Time Synchronisation

Time synchronisation is now a critical aspect of network management enabling time sensitive applications to be conducted from across the globe. Without correct synchronisation computer systems would be unable to communicate with each other and transactions such as seat reservation, Internet auctions and online banking would be impossible.

For effective time synchronisation the global timescale UTC (Coordinated Universal Time) is a prerequisite. While a computer network can be synchronised to any single time source, UTC is employed by computer networks all over the world. By synchronising to a UTC time source a computer network can therefore be synchronised to every other computer network across the globe that also use UTC as their time source.

Receiving a reliable UTC time source is not as easy as it sounds. Many network administrators opt to use a UTC Internet time source. Whilst many of these time sources are accurate enough, they can be too far away to provide reliability and there are plenty of Internet time sources that are vastly inaccurate.

Another reason why Internet time sources should not be used as a source of time synchronisation is because an Internet time source is outside of a firewall and leaving a gap in the firewall to receive timing information can leave a system open to abuse.

So that UTC time can be opted as a civil time throughout the world several national physics laboratories broadcast a UTC timing signal that can be received and utilised as a network time source. Unfortunately, however, these time signals are not available in every country and even in those areas where a signal exists; they can be quite often obstructed by interference and local topography.

Another method for receiving a source of UTC time is to use the GPS satellite network. Strictly speaking the Global Positioning System (GPS ) does not relay UTC but it is a time based on International Atomic Time (TAI) with a predefined offset. A GPS NTP clock can simply convert the GPS time into UTC for synchronisation purposes.

The main advantage of using GPS is that a GPS signal is available anywhere on the planet providing that there is a clear view of the sky above (GPS transmissions are broadcast via line-of-sight) so UTC synchronisation can be conducted anywhere.

Network Time Protocol is an Internet protocol used to synchronise computer clocks to a stable and precise time reference. NTP was originally developed by Professor David L. Mills at the University of Delaware in 1985 and is an Internet standard protocol.

NTP was developed to solve the problem of multiple computers working together and having the different time. Whilst, time usually just advances, if programs are running on different computers time should advance even if you switch from one computer to another. However, if one system is ahead of the other, switching between these systems would cause time to jump forward and back.

As a consequence, networks may run their own time, but as soon as you connect to the Internet, effects become visible. Just Email messages arrive before they were sent, and are even replied to before they were mailed!

Whilst this sort of problem may seem innocuous when it comes to receiving email, however, in some environments a lack of synchronisation can have disastrous results this is why air traffic control was one of the first applications for NTP.

NTP uses a single time source and distributes it amongst all devices on a network it does this by using an algorithm that works out how much to adjust a system clock to ensure synchronisation.

NTP works on a hierarchical basis to ensure there are no network traffic and bandwidth problems. It uses a single time source, normally UTC (coordinated universal time) and receives time requests from the machines on the top of the hierarch which then pass the time on further down the chain.

Most networks that utilise NTP will use a dedicated NTP  time server to receive their UTC time signal. These can receive the time from the GPS network or radio transmissions broadcast by national physics laboratories. These dedicated NTP time servers are ideal as they receive time direct from an atomic clock source they are also secure as they are situated externally and therefore do not require interruptions in the network firewall.

Network time servers are devices that use the ultra precise timing of an atomic clock to synchronise a computer network to. Unfortunately atomic clocks are both highly expensive and extremely sensitive and are therefore not the most pragmatic tool to have lingering around your server room.

However, network time servers can still receive the ultra precise timing from these machines as several atomic clocks in national physics laboratories have their time broadcast via long wave radio transmissions.

An international timescale known as UTC (Coordinated Universal Time) has been developed based on this time, meaning the entire globe can synchronise to the exact same, accurate time.

Not all national physics laboratories, however, broadcast this signal and for those residents in a country that are lucky enough to have one of these time and frequency transmissions it is not necessarily guaranteed that it can be received. The 60 kHz signal can be blocked by buildings, local topography and is susceptible to interference from other electrical equipment.

The signals that are broadcast are all transmitted at roughly the same frequency but are known as different things form country to country. In the UK the signals are transmitted by the National Physical Laboratory in Cumbria and are referred to as the MSF signal. In the USA, NIST (National Institute for Standards and Time) broadcast their WWVB timing signal from Fort Collins, Colorado while the Germans have a similar system (DSF) transmitted near to Frankfurt.

A network time server is just a name given to any device that controls time on a network. This does not necessarily need to be a dedicated server as any workstation, equipped with a time protocol such as NTP (Network Time Protocol). However, for most network administrators, for reasons of security and accuracy, opt to use a dedicated network time server. For the purposes of this article it is these dedicated devices that we will discuss.

The types of dedicated network time servers really come down to how the time severe receives its timing source. While it is possible for a workstation to receive an Internet timing source that relays UTC (coordinated universal time) that is neither, accurate or secure for any serious time synchronisation needs.

There are two secure and accurate methods that a network time server can receive a UTC timing source. The first is from the GPS network. This is possibly the most reliable and accurate method or receiving UTC time. As long as a GPS antenna is connected and has a good clear view of the sky accuracy to within a few milliseconds of UTC should be expected.

The other method is to use the national time and frequency transmissions that are broadcast by several national physics laboratories. These are only available in certain countries (USA, UK, Germany, France, Switzerland and Japan) and the long-wave transmissions are vulnerable to local topography.

The two types of dedicated network time server are therefore radio clock/receivers and GPS clock/receivers.  For those really serious about time synchronisation there are dedicated network time servers that can receive both radio and GPS signal ensuring higher levels of accuracy and reliability.