A network time server is just a name given to any device that controls time on a network. This does not necessarily need to be a dedicated server as any workstation, equipped with a time protocol such as NTP (Network Time Protocol). However, for most network administrators, for reasons of security and accuracy, opt to use a dedicated network time server. For the purposes of this article it is these dedicated devices that we will discuss.
The types of dedicated network time servers really come down to how the time severe receives its timing source. While it is possible for a workstation to receive an Internet timing source that relays UTC (coordinated universal time) that is neither, accurate or secure for any serious time synchronisation needs.
There are two secure and accurate methods that a network time server can receive a UTC timing source. The first is from the GPS network. This is possibly the most reliable and accurate method or receiving UTC time. As long as a GPS antenna is connected and has a good clear view of the sky accuracy to within a few milliseconds of UTC should be expected.
The other method is to use the national time and frequency transmissions that are broadcast by several national physics laboratories. These are only available in certain countries (USA, UK, Germany, France, Switzerland and Japan) and the long-wave transmissions are vulnerable to local topography.
The two types of dedicated network time server are therefore radio clock/receivers and GPS clock/receivers. For those really serious about time synchronisation there are dedicated network time servers that can receive both radio and GPS signal ensuring higher levels of accuracy and reliability.