Ntp And Network Timing Terminology Explained
Much of the terminology associated with the Network Time Protocol (NTP) and network timing systems in general can at best be described as obscure
. This article attempts to explain in clear, concise, language some of the jargon used in the computer time synchronisation industry.
NTP: The acronym NTP stands for Network Time Protocol. It is a standard protocol for the transfer of precise time around computer networks. Widely used for Internet time synchronisation, it is one of the oldest Internet protocols still in regular use today. NTP is used to synchronise network time clients to a server that maintains precise time from an external hardware reference clock.
SNTP: The acronym SNTP stands for Simple Network Time Protocol. SNTP is simplified version of the Network Time Protocol. The protocol was originally developed to be used on computer systems with less processing power or which does not require the high degree of precision provided by the full NTP protocol. Many of the complex synchronisation algorithms of NTP are removed or simplified. The Microsoft Windows operating system utilises SNTP for synchronisation.
NTP Server: A NTP Server is a device that obtains accurate timing information from an external hardware clock such as GPS or radio time references and provides a precise timing resource to a computer network.
Stratum: The Stratum of a NTP server denotes its level in the timing hierarchy. A stratum 1 NTP server obtains time from an external hardware clock timing reference. Stratum 2 devices obtain time from stratum 1 devices and pass timing information to the next level and so on. The most accurate NTP servers are stratum 1 devices; each lower level in the hierarchy is slightly less accurate than the one above.
Round-Trip Delay: The time taken from a client requesting time to a response being received from a time server is known as the round-trip delay. It is a value of the latency of a timing message passed from client to server and back again. The delay is an important value, so that network delays can be calculated and accounted for by a time client to improve timing accuracy.
Offset: In computer timing terminology, offset, indicates the time difference between an external timing reference and local system time of the host machine. The greater the offset, the more inaccurate the timing source is. Synchronised NTP servers will generally have a very low offset. Offset is generally measured in milliseconds.
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 measured in milliseconds. A GPS referenced time server typically has a jitter value in the region of a few microseconds.
Reference Clock: A NTP reference clock is an external hardware clock that a server utilises for precise 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 from a number of manufacturers.
WWVB: The call sign of the US radio time and frequency broadcast is WWVB. The transmission is available throughout the United States and much of Northern America. The radio signal is broadcast at a frequency of 60kHz from Colorado. Many computer time synchronisation systems utilise the WWVB time broadcast as a precision timing reference
MSF: The call sign of the UK radio time and frequency broadcast is MSF. The transmission is available throughout the British Isles and much of North-Western Europe. The radio signal is broadcast at a frequency of 60kHz from Anthorn, Cumbria.
DCF-77: The call sign of the German radio time and frequency broadcast is DCF-77.The signal is broadcast from Frankfurt, Germany at a frequency of 77.5Khz. The transmission can be received throughout Germany and much of Western Europe.
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 be received worldwide. GPS has a number of advantages over radio time and frequency broadcasts. It can be received anywhere in the world, rather than limited to national boundaries. It is also more accurate than radio time references, typically nanoseconds rather than milliseconds. However, a GPS antenna does need a good view of the sky, ideally rooftop mounted.
To summarise, much of the jargon relating to computer network time synchronisation and NTP server systems seems complex. However, most of these phrases are used to describe relatively simple and easily understood concepts.
by: Dave Evans
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