The NTP server is an integral part of the modern computer network. Without Network Time Protocol and NTP time servers many of the modern functionality of computers that we take for granted such as online reservation, Internet trading and satellite communication would be impossible.
Synchronisation in computers is dealt with by NTP. NTP and NTP servers use a single time reference to synchronise all machines on a network to that time. This time reference could in fact be anything such as the time on a wrist watch perhaps. However, synchronisation is pointless unless a UTC (coordinated universal time) time source is used as UTC has been developed to allow the whole world to synchronise to the same time, allowing truly global synchronisation.
UTC is based on the time told by atomic clocks although compensation measures such as Leap Seconds are added to UTC to keep it inline with Greenwich Meantime (GMT).
Atomic clocks are very expensive and extremely delicate pieces of equipment and not the sort of thing that can be housed in the office server room. Fortunately a NTP server can receive a UTC time source from several different locations.
The Internet is perhaps the most widely used source of time references. Unfortunately however, there are draw backs in using the Internet for a timing source. Firstly the Internet timing sources can’t be authenticated. Authentication is a security measure used by NTP to check that timing source is genuine. Secondly, to use an Internet timing reference means a hole has to be left open in the network’s firewall, again compromising security. Thirdly, Internet timing sources are notoriously inaccurate and those that aren’t can often be too far away from a client to provide any useful precision.
However, if security and high level of accuracy to UTC time is not required then the Internet can provide a simple and affordable solution.
A far more secure method of receiving a UTC timing reference is to use the specialist national time and frequency transmission broadcast by several countries. The UK (MSF), USA (WWVB), Germany (DCF) and Japan (JJY) all boast a long wave timing signal. While these signals are limited in range and strength, where available they make an ideal timing source as the radio receiver can pick these signals up from inside a building. These transmissions can also be authenticated providing a high level of security.
The third and perhaps simplest solution is to use a GPS NTP server. These use the signals sent from the Global Positioning System which contains timing information. This is ideal as the GPS signal can be received literally anywhere in the world so if there is no radio transmission your area then the GPS network will provide a secure and authenticated solution.
The only downside to GPS is that an antenna has to have a good view of the sky and therefore need to be positioned on the roof. This obviously has logistical drawbacks if the server room is in the basement of a sky-scraper.
In selecting a timing source, the most important thing to remember is where the NTP server is going to be situated. If it is indoors and there is no opportunity to run and antenna to the roof then the radio transmissions would be the best alternative. If there are no radio transmission in your country/area or the signals are blocked by local topography then the GPS is an ideal solution.
However, if accuracy and security are not an issue then the Internet would be the most obvious solution.