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A clock network or clock system is a set of synchronized
clock A clock or chronometer is a device that measures and displays time. The clock is one of the oldest Invention, human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month, a ...
s designed to always show exactly the same time by communicating with each other. Clock networks usually consist of a central master clock kept in sync with an official time source, and one or more slave clocks which receive and display the time from the master.


Synchronization sources

The master clock in a clock network can receive accurate time in a number of ways: through the
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
GPS satellite constellation, a
Network Time Protocol The Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-Network latency, latency data networks. In operation since before 1985, NTP is one of the oldest Intern ...
server, the
CDMA Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communicatio ...
cellular phone network, a
modem The Democratic Movement (, ; MoDem ) is a centre to centre-right political party in France, whose main ideological trends are liberalism and Christian democracy, and that is characterised by a strong pro-Europeanist stance. MoDem was establis ...
connection to a time source, or by listening to radio transmissions from WWV or
WWVH WWVH is the callsign of the U.S. National Institute of Standards and Technology's shortwave radio time signal station located at the Pacific Missile Range Facility, Pacific Missile Range Facility Barking Sands, in Kekaha, Hawaii, Kekaha, on the ...
, or a special signal from an upstream broadcast network. Some master clocks don't determine the time automatically. Instead, they rely on an operator to manually set them. Clock networks in critical applications often include a backup source to receive the time, or provisions to allow the master clock to maintain the time even if it loses access to its primary time source. For example, many master clocks can use the reliable
frequency Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
of the
alternating current Alternating current (AC) is an electric current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current (DC), which flows only in one direction. Alternating current is the form in w ...
line they are connected to.


Slave clocks

Slave clocks come in many shapes and sizes. They can connect to the master clock through either a cable or a short-range wireless signal. Between the late 19th and early 20th centuries, Paris used a series of pneumatic tubes to transmit the signal. Some slave clocks will run independently if they lose the master signal, often with a warning light lit. Others will freeze until the connection is restored.


Clock synchronization

Many master clocks include the capability to synchronize devices like computers to the master clock signal. Common features include the transmission of the time via
RS-232 In telecommunications, RS-232 or Recommended Standard 232 is a standard introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a ''DTE'' (''data terminal equipment'') such as a compu ...
, a Network Time Protocol, or a Pulse Per Second (PPS) contact. Others provide
SMPTE The Society of Motion Picture and Television Engineers (SMPTE) (, rarely ), founded by Charles Francis Jenkins in 1916 as the Society of Motion Picture Engineers or SMPE, is a global professional association of engineers, technologists, and e ...
time code outputs, which are often used in television settings to synchronize the video from multiple sources. Master Clocks often come equipped with programmable relay outputs to synchronize other devices such as lights, doors, etc.


Applications

One of the driving factors in developing clock networks was the broadcast industry.
Television Television (TV) is a telecommunication medium for transmitting moving images and sound. Additionally, the term can refer to a physical television set rather than the medium of transmission. Television is a mass medium for advertising, ...
, in particular, operates on a very strict schedule, where each second of airtime is planned ahead of time and must be executed precisely. A central clock system allows a television station's
master control Master control is the technical hub of a broadcast operation common among most over-the-air television stations and television networks. It is distinct from a production control room (PCR) in television studios where the activities such as swit ...
and production personnel to work within that schedule. A clock network synchronized to the standard
UTC Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communica ...
time also allows different television facilities to coordinate their activities without complicated out-of-band signaling. It also provides accurate timing to equipment in stations that are becoming increasingly automated. While television broadcasters were some of the first users of clock networks, the equipment is becoming increasingly useful in other industries. For example, the
National Emergency Number Association The National Emergency Number Association (NENA) is an organization whose mission it is to foster the technological advancement, availability, and implementation of a universal emergency telephone number system in the United States. In carrying o ...
issued directiv
NENA-04-002
offering standards in timekeeping for 911 dispatch centers throughout the United States. Other common clock network users include airports and schools.


History

One of the first clock networks was installed by Charles Shepherd for the
Great Exhibition The Great Exhibition of the Works of Industry of All Nations, also known as the Great Exhibition or the Crystal Palace Exhibition (in reference to the temporary structure in which it was held), was an international exhibition that took ...
, held in London in 1851. Shepherd's technology was then installed at the
Royal Greenwich Observatory The Royal Observatory, Greenwich (ROG; known as the Old Royal Observatory from 1957 to 1998, when the working Royal Greenwich Observatory, RGO, temporarily moved south from Greenwich to Herstmonceux) is an observatory situated on a hill in G ...
, and a replica of his Shepherd Gate Clock outside the gate is still working, the original being severely damaged in a WWII air raid. In the period before the universal availability of A.C. mains or
atomic clock An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...
s, many clock networks were installed using a highly accurate
pendulum clock A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is an approximate harmonic oscillator: It swings back and forth in a precise time interval dep ...
as a master clock. This clock resembled a longcase clock, but had a very robust mechanism and a less ornate case. Electrical contacts attached to the mechanism generated minute, half minute and sometimes one second electrical pulses which were fed to the slave clocks on pairs of wires. The devices driven could be wall clocks, employee time clocks, tower clocks and occasionally clock chiming mechanisms. Some master clocks were set up to control the frequency of a generating authority's mains output; others, in the UK, were arranged to synchronise themselves with the
Greenwich Time Signal The Greenwich Time Signal (GTS), popularly known as the pips, is a series of six short tones (or "pips") broadcast at one-second intervals by many BBC Radio stations to mark the precise start of each hour. The pips were introduced in 1924, gene ...
pips.


Clock strata

There is a hierarchy of clocks in a network, reflecting their quality. For example: * NTP clock strata * Synchronization in telecommunications#Components


See also

*
Radio clock A radio clock or radio-controlled clock (RCC), and often colloquially (and incorrectly) referred to as an "atomic clock", is a type of Quartz clock, quartz clock or watch that is automatically Synchronization, synchronized to a time code transmi ...
* Master clock * Slave clock *
Time clock A time clock, sometimes known as a clock card machine, punch clock, or time recorder, is a device that records start and end times for hourly employees (or those on flexi-time) at a place of business. In mechanical time clocks, this was accompl ...
*
Time signal A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day. Church bells or voices announcing hours of prayer gave way to automatically operated chimes on public clocks; however, au ...
*
Time standard A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of cu ...
*
Clock distribution network In electronics and especially synchronous digital circuits, a clock signal (historically also known as ''logic beat'') is an electronic logic signal (voltage or current) which oscillates between a high and a low state at a constant frequency and ...


References

{{reflist


External links


ClockHistory.com
Clocks Broadcast engineering