Network Time Protocol is used to synchronize the clocks of computers on a network. How accurately it synchronizes the clocks of client computers depends on a number of factors. However, the following timings can be used as a guide:
- Public Internet NTP Servers: <10ms.
- Public Internet Servers over asymmetric connection (ADSL): <100ms.
- NTP Servers on a Local Area Network (LAN): <1ms.
Factors That Affect Timing
The accuracy to which NTP clients can synchronize their clocks to the correct time is dependent on a number of factors.
At the highest levels of the timing hierarchy, the accuracy of the root time source (Stratum 0) is important. At lower levels, the physical distance between client and server is a major factor.
Root Time Source
NTP servers rely on a source of accurate timing information, such as GPS satellites or long-wave radio broadcasts. The time source is referred to as a Stratum 0 reference. Typical accuracies of common timing references are as follows:
- Global Navigation Satellite System (GPS, Galileo, GLONASS): <50ns.
- Long wave radio time broadcasts (MSF, DCF, WWVB): <2ms.
Stratum Of Synchronizing Server
NTP implements a hierarchical system of time servers. Stratum denotes the hierarchy level of a server and represents the distance, or number of levels, between a server and the source reference clock.
At the highest level, Stratum 0 is a hardware reference clock, such as GPS or radio time sources.
Stratum 1 represents a NTP server with a hardware connection to a Stratum 0 clock.
For lower Stratum references, a server synchronized to a stratum N server automatically becomes stratum N+1. Stratum 2 servers synchronize stratum 3 servers, which in turn synchronize stratum 4 servers and so on.
The lower a server is in the stratum hierarchy, the further away it is from the root time source, the poorer accuracy becomes. Synchronizing to a higher stratum device, such as stratum 1, is best.
NTP incorporates complex algorithms to calculate network packet round-trip delays and latency in order to maintain accurate time. It assumes that network communication is symmetrical, i.e. that the transmission time of a packet is the same as the receive time.
NTP timing can be adversely affected by asymmetric networks, such as ADSL, where the transmit time is often much longer than the receive time.
Accuracy degrades the further the signal distance of the synchronizing server is from the client. If using public Internet servers, choose one as close as possible to the location of the client. Preferably in the same country, possibly even the same city.
A GPS NTP server on local area network with low network congestion provides best results. In such circumstances, client synchronization to better than 1ms is achievable.
Heavy network congestion can also adversely affect the client timing accuracy. A fast symmetric network with with plenty of spare bandwidth is best for timing.
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