Time Synchronisation

UTC – Coordinated Universal Time (from the French: Universel Temps Coordonné) is a global timescale based on Greenwich Meantime (GMT – from the Greenwich Meridian line where the sun is above at 12 noon). But accounts for the natural slowing of the Earth’s rotation. It is used globally in commerce, computer networks via a NTP server, air-traffic control and the World’s stock exchanges to name but a few of its applications.

UTC is really the only solution for time synchronisation needs. While it is just as possible to synchronise a computer network with an NTP server to a time other than UTC it is pointless. As UTC is utilised by computer networks all across the globe by using a UTC time source that means your network can synchronise with every other network in the world that is synchronised to UTC.

UTC is most commonly received from across the Internet, however, this can only be recommended for small network users where either accuracy or security is an issue. An Internet based UTC source is external to the firewall so will leave a potential hole for malicious users to exploit.

Two secure methods of receiving UTC are commonly available. These are either the GPS network (Global Positioning System) or specialist radio transmission broadcast on long wave from several of the world’s national physics laboratories. The two methods have both advantages and disadvantages which need to be ascertained before a method is selected.

A radio transmission such as the UK’s MSF, the German DCF-77 or the USA’s WWVB signal are vulnerable to local topography although many of these signals can be picked up indoors. Whilst not every country transmits a UTC radio signal around the neighbouring countries that do it is possible to still receive it.

GPS on the other hand is available literally anywhere on the globe. The signal comes directly from above and as long as the antenna has a good clear view of the sky it can be received anywhere. However, as the antenna has to be on a roof looking up this can have logistical problems (particularly for very tall buildings).

Specialist dedicated network time servers are available that can actually receive both methods of UTC but whether using GPS or a radio transmissions synchronisation of a network to within a few milliseconds is possible.

NTP – Network Time Protocol

SNTP – Simple Network Time Protocol

GPS – Global Positioning System

UTC – Coordinated Universal Time

MSF – Radio Time Signal for United Kingdom

WWVB – Radio Time Signal for American

DCF – Radio Time Signal for Germany

LAN – Local Area Network

UDP – User Datagram Protocol

TCP – Transmission Control Protocol

IP – Internet Protocol

TDF – Radio Time Signal for France

CHU – Radio Time Signal for Canada

JJY – Radio Time Signal for Japan

HBG – Radio Time Signal for Switzerland

USB – Universal Serial Bus

RTC – Real Time Clock

AM – Amplitude Modulation

APM – Automatic Power Management

DES – Data Encryption Standard

ESD – Electrostatic Discharge

FM – Frequency Modulation

IETF – Internet Engineering Task Force

IRIG – Inter-Range Instrumentation Group

MD5 – Message Digest

PPM – Part Per Million

PPS – Pulse Per Second

RFC – Request For Comments

SA – Selective Availability

TAI – International Atomic Time

SI – International System of Units

The NTP server is an essential network tool. Whilst other protocols do exist, NTP is by far the standard time synchronisation protocol and is utilised in the majority of time servers.

A NTP server is reliant on a single time source it is this time reference that it uses to distribute amongst the network and synchronise to. This timing reference tends to be a UTC time source (coordinated universal time) which is a global time source based on the time told by atomic clocks.

There are only two viable options for receiving a UTC timing source. Although the Internet can be used, the signal can’t be authenticated this is a security measure used by NTP to ensure the reference is what it says it is. Also by using an Internet time source a hole must be left open in the network firewall to allow for communication to the server, this has its own security risks.

The only two secure methods for receiving a UTC time signal is to either use the GPS network or national time and frequency transmissions that are broadcast by several countries’ national physics laboratories.

In selecting a timing source for a NTP server, location is the key consideration. The national time and frequency transmissions are not available in every country. Whilst the USA, UK, Germany, France, Japan and Finland have a signal there are many countries that do not. Furthermore being a long wave radio transmission it can easily be blocked by local topography, although the radio aerial can pick op a signal indoors which is something a GPS NTP server can’t do.

GPS antennas have to be situated on a roof. This can have logistical problems if the server room is in the basement of a high storey building but on the plus inside the GPS signal can be received literally anywhere in the world.

An NTP server (network time protocol) is a device to ensure all machines on a computer network are running the exact same time. Without an NTP server time would be inconsistent between devices which could problems for the network, seconds could be lost here or gained there which could cause major confusion as well as leaving your network vulnerable.

Time, in the form of timestamps, provides the only frame of reference between all devices on a network and the way an NTP server works is pretty straightforward. The timestamp relayed to the server is in the form of an ever increasing number that started from a set point in time, this is known as the prime epoch and for most systems this started on 1 January, 1900.

The NTP server checks the time stamp from an authoritative source, normally a UTC source (Coordinated Universal Time, a global timescale based on the time told by atomic clocks), from either the Internet, a radio transmission or via the GPS network.

The NTP server uses the timestamp to calculate if the network clocks are drifting and adds or subtracts a second to match the reference clock. The NTP server will do this at set intervals, normally every fifteen minutes to ensure perfect accuracy.

NTP is accurate to within 1/100th of a second (10 milliseconds) over the public Internet and can perform even better over LANs and WANS with accuracies of 1/5000th of a second (200 microseconds) not unheard of.

To ensure further accuracy the NTP service (or daemon on Linux) which runs in the background and does not believe the time it is told until after several exchanges and each one has passed a protocol specification (a test), the server is then considered. It usually takes about five good samples) until a NTP server is accepted as a timing source.

We may think of their being only one time and therefore one timescale. Sure, we’re all aware of time zones where the clock has to be pushed back an hour but we all obey the same time surely?

Well actually we don’t. There are numerous different timescales all developed for different reasons are too numerous to mention them all but it wasn’t until the nineteenth century that the idea of a single timescale, used by everybody came into effect.

It was the advent of the railway that provoked the first national timescale in the UK (Railway time) before then people would use noon as a basis for time and set their clocks to it. It rarely mattered if your watch was five minutes faster than your neighbours but the invention of the trains and the railway timetable soon changed all that.

The railway timetable was only useful if people all used the same time scale. A train leaving at 10.am would be missed if a watch was five minutes slow so synchronisation of time became a new obsession.

Following railway time a more global timescale was developed GMT (Greenwich Meantime) which was based on the Sun’s position at noon which fell over the Greenwich Meridian line (0 degrees longitude). It was decided during a world conference in 1884 that a single world meridian should  replace the numerous one’s already in existence. London was perhaps the most successful city in the world so it was decided the best place for it.

GMT allowed the entire world to synchronise to the same time and while nations altered their clocks to adjust for time-zones their time was always based on GMT.

GMT proved a successful development and remained the world’s global timescale until the 1970’s. By then that atomic clock had been developed and it was discovered in the use of these devices that Earth’s rotation wasn’t a reliable measure to base our time on as it actually alters day by day (albeit by fractions of a second).

Because of this a new timescale was developed called UTC (Coordinated Universal Time). UTC is based on GMT but allows for the slowing of the Earth’s rotation by adding additional ‘Leap Seconds’ to ensure that Noon remains on the Greenwich Meridian.

UTC is now used all over the World and is essential for applications such as air traffic control, satellite navigation and the Internet. In fact computer networks across the globe are synchronised to UTC using NTP time servers (Network Time Protocol). UTC is governed by a constellation of atomic clocks controlled by national physics laboratories such as NIST (National Institute of Standards and Time) and the UK’s NPL.

Network time servers are essential tools for keeping computer networks synchronised and secure. Lack of synchronisation can have many consequences and can leave a computer network vulnerable to security threats and fraud.

Whilst synchronisation over the Internet is available this is not recommended by Microsoft among others are they can leave a system vulnerable to security threats (being as they are external to the network firewall).

A network time server can receive a time signal from two sources: the GPS network (Global Positioning System) a highly accurate method of receiving UTC (Coordinated Universal Time) and available everywhere on the globe (with a good clear view of the sky).

An alternative to the GPS signal in the UK and northern Europe is the MSF long wave signal transmitted by the British National Physical Laboratory (NPL) from Cumbria in Northern England.

The MSF signal provides and accurate, reliable and secure method of receiving UTC and is available across Britain and many parts of northern Europe. With a dedicated MSF compatible network time server a computer network can be synchronised to within a few milliseconds of UTC.

A time server is an essential piece of equipment responsible for ensuring all devices on a computer network are running the same time. Most time servers are dedicated devices that receive a time signal, normally UTC (Coordinated Universal Time), and distribute it to all devices on a network.

Most time servers use the Internet protocol NTP (Network Time Protocol) to synchronise all devices and are often referred to as NTP servers. NTP distributes a single time source throughout the network which is normally a UTC source (Coordinated Universal Time).

There are several places a time server can receive a time signal from. The internet is an obvious source for many although online time sources are not very accurate, can be too far away to give any useful precision, and more importantly are not secure being as they are external to the firewall.

As a dedicated time server is an external device they are extremely secure and impossible for malicious users to tamper with. Dedicated time server can receive a time signal from two sources the GPS network (Global Positioning System) , a highly accurate method and available everywhere on the globe with a good view of the sky; or the specialist long wave radio transmissions broadcast by national physics laboratories.

In Europe the two main radio transmissions are the UK’s MSF signal broadcast by NPL (National Physical Laboratory) in Cumbria, England and the German DCF-77 broadcast near Frankfurt.

These long wave signals are also highly accurate and can be picked up in most neighbouring countries too. The USA has a similar system called WWVB,  transmitted by the National Institute for Standards and Technology (NIST) from Boulder, Colorado.

Synchronising a network is often considered a headache by network administrators who fear that getting it wrong can lead to disastrous results and while there is no deny that a lack of synchronisation can cause unforeseen problems particularly with time sensitive transactions and security, perfect synchronisation is simple if these steps are followed:

1. Use a dedicated NTP server. The NTP server is a device that receives a single time source then distributes it amongst a network of computers using the protocol NTP (Network Time Protocol) one of the oldest Internet based protocols and by far the most widely used time synchronisation software. NTP is often packaged with modern operating systems such as Windows or Linux although there is no substitute for a dedicated NTP device.

2. Always use a UTC time source (Coordinated Universal Time). UTC is based on GMT (Greenwich Meantime) and International Atomic Time (TAI) and is highly accurate. UTC is used by computer networks all over the world ensuring that commerce and trade are all using the same timescale.

3. Use a secure an accurate time signal. Whilst time signals are available all over the Internet they are unpredictable in their accuracy and while some may offer decent enough precision an Internet time server is outside a networks firewall which if left open to receive a timecode will cause vulnerabilities in the security of the network. Either GPS (global positioning system) or a dedicated radio signal such as those transmitted by national physics laboratories (such as MSF – UK, WWVB – USA, DCF –Germany) offer secure and reliable methods of receiving a secure and accurate time signal.

4. Organise a network into stratum, levels. Strata ensure that the NTP server is not inundated with time requests and that the network bandwidth doesn’t become congested. A stratum tree is organised by a few select machines being stratum 2 devices in that they receive a time signal from the NTP server (stratum 1 device) these in turn distribute the time to other devices (stratum 3) and so on.

5. Ensure all machines are utilising UTC and the NTP server tree. A common error in time synchronisation is to not ensure all machines are properly synchronised, just one machine running inaccurate time can have unforeseen consequences.

There are several timescale used throughout the world. Most NTP servers and other network time servers use UTC as a base source however, there are others:

When we are asked the time it is very unlikely we would respond with ‘for which timescale’ yet there are several timescales used all over the globe and each is based on different methods of keeping track of the time.

GMT

Greenwich Mean Time (GMT) is the local time on the Greenwich meridian based on the hypothetical mean sun. As the Earth’s orbit is elliptical and its axis is tilted, the actual position of the sun against the background of stars appears a little ahead or behind the expected position. The accumulated timing error varies through the year in a smoothly periodic manner by up to 14 minutes slow in February to 16 minutes fast in November. The use of a hypothetical mean sun removes this effect. Before 1925 astronomers and navigators measured GMT from noon to noon, starting the day 12 hours later than in civil usage which was also commonly referred to as GMT. To avoid confusion astronomers agreed in 1925 to change the reference point from noon to midnight, and a few years later adopted the term Universal Time (UT) for the “new” GMT. GMT remains the legal basis of the civil time for the UK.

UT

Universal Time (UT) is mean solar time on the Greenwich meridian with 0 h UT at mean midnight, and since 1925 has replaced GMT for scientific purposes. By the mid-1950s astronomers had much evidence of fluctuations in the Earth’s rotation and decided to divide UT into three versions. Time derived directly from observations is called UT0, applying corrections for movements of the Earth’s axis, or polar motion, gives UT1, and removing periodic seasonal variations generates UT2. The differences between UT0 and UT1 are of the order of thousandths of a second. Today, only UT1 is still widely used as it provides a measure of the rotational orientation of the Earth in space.

The world time standard (UTC):

Although TAI provides a continuous, uniform, and precise time scale for scientific reference purposes, it is not convenient for everyday use because it is not in step with the Earth’s rate of rotation. A time scale that corresponds to the alternation of day and night is much more useful, and since 1972, all broadcast time services distribute time scales based on Coordinated Universal Time (UTC). UTC is an atomic time scale that is kept in agreement with Universal Time. Leap seconds are occasionally.

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.