But how accurate are all these clocks and does it matter if they are all telling different times? For our day-to-day business it probably doesn't matter too much if the office wall clock is faster than your wrist-watch your boss probably won't fire you for being a minute late. But in some environments accuracy and synchronisation are vital where a minute can make all the difference in something being sold or not or even something being stolen!



Time synchronisation in modern computer networks is essential. It not only provides the only frame of reference between all devices, it is critical in everything from securing, planning and debugging a network to providing a time stamp for applications such as data acquisition or email.

Most PC's and network devices internal clocks, called Real Time Clock chips (RTC) providing time and date information. The chips are battery backed so that even during power outages, they can maintain time. However, personal computers are not designed to be perfect clocks, their design has been optimized for mass production and low-cost rather than maintaining accurate time.

Therefore these internal clocks are prone to drift and although for many application this is can be quite adequate, often machines that work together on a network will become out of sync with each other and problems can arise particularly with time sensitive transactions. Can you imagine buying an airline seat only to be told at the airport that the ticket was sold twice because it was purchased afterwards on a computer that had a slower clock?

NTP time servers (Network Time Protocol) use a single time reference to synchronise all machines on the network to that time. This time reference can be either relative (a computer's internal clock or the time on a wrist-watch perhaps) or absolute such as an atomic clock that relays UTC time (Universal Coordinated Time) and is as accurate as is humanely possible.

Atomic clocks are the most absolute time-keeping devices accurate to a second every 1.4 million years. However, atomic clocks are extremely expensive and are generally only to be found in large-scale physics laboratories. However, NTP can synchronise networks to UTC time via an atomic clock by using either the Global Positioning system (GPS) network or specialist radio transmissions (MTF in the UK).

While some organisations have to synchronise their networks to UTC such as airlines and the stock exchange, a network can be synchronised to any time and still function, but there really is no substitute for UTC time. Not only is it more efficient to have network synchronised with the rest of the world, a UTC time source is vital in providing security against fraud, data loss and legal exposure and without it, organizations can be vulnerable and lose credibility.

NTP (version 4) can maintain time over the public Internet to within 10 milliseconds (1/100th of a second) and can perform even better over LANs with accuracies of 200 microseconds (1/5000th of a second) under ideal conditions.

Note: it is strongly recommended by Microsoft and others, that external based timing should be used rather than Internet based, as these can't be authenticated. Specialist NTP servers are available that can synchronise time on networks using either the MSF (or equivalent) or GPS time server signal.

Copyright (c) 2008 Richard Williams