Many of the terms and terminology utilised when describing the operation of NTP servers and computer timing can seem overwhelming. However, most of these terms describe fairly simple concepts. This article attempts to dig below the surface to attempt to explain many of the most commonly used terms in simple language. NTP: NTP is the short hand name given to the Network Time Protocol; it is a standard protocol for distributing time around the internet and computer networks. Precise time is passed from accurate time references to network time clients for the purpose of synchronisation.
NTP Server: A NTP Server is a device that obtains precise time from an external timing reference such as GPS and provides a precise timing resource to computer networks. Stratum: The Stratum or level of a NTP time server denotes its position in the network timing heirachy. A stratum 1 NTP server obtains time from an external timing reference. Stratum 2 devices obtain time from stratum 1 devices and pass timing information to the next level and so on. Jitter: The jitter associated with a timing reference indicates the magnitude of variance, or dispersion, of the signal. Different timing references have different amounts of jitter.
The more accurate a timing reference, the lower the jitter value. Jitter is usually measure in milliseconds. Offset: Offset generally refers to the difference in time between an external timing reference and time on a local machine. The greater the offset, the more inaccurate the timing source is.
Synchronised NTP servers will generally have a low offset. Offset is generally measured in milliseconds. Delay: Delay in a NTP server describes the round-trip delay or latency of a timing message passed from client to server and back again. The time delay is generally measured in milliseconds and is used to calculate adjustments required to account for network delays. Reference Clock: A NTP reference clock is an external hardware clock that a NTP server utilises for accurate time synchronisation. GPS and radio time and frequency broadcasts are commonly used as NTP reference clocks.
The standard NTP distribution includes source code for a large number of hardware reference clocks. MSF: MSF is a UK radio time and frequency broadcast available throughout the British Isles and much of North-Western Europe. The MSF signal is broadcast at 60kHz from Anthorn, Cumbria. NTP server systems utilise the MSF time transmission as a reference for providing accurate time. The term MSF is the call sign of the radio broadcast.
DCF-77: The DCF-77 radio time signal is broadcast from Frankfurt, Germany at 77.5Khz. The signal is a time and frequency broadcast that can be received throughout Germany and much of Western Europe. The broadcast is used by many stratum 1 NTP server systems as a precision timing reference. GPS: The Global Positioning System (GPS) is primarily intended as a global navigation system. However, GPS also provides a highly accurate precision timing reference that can operate worldwide.
GPS is a common source of time for stratum 1 NTP server systems. To conclude, most of the terminology used in computer timing can be confusing and vague. However, most of these phrases are used to describe a relatively simple concept.
Dave Evans is a technical author and engineer that has been involved in the computer timing industry for a number of years. Click here, if you would like more information about NTP server systems.
