Category: Time Synchronisation

Keeping Time with Network Time Protocol

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NTP (Network Time Protocol) is the most flexible, accurate and popular method of sending time over the Internet. It is perhaps the Internet’s oldest protocol having been around in one form or another since the mid 1980’s.

The main purpose of NTP is to ensure that all devices on a network are synchronised to the same time and to compensate for some network time delays. Across a LAN or WAN NTP manages to maintain an accuracy of a few milliseconds (Across the Internet, time transfer if far less accurate due to network traffic and distance).

NTP is by far the most widely used time synchronisation protocol (somewhere in the region of 95% of all time servers use NTP) and it owes much of its success to its continual updates and its flexibility. NTP will run on UNIX, LINUX, and Windows based operating systems (it is also free, another possible reason for its huge success).

NTP uses a single time source that it distributes among all devices on a network; it also checks each device for drift (the gaining or losing of time) and adjusts for each.  It is also hierarchical in that literally thousands of machines can be controlled using just one NTP server as each machine can in itself be used by neighbouring machines as a time server.

NTP is also highly secure (when using an external time reference not when using the Internet for a timing source) with an authentication protocol able to establish exactly where a timing source comes from.

For a network to be really effective most NTP time servers use an atomic clock as a basis for their time synchronisation. An international timescale based on the time told by atomic clocks has been developed for this very purpose. UTC (Coordinated Universal Time).

There are really two methods to receive a secure UTC atomic clock time signal to be utilised by NTP. The first being the time and frequency transmissions that several national physics laboratories broadcast on long wave around the world; the second (and by far the most readily available) is by using the timing information in the GPS satellite transmissions. These can be picked up anywhere on the globe and provide safe, secure and highly accurate timing information.

The importance of time synchronisation in the modern world

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Time has always played an important part in civilisation. Understanding and monitoring time has been one of the pre-occupations of mankind since prehistory and the ability to keep track of time was as important to the ancients as it is to us.

Our ancestors needed to know when the best time was to plant crops or when to gather for religious celebrations and knowing the time means making sure it is the same as everyone else’s.

Time synchronization is the key to accurate time keeping as arranging an event at a particular time is only worthwhile if everybody is running at the same time. In the modern world, as business has moved from a paper-based system to an electronic one, the importance of time synchronisation and the search for ever better accuracy is even more crucial.

Computer networks are now communicating with each other from across the globe conducting billions of dollars worth of transactions every second, millisecond accuracy is now part of business success.

Computer networks can be comprised of hundreds and thousands of computers, servers and routers and while they all have an internal clock, unless they are synchronised perfectly together a myriad of potential problems could occur.

Security breaches, data loss, frequent crashes and breakdowns, fraud and customer credibility are all potential hazards of poor computer time synchronisation. Computers rely on time as the only point of reference between events and many applications and processes are time dependent.

Even discrepancies of a few milliseconds between devices can cause problems particularly in the world of global finance where millions are gained or lost in a second. For this reason most computer networks are controlled by a time server. These devices receive a time signal from an atomic clock. This signal is then distributed to every device on the network, ensuring that all machines have the identical time.

Most synchronisation devices are controlled by the computer program NTP (Network Time Protocol). This software regularly checks each device’s clock for drift (slowing or accelerating from the desired time) and corrects it ensuring the devices never waver from the synchronised time.

The MSF Time Signal

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The MSF time signal is a dedicated radio broadcast providing an accurate and reliable source of UK civil time, based on the global time scale UTC (Coordinated Universal Time), the MSF signal is broadcast and maintained by the UK’s National Physical Laboratory (NPL).

The MSF time signal can be utilised by anyone requiring accurate timing information its main use however is as a source of UTC time for administrators synchronising a computer network with a radio clock. Radio clocks are really another term for a network time server that utilises a radio transmission as a timing source.

Most radio based network time servers use NTP (Network Time Protocol) to distribute the timing information throughout the network.

The MSF signal is broadcast from Anthorn Radio station in Cumbria by VT communications under contract to the NPL.  It is available 24 hours a day across the whole of the UK and beyond, although the signal is vulnerable to interference and local topography. Users of the MSF service receive predominantly a ‘ground wave’ signal. However, there is also a residual ‘sky wave’ which is reflected off the ionosphere and is much stronger at night; this can result in a total received signal that is either stronger or weaker.

The MSF signal is carried on a frequency of 60 kHz (to within 2 parts in 1012) and is controlled by a Caesium atomic clock based at the radio station.

The antenna at Anthorn is at 54° 55′ N latitude, and 3° 15′ W longitude. The signal’s field strength exceeds 100 µV/m(micro volts a metre) at a distance of 1000 km from Anthorn, covering the whole of the UK, and can even be received throughout some of northern and western Europe.

The MSF transmits a simple binary code containing time and date information The MSF time and date code includes the following information: year, month, day of month,  day of week,  hour, minute, British Summer Time (in effect or imminent),  DUT1 (a parameter giving UT1-UTC)

Five Reasons Why You Should Never Use an Internet Timing Source

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Time synchronisation is now an integral part of network administration. Networks that are not synchronised to UTC time (Coordinated Universal Time) become isolated; unable to process time sensitive transactions or communicate securely with other networks.

UTC time has been developed to allow the entire globe to communicate under a single time-frame and it is based on the time told by atomic clocks.

To synchronise to UTC time many network administrators simply connect to an Internet timing source and assume they are receiving a secure source of UTC time. However, there are pitfalls to this and any network that requires security should NEVER use the Internet as a timing source:

1.    To use an internet timing source a port needs to be forwarded in the firewall. This ‘hole’ to allow the timing information to pass through can be utilised by anybody else too.
2.    NTP (Network Time Protocol) has an inbuilt security measure called authentication that ensures a timing source is exactly who it says it is, this can’t be utilised over the Internet.
3.    Internet timing sources are wholly inaccurate. A survey by Nelson Minar of MIT (Massachusetts  Institute of Technology) discovered less than half were close enough to UTC time to be described as reliable (some where minutes and even hours out!).
4.    Distance across the Internet can render even an extremely accurate Internet timing source useless as the distance to client could cause delay.
5.    A dedicated time server will use a radio of GPS timing signal which can be audited to guarantee its accuracy, providing security and legal protection; internet timing sources cannot.

Dedicated NTP time servers not only offer greater protection and security than Internet time sources. They also offer unbridled accuracy with both the GPS and time and frequency radio transmissions (such as MSF, DCF or WWVB) accurate to within a few milliseconds of UTC time.

GPS Time Server Receiving Time from Space

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GPS time servers are network time servers that receive a timing signal from the GPS network and distribute it amongst all devices on a network ensuring that the entire network is synchronised.

GPS is an ideal time source as a GPS signal is available anywhere on the globe. GPS stands for Global Positioning System, the GPS network is owned by the US military and controlled and run by the US air force (space wing). It is however, since the late 1980’s been opened up to the world’s civilian population as tool to aid navigation.

The GPS network is actually a constellation of 32 satellites that orbit the Earth, they do not actually provide positioning information (GPS receivers do that) but transmit from their onboard atomic clocks a timing signal.

This timing signal is what is used to work out a global position by triangulating 3-4 timing signals a receiver can work out how far and therefore the position you are from a satellite. In essence then, a global positioning satellite is just an orbiting clock and it is this information that is broadcast that can be picked up by a GPS time server and distributed amongst a network.

Whilst strictly speaking GPS time is not the same as the global timescale UTC (coordinated universal time), a GPS time server will automatically convert the time format into UTC.

A GPS time server can provide unbridled accuracy with networks able to maintain accuracy to within a few milliseconds of UTC.

Common NTP Server Time Reference Problems

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The NTP server (Network Time Protocol) is one of the most used but least understood computer networking hardware items.

A NTP Server is just a time server that uses the protocol NTP. Other time protocols do exist but NTP is by far the most widely used. The terms ‘NTP server’, ‘time server’ and ‘network time server’ are interchangeable and often the terms ‘radio clock’ or ‘GPS time server’ are used but these simply describe the method which the time servers receive a time reference.

NTP servers receive a time source that they can then distribute amongst a network. NTP will check a devices system clock and advance or retreat the time depending on how much it has drifted. By regularly checking the system clock with the time server, NTP can ensure the device is synchronised.

The NTP server is a simple device to install and run. Most connect to a network via an Ethernet cable and the software included is easily configured. However, there are some common troubleshooting problems associated with NTP servers and in particular with receiving timing sources:

A dedicated NTP server will receive a time signal from various sources. The Internet is probably the most common sources of UTC time (Coordinated Universal Time), however, using the Internet as a timing source can be a cause for several time server problems.

Firstly Internet timing sources can’t be authenticated; authentication is NTP’s in-built security measure and ensures that a timing reference is coming from where it says it is. On a similar note to use an Internet timing source would mean that a gap would have to be created in the network firewall, this can obviously cause its own security issues.

Internet timing sources are also notoriously inaccurate. A survey by MIT (Massachusetts Institute of Technology) found less than a quarter of Internet timing sources were any where near accurate and often those that were, were too far away from clients to provide a reliable timing source.

The most common, secure and accurate method for receiving timing source is the GPS system (Global Positioning System). While a GPs signal can be received anywhere on the planet there are still common installation issues.

A GPS antenna has to have a good clear view of the sky; this is because the GPs satellite broadcast their signal by line of sight. He signal can not penetrate buildings and therefore the antenna has to be situated on the rood. Another common issue with a GPS time server is that they need to be left for at least 49 hours to ensure the GPS receiver gets a good satellite fix. Many users find that they are receiving an intermittent signal this is normally due to impatience and not letting the GPS system obtain a solid fix.

The other secure and reliable method for receiving a timing signal is the national radio transmissions. In the UK this is called MSF but similar systems exist in the US (WWVB), Germany (DCF) and several other countries. There are usually less problems faced when using the MSF/DCF/WWVB signal.

Although the radio signal can penetrate buildings it is susceptible to interference from topography and other electrical appliances.  Any issues with a MSF time server can normally be resolved by moving the server to another locale or often just angling the server so its ib-built antenna is perpendicular to the transmission.

Time Synchronisation What is time?

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Time servers are common apparatus in modern server rooms but time synchronisation has only become possible thanks to ideas of physicist of the last century and it is our these ideas of time that has made many of the technologies of the last few decades possible.

Time  is one of the most difficult of concepts to understand. Until the last century it was thought that time was a constant but it wasn’t until the ideas of Einstein that we discovered time was relative.
Relative time was a consequence of Einstein’s most popular theory the ‘General Theory of Relativity’ and its famous equation E=MC2.

What Einstein discovered was that the speed of light was the only constant in the Universe (in a vacuum anyway) and that time will differ for different observers. Einstein’s equations demonstrated that the faster an observer travelled towards the speed of light the slower time would become.

He also discovered that time wasn’t a separate entity of out universe but was part of a four dimensional space-time and that the effects of gravity would warp this space time causing time to slow.

Many modern technologies such as satellite communication and navigation have to take these ideas into account otherwise satellites would fall out of orbit and it would be impossible to communicate across the globe.

Atomic clocks are so accurate they can lose less than a second in 400 million years but consideration to Einstein’s ideas have to be taken into account as atomic clocks based at sea level run slower that those at higher altitude because of the Earth’s gravity warping spacetime.

A universal time scale has been developed called UTC (Coordinated Universal Time) which is based on the time told by atomic clocks but compensates for the minute slowing of the Earth’s rotation (caused by the gravity of the Moon) by adding Leap Seconds every year to prevent day from creeping into night (albeit in a millennia or two).

Thanks to atomic clocks and UTC time computer networks all over the world can receive a UTC time source over the Internet, via a national radio transmission or through the GPS network. A NTP server (Network Time Protocol) can synchronise all devices on a network to that time.

Time Server FAQ on British Time

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Time servers are used throughout UK industry. Many of which receive the MSF signal from the National Physical Laboratoruy in Cumbria. Here are some FAQ’s about British time and the MSF signal:

Who decides when clocks should go forward or back for summer time?

If you live in Europe, the time at which summer time begins and ends is given in the relevant EU Directive and UK Statutory Instrument as 1 a.m. Greenwich Mean Time (GMT).

Does ‘midnight’ belong to the day before or the day after?

The use of the word midnight is heavily dependent on its context but 00.00 (often called 12 am) is the start of the next day. There are no standards established for the meaning of 12 a.m. and 12 p.m. and often a 24 hour time is less confusing.

Is there an approved way to represent dates and times?

The standard notation for the date is the sequence YYYY-MM-DD or YY-MM-DD although in the USA it is the convention to have days and months the other way around.

When did the new millennium really begin?

A millennium is any period of a thousand years. So you could say that the next millennium begins now. The third millennium of the Christian Era began at the start of the year 2001 A.D.

How do you know atomic clocks keep better time?

If you look at several atomic clocks all set to the same time you’ll find that they still agree within ten millionths of a second after a week.

What is the accuracy of the ‘speaking clock’?

Even allowing for the delay in the telephone network, you can probably expect the starts of the seconds pips to be accurate seconds markers within about one-tenth of a second.

Why did my radio-controlled clock move to summer time at 2 a.m., one hour late?

Battery powered radio-controlled clocks typically check the time only every hour or two, or even less, This is to conserve the battery.

Why does my radio-controlled clock receive the MSF signal less well at night?

Users of the MSF service receive predominantly a ‘ground wave’ signal. However, there is also a residual ‘sky wave’ which is reflected off the ionosphere and is much stronger at night, this can result in a total received signal that is either stronger or weaker.

Is there a permanent one-hour difference between MSF time and DCF-77 time?

Since 1995 October 22 there has been a permanent one-hour difference between British time (as broadcast by MSF) and Central European Time, as broadcast by DCF-77 in Germany.

What does MSF stand for?

MSF is the three-letter call sign used to designate the UK’s 60 kHz standard-frequency and time signal.

Thanks to the National Physical Laboratory for their help with this blog.

NTP Time Server Packet Header Explained

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Most time servers use Network Time Protocol and like other Internet based protocols NTP contains a packet header. A packet header, put simply, is just is a formatted unit of data that describes the information contained in the packet.

The NTP packet header consists of a number of 32-bit words. Here is a list of the most common packet header terms and their meaning:

IP address – the address of the NTP Time Server

NTP Version – which version of NTP (currently version 4 is the most recent)

Reference timestamp (the prime epoch ) used by NTP to work out the time from this set point (normally January 01 1900

Round trip delay (the time it takes request to arrive and come back in milliseconds)

Local clock offset – time difference between host and client

Leap indicator (if there is to be a leap second that day –normally only on 31 December)

Mode3  –  a three bit integer which values represent: 0=reserved, 1=symmetric active, 2= symmetric passive, 3=client, 4=server, 5=broadcast, 6=NTP control message, 7=reserved for private use.

Stratum level – which stratum level the NTP server is (a stratum 1 server receives the time from an atomic clock source a stratum 2 server receives the time from a stratum 1 server)

Poll Interval (How many requests is made and their intermittence)

Precision – how accurate in milliseconds is the system clock

Root Delay – This is a signed fixed-point number indicating the total roundtrip delay to the primary reference source at the root

Root dispersion (in milliseconds)- The root dispersion is the maximum (worst case) difference between the local system clock and the root of the NTP tree (stratum 1 clock)

Ref ID – 32 bit identifying the reference clock

Originate time stamp (time before synchronisation request)

Receive timestamp – the time the host/NTO time Server got the request

Transmit timestamp – the time the host sent back the request

Valid  response– is the system clock  synchronised or not

NTP Server History and Implementation

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Network Time Protocol (NTP) was, invented by Dr David Mills from the University of Delaware, it has been in utilized since 1985 and is still in constant development. NTP is a protocol designed to synchronize the clocks on computers and networks across the Internet or Local Area Networks (LANs). Most networks are synchronised via NTP to a UTC time source (coordinated universal time)

UTC is based on the time told by atomic clocks and is used globally as standardized time source.

NTP (version 4) can maintain time over the public Internet to within 10 milliseconds (1/100th of a second)  of UTC time 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, time synchronisation with NTP is relatively simple, it synchronises time with reference to a reliable UTC source and then distributes this time to all machines and devices on a network.

Microsoft and others recommend that only external based timing should be used rather than Internet based, as these can’t be authenticated and can leave a system open to abuse, especially since an Internet timing source is beyond the firewall. Specialist NTP servers are available that can synchronise time on networks using either the MSF, DCF or WWVB radio transmission. These signals are broadcast on long wave by several national physics laboratories.

In the UK, the MSF national time and frequency radio transmissions used to synchronise an NTP server 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).

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. The antenna 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.

A radio referenced NTP server is easily installed and can provide an organization with a precise time reference enabling the synchronization of entire networks. The NTP server will receive the time signal and then distribute it amongst the network devices.