Time Synchronisation

To receive and distribute and authenticated UTC time source there are currently two types of NTP server, the GPS NTP server and the radio referenced NTP server. While both these systems distribute UTC in identical ways the way they receive the timing information differs.

A GPS NTP time server is an ideal time and frequency source because it can provide highly accurate time anywhere in the world using relatively cheap components.  Each GPS satellite transmits in two frequencies L2 for the military use and L1 for use by civilians transmitted at 1575 MHz, Low-cost GPS antennas and receivers are now widely available.

The radio signal transmitted by the satellite can pass through windows but can be blocked by buildings so the ideal location for a GPS antenna is on a rooftop with a good view of the sky. The more satellites it can receive from the better the signal. However, roof-mounted antennas can be prone to lighting strikes or other voltage surges so a suppressor is highly recommend being installed inline on the GPS cable.

The cable between the GPS antenna and receiver is also critical. The maximum distance that a cable can run is normally only 20-30 metres but a high quality coax cable combined with a GPS amplifier placed in-line to boost the gain of the antenna can allow in excess of 100 metre cable runs. This can provide difficulties in installation in larger buildings if the server is too far from the antenna.

An alternative solution is to use a radio referenced NTP time server. These rely on a number of national time and frequency radio transmissions that that broadcast UTC time. In Britain the signal (called MSF) is broadcast by the National Physics Laboratory in Cumbria which serves as the United Kingdom’s national time reference, there are also similar systems in the USA (WWVB) and in France, Germany and Japan.

A radio based NTP server usually consists of a rack-mountable time server, and an antenna, consisting of a ferrite bar inside a plastic enclosure, which receives the radio time and frequency broadcast. It should always be mounted horizontally at a right angle toward the transmission for optimum signal strength. Data is sent in pulses, 60 a second. These signals provides UTC time to an accuracy of 100 microseconds, however, the radio signal has a finite range and is vulnerable to interference.

Network Time Protocol (NTP) is one of the Internet’s oldest protocols still utilised. Invented by Dr David Mills from the University of Delaware it has been in use since 1985. NTP is a protocol designed to synchronize the clocks on computers and networks across the Internet or Local Area Networks (LANs).

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.

NTP works within the TCP/IP suite and relies on UDP, a less complex form of NTP exists called Simple Network Time Protocol (SNTP) that does not require the storing of information about previous communications, needed by NTP. It is used in some devices and applications where high accuracy timing is not as important.

Time synchronisation with NTP is relatively simple, it synchronises time with reference to a reliable clock source. This source could be relative (a computer’s internal clock or the time on a wrist-watch) or absolute (A UTC – Universal Coordinated Time – clock source that is accurate as is humanely possible).

Atomic clocks are the most absolute time-keeping devices. They work on the principle that the atom, caesium-133, has an exact number of cycles of radiation every second (9,192,631,770). This has proved so accurate the International System of Units (SI) has now defined the second as the duration of 9,192,631,770 cycles of radiation of the caesium-133 atom.

However, atomic clocks are extremely expensive and are generally only to be found in large-scale physics laboratories. However, NTP can synchronise networks to an atomic clock by using either the Global Positioning System (GPS) or a specialist radio transmission.

The most widely used is the GPS system which consists of a number of satellites providing accurate positioning and location information. Each GPS satellite can only do this by utilising an atomic clock which in turn can be can be used as a timing reference.

A typical GPS receiver can provide timing information to within a few nanoseconds of UTC as long as there is an antenna situated with a good view of the sky.

There are also a number of national time and frequency radio transmissions that can be used to synchronise a NTP server. In Britain the signal (called MSF) is broadcast by the National Physics Laboratory in Cumbria which serves as the United Kingdom’s national time reference, there are also similar systems in Colorado, US (WWVB) and in Frankfurt, Germany (DCF-77). These signals provides UTC time to an accuracy of 100 microseconds, however, the radio signal has a finite range and is vulnerable to interference.

The distance from the reference clock is known as the stratum levels and they exist to prevent cycles in the NTP. Stratum 0, are devices such as atomic clocks connected directly to a computer. Stratum 1, are computers attached to stratum 0 devices, while Stratum 2 are computers that send NTP requests to Stratum 1 servers. NTP can support up to 256 strata.

All Microsoft Windows versions since 2000 include the Windows Time Service (w32time.exe) which has the ability to synchronise the computer clock to an NTP server (or an SNTP server – a simplified version of NTP) Many LINUX and UNIX based operating systems also have a version of NTP but the source code is free to download (current version 4.2.4) at the NTP website (NTP.org).

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 time servers are available that can synchronise time on networks using either the MSF (or equivalent) or GPS signal.

All PC’s and networking devices use clocks to maintain an internal system time. These clocks, called Real Time Clock chips (RTC) provide 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.

These internal clocks are prone to drift and although for many application this can be quite adequate, often machines need to work together on a network and if the computers drift at different rates the computers will become out of sync with each other and problems can arise particularly with time sensitive transactions.

Network Time Protocol (NTP) is one of the Internet’s oldest protocols still used, invented by Dr David Mills from the University of Delaware, it has been in utilized since 1985. NTP is a protocol designed to synchronize the clocks on computers and networks across the Internet or Local Area Networks (LANs).

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.

NTP works within the TCP/IP suite and relies on UDP, a less complex form of NTP exists called Simple Network Time Protocol (SNTP) that does not require the storing of information about previous communications, needed by NTP. It is used in some devices and applications where high accuracy timing is not as important.

Many operating systems including Windows, UNIX and LINUX can utilize NTP and SNTP  and time synchronisation with NTP is relatively simple, it synchronises time with reference to a reliable clock source. This source could be relative (a computer’s internal clock or the time on a wrist-watch) or absolute (A UTC – Universal Coordinated Time – clock source that is accurate as is humanely possible).
All Microsoft Windows versions since 2000 include the Windows Time Service (w32time.exe) which has the ability to synchronise the computer clock to an NTP server.

There are a large number of Internet hosted NTP servers that synchronise with external UTC references such as time.nist.gov or NTP.my-inbox.co.uk but it must be noted that Microsoft and others recommend that an external source is used to synchronise your machines, as Internet based references can’t be authenticated. Specialist NTP time servers are available that can synchronise time on networks using either the MSF (or equivalent) or GPS signal.

The most widely used are the GPS time servers which use the GPS system to relay accurate time. The GPS system consists of a number of satellites providing accurate positioning and location information. Each GPS satellite can only do this by utilising an atomic clock which in turn can be can be used as a timing reference.

A typical GPS receiver can provide timing information to within a few nanoseconds of UTC as long as there is an antenna situated with a good view of the sky.

There are a number of national time and frequency radio transmissions that can be used to synchronise a NTP server. In Britain the signal (called MSF) is broadcast by the National Physics Laboratory in Cumbria which serves as the United Kingdom’s national time reference, there are also similar systems in Colorado, US (WWVB) and in Frankfurt, Germany (DCF-77). These signals provides UTC time to an accuracy of 100 microseconds, however, the radio signal has a finite range and is vulnerable to interference.

What time is it? One of the commonest questions uttered around the World but what exactly are we asking? You ask someone in China what the time is then you will certainly get a different answer if you ask an American, obviously their time-zones are on the opposite side of the world.

But what if you ask two people in the same room as you? You may get the same answer from them both but then again one person’s watch may be a minute or two faster.

When we ask the time then what we are really asking for is a rough estimate for the time zone that we are in. Some watches are more accurate than others but it is often enough for our day to day needs.

But what if you need to know the exact time and what if you need to know what that time is another country too. Perhaps you have bought an airline ticket; it would be disappointing to turn up at the airport only to be told that your ticket was sold to somebody else in as the clock at their travel agent was slower than the one where you bought your ticket.

So how does global industry keep accurate time with one another? The answer is quite simple and it is called Coordinated Universal Time or UTC.

The International Bureau of Weights and Measures (BIPM) acts as the official time-keeper for the globe and started UTC in 1972 after the development of atomic clocks.

The atomic clock was first developed in the late 50’s when it was discovered the atom caesium-133 resonates at an exact frequency of 9,192,631,770 every second. This frequency was so exact that atomic clocks developed an accuracy of one second in 1.4million years and The International System of Units defined the second as the frequency of the caesium-133 atom and an international unit for measuring time was born.

However, atomic clocks are even more accurate than the Earth itself which is actually slowing in its rotation. This slowing is only small but if the standard system of time, UTC, didn’t compensate for it, eventually midnight would fall in the middle of the day (although that would take a millennia or two) so leap seconds are added every few years to compensate.

The only problem with UTC timepieces is that atomic clocks are enormous in both size and cost. In fact they are generally only to be found in large scale physics laboratories such as NPL (National Physics Laboratory, UK) or MIT (Massachusetts Institute of Technology, US).

Then how does the rest of the world keep track of UTC time? The time told on these vast atomic clocks is broadcast via radio broadcasts or the GPS satellite system (Satellite Navigation is reliant on UTC as without it a satellite can’t tell exactly where a receiver is).

Most computer networks are sycnhronised to UTC time either over the Internet (which isn’t secure and only recommended for home users) or through specialist GPS or radio time servers. These time servers make use of NTP (Network Time Protocol) which has been developed over the last 25 years to keep computer networks synchronized so they do not have to rely on their inaccurate internal clocks.

NTP servers and UTC have allowed industry to become truly global and made possible technologies such as communication satellites, mobile phones, sat-nav and ATM’s that we all take for granted.