Time Synchronisation

Distributed networks rely completely on the correct time. Computers need timestamps to order events and when a collection of machines are working together it is imperative they run the same time.

Unfortunately modern PC’s are not designed to be perfect timekeepers. Their system clocks are simple electronic oscillators and are prone to drift. This is not normally a problem when the machines are working independently but when they are communicating across a network all sorts of problems can occur.

From emails arriving before they have been sent to entire system crashes, lack of synchronisation can causes untold problems across a network and it is for this reason that network time servers are used to ensure the entire network is synchronised together.

Network time servers come in two forms – The GPS time server and the radio referenced time server. GPS NTP servers use the time signal broadcast from GPS satellites. This is extremely accurate as it is generated by an atomic clock on board the GPS satellite. Radio referenced NTP servers use a long wave transmission broadcast by several national physics laboratories.

Both these methods are a good source of Coordinated Universal Time (UTC) the world’s global timescale. UTC is used by networks across the globe and synchronising to it allows computer networks to communicate confidently and partake of time sensitive transactions without error.

Some administrators use the Internet to receive a UTC time source. Whilst a dedicated network time server is not required to do this it does have security drawbacks in that a port is needed to be left open in the firewall for the computer to communicate with the NTP server, this can leave a system vulnerable and open to attack. Furthermore, Internet time sources are notoriously unreliable with many either too inaccurate or too far away to serve any useful purpose.

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.

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.