Time servers are like other computer servers in the sense they are usually located on a network. A time server gathers timing information, usually from an external hardware source and then synchronises the network to that time.
Often time servers are synchronised to a UTC (Coordinated Universal time) source which is the global standard time scale and allows computers all over the world to synchronised to exactly the same time. This has obvious importance in industries where exact Timing is crucial, as the Stock Exchange or airline industry.
There are various sources that can be a time server to use as a timing reference. The Internet is an obvious, but Internet timing references from the Internet as nist.gov windows.time and can not be authenticated, so that the time server and thus the network vulnerable to security risks.
There are alternatives for authentication on the Internet, utilizing the most common to the GPS network. As the GlobalPositioning System is reliant on knowing exactly what time it is to ensure reliable location information, this information can be utilised by a time server.
A simple GPS antenna connected to the time server will allow the GPS timing reference to be regularly checked by the time server. A GPS time server will be accurate to within a few hundred nanoseconds (a nanosecond = a billionth of a second).
There are also a number of national radio broadcasts such as the WWVB signal from Colorado in the U.S., the MSF signal from Cumbria in the UK and the DCF-77 signal from Frankfurt am Main in Germany.
These signals are limited in their scope, even in big cities like London can be difficult to receive a decent enough signal.
Most of timing servers use NTP (Network Time Protocol), there are other protocols available, but NTP is mostly used and is intended as a standard for the timing protocols. NTP has over 25 years and is currently onversion 4 but is always being updated which is probably why it is by fat the most common timing protocol.
NTP time servers work within the TCP/IP suite and rely on UDP (User Datagram Protocol). A less complex form of NTP - Simple Network Time Protocol (SNTP) is used in some devices and applications where high accuracy timing is not as important and is also included as standard in Windows software (although more recent versions of Microsoft Windows have the full NTP installed and the Source code is free and easy) over the Internet from ntp.org.
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