based time server does require a port to be left open in your firewall, which can pose a significant risk to network security.
Furthermore, the accuracy of the timing information provided has been questioned by some agencies and also the service can be withdrawn at any time, without prior notification.

Utilising a local time server is a more secure and reliable solution. A local time server resides inside your fire wall, thus maintaining network security. Also you have sole responsibility for the configuration of the time server, ensuring it is correct and providing you with precise time information. Furthermore as you have become the service provider yourself, continuation of service is under your own control.

Leap Seconds

Coordinated Universal Time, the primary standard for timekeeping worldwide, references extremely accurate atomic clocks. A leap second is the occasional adjustment of one second to UTC time which is necessary to keep it close to mean solar time.

The slowing of the Earth’s rotation means solar time ebbs away from the highly precise time measurements of atomic clocks and UTC. The implementation of leap seconds have been carried out since 1972, and are authorised by the International Earth Rotation and Reference Systems Service. Adjustments only take place at the end of the month of June or December and are made at the end of the day.

Master Clock

A master clock is a central point of reference for obtaining accurate time information that has been received from an external reference clock, for instance GPS or a radio time and frequency transmission, and which is then shared across a network to other network devices.

For modern business environments operating over large networks and carrying out shared tasks a master clock system is essential for the accurate logging of events that have taken place over that network. The many time critical applications of today require accurate time synchronisation for the purpose of legal traceability.


MSF is a UK based radio time broadcast which since 2007 has been transmitted from Anthorn, Cumbria. Historically it was known as the ‘Rugby Clock’ due its original location from 1950 to 2007 being Rugby, Warwickshire

The MSF signal operates at a frequency of 60kHz and is typically available 24 hours a day, though it is taken off-air on occasion for maintenance works to be carried out. Radio signal reception is often limited to national boundaries. Geographical location can have a significant influence on signal reception and reliability.

Network Time Protocol

Network Time Protocol is an Internet protocol utilised to distribute accurate time information to computers and computer networks.

Network Time Server

This is a computer server which operates to supply a precise time stamp to other computer network equipment for the purpose of synchronisation.

The network time server synchronises its internal system clock to an external reference clock – either the GPS system or a radio time and date transmission. Utilising NTP the time server’s accurate time stamp is then shared with nominated network time clients who synchronise their internal system clock to the precise time information that has been provided.


NTP stands for Network Time Protocol and is an Internet standard protocol used by computer equipment to synchronise their internal system time to an accurate time reference.

For more information see Network Time Protocol.

NTP Clock

A NTP clock is an appliance that utilises the network time protocol over an ethernet network to provide an accurate time display using either large LED digits or an analog clock face.

NTP Server

A NTP server, also known as a Network Time Server, is a computer network device which features precise time-keeping. The NTP server operates to distribute precise time information to other computer equipment across a computer network utilising the Network Time Protocol

For further information see Network Time Server


Acronym for Pulse Per Second

Pulse Per Second

An electrical signal that accurately pulses once every second for precise time measurement.

Radio Clock Reference

A radio clock reference is a source of precise time information received through a national radio time and frequency transmission.

The UK offers a radio clock reference through the MSF signal broadcast from Anthorn, Cumbria. The UK transmission was formally known as the Rugby Clock as its previous location was Rugby, Warwickshire. Other nations also offer radio time transmissions, including Germany which operates the DCF-77 signal, USA, Canada and Japan. Operating a different frequencies, reception of radio transmissions are often limited to national boundaries.


RS232 is a telecommunications standard. RS232 ports are featured on computers and computer network equipment for connection/communication to other equipment.

Slave Clock

The term slave clock is used for a devices which does not control its own internal system time, but synchronises to a master clock.


SNMP stands for Simple Network Management Protocol. It is an Internet standard for managing network equipment, which highlights and reports on any problems or issues which require intervention by IT personnel.


SNTP is an acronym for Simple Network Time Protocol, and is a variant of NTP. The internal algorithms encompassed in NTP can be too complex for some systems. SNTP addresses this issue by providing a less complex version of NTP. SNTP does however exhibit a lower degree of accuracy.


NTP is defined in a hierarchical format, the various levels in the NTP hierarchy are known as a ‘stratum’ and are allotted a number. The number attached to the stratum signifies how far down the chain a time client is to the reference clock.

Stratum 0 is the highest level of the hierarchy and denotes the external time clocks that are used as precise time references, these include atomic clocks, the GPS system, and radio time broadcasts. The level below is stratum 1 and devices here synchronise directly with the accurate time sources of stratum 0.

As you continue down the hierarchy each stratum synchronises to the stratum level above it. As such stratum 2 devices synchronise to stratum 1 devices, and stratum 3 devices synchronise to stratum 2.

Temperature Controlled Crystal Oscillator – TCXO

A TCXO is a type of crystal oscillator, which is used as a precision frequency source. As the name suggests a TCXO is a crystal oscillator which features a temperature control to improve performance when compared to the standard crystal oscillators which can be found in quartz watches and clocks.

Featured in the top of the range models of TimeTools SR and SC Series NTP Time Servers, the purpose of the TCXO is to maintain accurate time providing extended holdover should the primary time source be lost.

Time Server

A time server is a computer network device which operates to provide clients across that network with precise time information to which they can synchronise their internal system clock.
Modern business environments typically operate a high volume of devices across a network. To effectively manage multiple devices across a network is has become increasingly important to have consistent, and precise time stamps. TimeTools SR and SC series NTP Time Servers reference precise time information from the GPS system or radio time and frequency broadcasts, for accurate time synchronisation of computers and computer networks.

Time Servers are available on the Internet, but these require a port to be left open in your firewall placing network security potentially at risk.

Time Synchronisation

The internal system clocks of individual computers, are not intended to be very accurate and each computer, if left to their own devices, will feature a slightly different time stamp. Time synchronisation introduces a master clock system whereby participating computers all reference time from a central point, namely the master clock, to which they synchronise their internal system time.


UTC is the abbreviated term for Coordinated Universal Time, and has become the primary standard to regulate time throughout the world. Greenwich Mean Time previously dominated as the world time standard, however time zones around the world are now determined as offsets of UTC time, which is based on highly accurate atomic clocks – the most accurate devices for time measurement available today.

For further information see Coordinated Universal Time.














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