Time Synchronisation

Time synchronisation is often described as a ‘headache’ by network administrators. Keeping computers on a network all running the same time is increasingly important in modern network communications particularly if a network has to communicate with another network running independently.

For this reason UTC (Coordinated Universal Time) has been developed to ensure all networks are running the same accurate timescale. UTC is based on the time told by atomic clocks so it is highly precise, never losing even a second. Network time synchronisation is however, relatively straight forward thanks to the protocol NTP (Network Time Protocol).

UTC time sources are widely available with over a thousand online stratum 1 servers available on the Internet. The stratum level describes how far away a time server is to an atomic clock (an atomic clock that generates UTC is known as a stratum 0 device). Most time servers available on the Internet are in fact not stratum 1 devices but stratum in that they get their time from a device that in turn receives the UTC time signal.

For many applications this can be accurate enough but as these timing sources are on the Internet there is very little you can do to ensure both their accuracy and their precision. In fact even if an Internet source is highly accurate the distance away form it can cause delays int eh time signal.

Internet time sources are also unsecure as they are situated outside of the firewall forcing the network to be left open for the time requests. For this reason network administrators serious about time synchronisation opt to use their own external stratum 1 server.

These devices, often called a NTP server, receive a UTC time source from a trusted and secure source such as a GPS satellite then distribute it amongst the network. The NTP server is far more secure than an Internet based time source and are relatively inexpensive and highly accurate.

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.