Time Synchronisation

Using a long wave time and frequency transmission is perhaps the simplest and most efficient way of receiving an accurate and secure UTC timing reference (coordinated universal time). Dedicated NTP servers are available that receive a time code this way and distribute the timing information to a network. Often these time servers are referred to as radio clocks, although this title is a little misleading.

The long wave transmissions are usually broadcast at 60 khz but are not available everywhere. Only certain countries have these broadcasts and most come from their country of origin’s national physics laboratory.

In the UK the signal is known as MSF as is broadcast by the NPL (National Physical Laboratory) in Cumbria. The USA signal, WWVB, is broadcast Near Fort Collins in Colorado while the signal in Germany is known as DCF and is broadcast near to Frankfurt. Other nations such as Switzerland, Japan and Finland also have their own signals.

These transmissions are not however, available everywhere. While in many neighbouring countries it is possible to receive one of these transmissions, the long-wave signal is finite in range and susceptible to interference from topography and other electrical devices

However, where they are available, these time and frequency signals make an ideal source for a NTP server to synchronise a network too making them a logical choice for securing UTC time.

Time servers are incredibly important but an often overlooked part of a computer network. They are essential devices ensuring that all computers and devices connected to the network are synchronised to the same time. This prevents any problems from occurring when networks run sporadic time such as emails arriving before they were sent or even more serious security issues.

Time servers are similar to other servers but their sole role is to receive the time from a trusted source and distribute it amongst the network. The network time server can receive this time source from several places but in doing so one must have in mind two questions: How accurate is the time reference supplying the time? And how secure is it?

There are two highly accurate and completely secure methods of receiving the time for a time server. Both methods supply UTC (Coordinated Universal Time) which is a global timescale maintained by highly accurate atomic clocks. The first is to utilise the Global Positioning System (GPS) whose satellites all have onboard an atomic clock or the second is to use the national physics’ laboratories time and frequency transmissions.

Once a timing reference is received it is distributed to machines on a network using the protocol NTP (Network Time Protocol). NTP is based on an algorithm that not only corrects the time on any device but also ensures that the UTC time being received is secure, stable and precise.

NTP is one of the oldest Internet based protocols having been around since the 1980’s yet it is a testament to its developers that over twenty years on it is not only still in use but is by far the most common time protocol in use.

GPS time servers are often called many things: NTP time servers, GPS network time servers, GPS NTP servers etc. A time server is merely a device to that computer’s can contact to receive timing information from for purposes of time synchronisation.

The way a time server receives the time is what defines it. A radio referenced time server will receive a time signal from a national physics laboratory via a long wave radio signal. A GPS time server receives a time signal from the Global Positioning System a constellation of satellites designed to provide navigation information.

What makes GPS possible is that onboard each global positioning satellite there is an atomic clock. The time from this clock is broadcast along with the position and velocity of the satellite. It is this information that a satellite navigation receiver uses to work out position by triangulation. It receives the same data from three or more satellites and works out by the time it takes for the transmission from each satellite to reach the receiver.

While the atomic clocks onboard the GPS satellites do not broadcast UTC (Coordinated Universal Time – the civil global timescale) because it is an atomic clock signal and therefore extremely reliable, a GPS time server can easily translate the GPS time into UTC.

Time servers are like other computer servers in the sense they are usually located on a network. A time server gathers timing information, usually from an external hardware source and then synchronises the network to that time.

Often time servers are synchronised to a UTC (Coordinated Universal time) source which is the global standard time scale and allows computers all over the world to synchronised to exactly the same time. This has obvious importance in industries where exact timing is crucial such as the stock exchange or airline industry.

There are various sources that a time server can use as a timing reference. The Internet is an obvious source, however, internet timing references from the Internet such as nist.gov and windows.time can not be authenticated, leaving the time server and therefore the network vulnerable to security threats.

There are authenticated alternatives to the Internet, the most common being to use the GPS network. As the Global Positioning System is reliant on knowing exactly what time it is to ensure reliable location information, this information can be utilised by a time server.

A simple GPS antenna connected to the time server will allow the GPS timing reference to be regularly checked by the time server. A GPS time server will be accurate to within a few hundred nanoseconds (a nanosecond = a billionth of a second).

There are also a number of national radio broadcasts such as the WWVB signal from Colorado in the US, the MSF signal from Cumbria in the UK and the DCF-77 signal from Frankfurt in Germany.

These radio signals are limited in their range though and even in major cities such as London it can be difficult to receive a decent enough signal.

Most timing servers use NTP (Network Time protocol) there are other protocols available but NTP is predominately used and is thought of as the standard for timing protocols. NTP has been around for over 25 years and is currently on version 4 but is always being updated which is probably why it is by fat the most common timing protocol.

NTP time servers work within the TCP/IP suite and rely on UDP (User Datagram Protocol). A less complex form of NTP – Simple Network Time Protocol (SNTP) is used in some devices and applications where high accuracy timing is not as important and is also included as standard in Windows software (although more recent versions of Microsoft Windows have the full NTP installed and the source code is free and readily available on the Internet from NTP.org).