This article explores how to use national time and frequency radio transmissions for network time synchronisation.
The importance of an authenticated timing reference to synchronise a computer network to, cannot be stressed highly enough.
While there are hundreds and quite possibly thousands of internet based timing sources these can’t be authenticated leaving a system open to viruses, malicious hackers or malware.
Furthermore, a survey by MIT (Massachusetts Institute of Technology) found that nearly half of internet timing sources were offset by over ten seconds and only a third could be regarded as being ‘useful ’ also it was discovered that many were too far away from peers to provide any useful accuracy.
Most dedicated network time servers are designed to receive a timing signal from the GPS (Global Positioning System), primarily because it is the most accurate and can be received from anywhere on the globe.
However, there are situations where it may not be practical to use a GPS time server. A GPS antenna has to be situated on a rooftop and have a clear view of the sky which may prove difficult if the server is on the ground floor of a multi-storey sky-scraper. Many administrators also dislike the hassle and expense of having to run a cable up a building and install an antenna or if there are possibilities the server room maybe relocated and the process has to be repeated.
Fortunately many countries’ national physics laboratories broadcast a time and frequency signal from a radio transmitter. In the US the signal is referred to as WWVB and is broadcast by NIST (National Institute for Standards and Technology) in Colorado. In the UK the National Physical Laboratory (NPL) broadcasts the MSF signal from Cumbria and similar systems are broadcast in Germany (DCF-77), Japan (JJY) and France (TDF).
Unfortunately not every country transmits a national time and frequency broadcast so if a time server is to be located outside of the US, Germany, UK, France or Japan it may be doubtful that a signal could be received (although many of the these transmissions can be received in neighbouring countries).
Radio signals are also easily susceptible to atmospheric interference and can be blocked by mountains, sky-scrapers or other topography. However, an upside to using a radio receiver is that it will receive a signal inside a building.
While a radio transmission is not as accurate as a GPS time signal a dedicated network time server receiving a radio signal can still provide accuracy between 1 – 20 milliseconds (a millisecond is 1/1000 of a second) which is more than adequate for the needs of network synchronisation.