A NTP time server is essential if accurate time is to be maintained on a network. Network Time Protocol (NTP) uses the UDP (User Datagram Protocol) on port 123 to communicate. This prevents a the bandwidth from being clogged if the NTP Time Server goes down for any reason.
NTP works on a hierarchical basis each hierarchy is referred to as a strata. A stratum 0 server is an absolute time source such as an atomic clock, a stratum 1 server is the NTP time server that receives this time signal and server’s that get a time signal from a stratum 1 server are referred to as stratum 2 (and so up to 16 strata can be supported).
Arranging servers into a hierarchy like this prevents too much traffic from trying to communicate withe NTP time server allowing networks of hundreds and even thousands of devices to be synchronised.
It is good practice to synchronise the lower strata to multiple servers as this ensures greater accuracy.
A dedicated NTP time server should get its timing information from a stratum 0 source. The most accurate and secure way of doing this is by using the time signal from the GPS network.
A GPS antenna should be connected to the NTP time server and the antenna must be mounted on the roof to ensure a clear view of the sky (the satellite signals work by line-of-sight). This of course can be impractical (especially if your server room is on the ground floor of a skyscraper).
An alternative is to use the specialist national time and frequency transmissions that are broadcast from certain countries. The USA, UK, France Germany and Japan all have their own signals. A radio receiver can pick up these transmissions even indoors although the accuracy is slightly reduced to that of the GPS system.
A dedicated NTP time server using GPS or a radio transmission can provide accuracy to within a few milliseconds of a timing source.