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.

Dedicated network time servers are on the whole reliable pieces of hardware and very rarely do they fail to receive and distribute a timing signal.  However, some common errors do occur:

Network time servers that receive a time signal from the GPS satellite system can often fail to find a signal if they have not been given enough time to lock on to a satellite. All GPS network time servers should be left on for at least 48 hours to ensure the time server can get a stable lock on a the satellites.

Network time servers that receive a radio signal can also fail, the most common reason is that the signal is being blocked by interference. Move the time server away from any other electrical equipment; even just tilting the network time server can increase the signal strength as the antenna should be perpendicular to the signal.

Radio signals are also disrupted for scheduled maintenance users of the UK’s MSF signal should check with the National Physical Laboratory for planned outages.

Network time servers are also vulnerable to over subscription if a network consists of a lot of machines all trying to receive a time signal then the bandwidth can be taken up with time requests. Ensure the correct use of stratum to alleviate the problem.

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.