Time Synchronisation

All versions of Windows Server since 2000 have included a time synchronization facility, called Windows Time Service (w32time.exe), built into the operating system. This can be configured to operate as a network time server synchronizing all machines to a specific time source.

Windows Time Service uses a version of NTP (Network Time Protocol), normally a simplified version, of the Internet protocol which is designed to synchronise machines on a network, NTP is also the standard for which most computer networks across the global use to synchronise with.

Choosing the correct time source is vitally important. Most networks are synchronized to UTC (Coordinated Universal Time) source. UTC is a global standardized time based on atomic clocks which are the most accurate time sources.

UTC can be obtained over the Internet from such places as time.nist.gov (us Naval Observatory) or time.windows.com (Microsoft) but it must be noted that internet time sources can not be authenticated which can leave a system open to abuse and Microsoft and others advise using an external hardware source as a reference clock such as a specialized network time server.

Network time servers receive their time source from either a specialist radio transmission from national physics laboratories which broadcast UTC time taken from an atomic clock source or by the GPS network which also relays UTC as a consequence of needing it to pin point locations.

NTP can maintain time over the public Internet to within 1/100th of a second (10 milliseconds) and can perform even better over LANs.

To configure Windows Time Service to use an external time source simply follow these instructions.

Locate the registry subkey.
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Parameters
Right click type then Modify the Value by inserting NTP in the Value Data box.
Right click ReliableTimeSource, then Modify the Edit DWORD Value box, by inserting 0 (zero).
Right-click NtpServer then Modify the Edit Value by typing the Domain Name System (DNS), (note each DNS must be unique).

Now locate the original subkey and right-click Period Modify the Edit DWORD Value box with the poll interval (how often a NTP server polls the time), under Value Data (recommended 24)

Run the following command line, Net stop w32time && net start w32time.
Now enable NTP by locating the subkey,
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\TimeProviders\NtpServer\
Right click Enabled and modify the Value data box by typing 1.

Right Click SpecialPollInterval in the right pain of the subkey
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\TimeProviders\NtpClient\SpecialPollInterval.
Edit the DWORD value box the time you want for each poll (900 will poll every 15 minutes)

Locate HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\config

To configure the time correction settings right click MaxPosPhaseCorrection, then modify the DWORD Value box with a time in seconds such (select decimal under base first, 3600 = one hour)

Now do the same for MaxNegPhaseCorrection the restart windows time service by running (or alternatively use the command prompt facility) net stop w32time && net start w32time.

To synchronise each machine simply type W32tm/ -s in the command prompt and the time server should now be working correctly (note it may take several polls before the correct time is displayed).

Computer networking can seem an intimidating undertaking. However, a computer network is really just a number of machines connected together for ease of data transfer and security. They can be very small such as two computers in a home network to really large networks consisting of hundreds and thousands of machines.

When a computer or device is connected to a network then there is only one point of reference that the computers can use to establish the order of events and applications and that is time.

Time, in the form of time stamps are used by most applications and this is when problems in computer networks can occur.

Computers tell the time by using a software clock. This is based on a system clock that keeps time when the computer is off. However, computers internal clocks are wholly inaccurate. They tend to drift up to several seconds a week. On a network when there is more than one machine, this can cause severe problems if the machines are drifting at different rates.

Emails may arrive before they have been sent and the whole network can be vulnerable to security threats and even fraud!

A network time server is used to synchronize a computer network to a single time source. This time source can be anything from an internal clock on a computer to the time told by a wrist watch. However, to ensure perfect accuracy and to keep a network synchronized to the rest of the world then a UTC time source should be used.

UTC (Coordinated Universal Time) is a global timescale based on the time told by atomic clocks. A network time server can receive a UTC time source from across the Internet (although unsecured), via the GPS (global positioning system) network or via specialist radio transmission from national physics laboratories.

Most network time servers use NTP (Network Time Protocol) to distribute the timing reference throughout the network. NTP is not the only timing protocol designed to do this although it is, however, by far the most widely used.

A network time server can be one of the most crucial devices on a computer network as timestamps are vital for most computer applications from sending and email to debugging a network.

Tiny inaccuracies in a timestamp can cause havoc on a network, from emails arriving before they have technically been sent, to leaving an entire system vulnerable to security threats and even fraud.

However, a network time server is only as good as the time source that it synchronises to. Many network administrators opt to receive a timing code from the Internet, however, many Internet time sources are wholly inaccurate and often too far away from a client to provide any real accuracy.

Furthermore, Internet based time sources can’t be authenticated. Authentication is  a security measure used by NTP (Network Time Protocol which controls the network time server) to ensure the time server is exactly what it says it is.

To ensure accurate time is kept it is vital to select a time source that is both secure and accurate. There are two methods which can ensure a millisecond accuracy to UTC (coordinated universal time – a global timescale based on the time told by atomic clocks).

The first is to use a specialist national time and frequency transmission broadcast in several countries including the UK, USA, Germany, France and Japan. Unfortunately these broadcasts can’t be picked up everywhere but the second method is to use the timing signal broadcast by the GPS network which is available literally everywhere on the face of the planet.

A network time server will use this timing code and synchronise an entire network to it using NTP (Network Time Protocol) it will continually adjust the network’s clocks ensuring there is no drift.

In selecting a network time server there are only really two considerations – where are you going to get your timing source from and where the network time server is going to be located.

There are three methods of receiving an accurate time reference for a network time server. one of the most common is using one of the hundreds of Internet based timing  sources. These can vary in their accuracy from being close to UTC (coordinated universal time based on the time told by atomic clocks) to being seconds, minutes and even hours off.

An Internet timing source should be selected not just on its accuracy but also the distance away, if it is too far away then there is no realistic prospect of achieving any useful accuracy. However, one of he biggest problems of using an Internet based timing source is that the y can’t be authenticated.

Authentication is a security measure provided by NTP (Network Time Protocol – the software that a network time server uses to synchronise with)  to ensure the time server is where and what it says it is.

Fortunately there are two other methods of receiving a UTC time reference for a network time server. the first is using the specialist time and frequency radio transmission broadcast by several countries. This has the advantage of allowing a network time server to receive the signal even indoors, however the signals are not available everywhere and local topography can block them.

The other alternative is to use the time signal broadcast by the GPS network. This can fortunately be picked up anywhere on the globe. However, a GPS network time server does need to have an antenna situated on a roof to ensure a signal can be received, this can of course be impractical and expensive.

Keeping track of the time is essential in the day-to-day running of our lives. We need to know when to start work, when to leave and even when to get up in the morning.

There are of course a myriad of devices that help us keep track of the time of day from wrist-watches and mobile phones to the display on our DVD player and in our cars, clocks, of one sort or another are everywhere.

Computers are no different. Time is essential for the functioning of computer applications, everything from sending and email to debugging a system is reliant on a time stamp.

Computers have their own clocks, real time chips, but unfortunately these on-board clocks are not very accurate timepieces and are prone to drift and the subsequent differences in time on a network can lead to all sorts of problems.

A network time server gets around this problem by using an absolute time source, that is a time source that is so accurate that it would barely lose a second in millions of years.

The only clocks capable of supplying such accurate time are atomic clocks but unfortunately these are highly expensive and delicate machines only to be found in large scale physics laboratories. Thankfully a network time server can receive the time told by these devices by using either the Internet (although that can be inaccurate and cause security problems, the GPS network or specialist radio transmissions.

These time signals broadcast UTC – coordinated universal time, which is a global timescale developed to ensure networks around the world are keeping the same time.

The network time server will synchronise the entire network of computers to this time, continually checking for drift and ensuring millisecond accuracy.

By using a network time server, security is increased and time-sensitive applications will run without error.

A network time server is responsible for ensuring that all devices on a network are synchronised to the same time. Without synchronisation problems with time sensitive applications can occur and can leave a network open to security issues and even fraud.

A network time server can synchronise a network to any timing source but to ensure security and accuracy a UTC (Coordinated Universal Time) time source is essential.

UTC was developed after the invention of the atomic clock. It is based on International Atomic Time (TAI) and Greenwich Mean Time (GMT). After atomic clocks were developed and the accuracy of a few nanoseconds that they can maintain it was discovered that the Earth was not as reliable in its rotation as the clocks.

UTC allows for the adding of leap seconds (and potential subtracting of them although that hasn’t happened yet) to allow UTC time to match up with GMT. If these leap seconds were not added then eventual day would creep into night (albeit in several millennia)

There are several possible sources for a UTC time source. Either the Internet, although these sources vary in accuracy and are not secure, the GPS network (Global Positioning System) through a roof mounted aerial or a national time and frequency transmission that are broadcast throughout several countries including the US, UK, Germany, France and Japan.

A network time server uses the protocol NTP (Network Time Protocol) to synchronise devices to UTC. NTP works by accounting for drift on the system clock and then adding or subtracting time depending on the difference. By utilising a network time server that uses a timing source from either the GPS network or radio transmission millisecond and even nanosecond accuracy to UTC is possible.

A network time server is one of the most underrated yet vital pieces of equipment on a network. It provides synchronisation, ensuring all computers and devices on a network are running the same time.

Most networks are synchronised to UTC (Coordinated Universal Time) a global timescale based on the time told by atomic clocks. This has the advantage of not only assuring your own network is running as accurate time as possible but also that you are running the exact same time as the rest of the global community.

A network time server uses NTP (Network Time Protocol) one of the Internet’s oldest protocols to synchronise all machines on a network to the timing reference.

UTC timing reference can come from several sources either the Internet, specialist national radio transmissions or the GPS network. It must be mentioned however, that Internet sources can vary in their accuracy and many can be too far away to provide accuracy. It should also be mentioned that Microsoft andNovell recommend they are not used as they can’t be authenticated. Authentication is a security measure used by NTP to ensure the server is what it says it is.

NTP works on a hierarchical basis. The network time server is called a stratum 1 device. devices that get time synchronised from a network time server are stratum 2 devices and devices that get their time from these are stratum 3 (and so on up to 16 strata).

By using strata a single time server can synchronise thousands of devices indirectly and not get swamped with requests.

NTP (Network Time Protocol) uses a single time reference to synchronise all machines on the network to that time. This time reference can be either relative (the server’s internal clock or the time on a wristwatch perhaps) or absolute such as an atomic clock that relays time that is as accurate as humanely possible.

For closed computer networks, a relative time source will function adequately enough, however, if communication with another network or transactions over the Internet is required then it is important that the networks are synchronised to the same time otherwise the above mentioned problems can occur.

But how do you know what time another network is running? Coordinated Universal Time (UTC) is a global time scale, developed to enable computer networks all over the world to synchronise to the exact same time.

UTC arose after the development of atomic clocks. Atomic clocks use the resonance of an atom (normally caesium which resonates at 9,192,631,770 every second) to maintain time. As this resonance never changes, atomic clocks can maintain time for millions of years without losing a second.

Unfortunately, atomic clocks are highly expensive and are normally only found in large-scale physics laboratories. However, the time told by these machines is readily available to synchronise to from several sources. The easiest method is to use a dedicated NTP time server that receives a time signal from either the GPS network or a specialist national radio broadcast.

Network Time Protocol – is just that a protocol designed to deal with delivering time across a network. Even when the Internet was in its infancy with just a few hundred machines connected together it was clear that some way of keeping machines miles apart synchronised was essential.

Many of the tasks we now employ the Internet to do all of which would be impossible without synchronisation. Trading in stocks and shares, airline reservation, Internet auctions and even sending and receiving email are all reliant on time being synchronised. If computers were not synchronised then emails would arrive before they were sent or airline seats could be double booked.

NTP was developed by Professor David Mills of the University of Delaware and has been in continuous use and constant development for the last twenty years. There are other time synchronisation protocols around but NTP is by far the most widely used in time server applications and is installed in most versions of Windows and Linux.

The time server’s job is to receive a timing signal. Normally this time reference comes direct from an atomic clock from either the GPS network or via specialist radio transmissions. NTP then distributes the time received by the time server around the network. Checking each client and advancing or retreating the system clock to ensure synchronisation with the time server.

In selecting a NTP time server it is important to think about where the device will be located and which timing reference would be most suitable for the applications it is required for. There are benefits and drawbacks to both the radio broadcasts and the GPS timing signals.

Unfortunately not every country transmits a national time and frequency broadcast so if a time server is to be located outside of the US, Germany, UK, France or Japan it may be doubtful that a signal could be received (although many of the these transmissions can be received in neighbouring countries).

Radio signals are also easily susceptible to atmospheric interference and can be blocked by mountains, sky-scrapers or other topography.

GPS on the other hand can be received literally anywhere on the planet. GPS is also a more accurate means of receiving a time code -  a typical GPS receiver can provide timing information to within a few nanoseconds of UTC (a nanosecond is a 1 billionth of a second) while national time and frequency transmissions are accurate to only 1 – 20 milliseconds (a millisecond is 1/1000 of a second).

However, one benefit of using a radio broadcast as a timing reference is that it is possible to receive the transmission from inside a building (although basements and rooms with lots of metal furnishings can cause problems), while a GPS antenna needs to be situated on the roof of a building and have a good view of the sky to be able to find and receive the signal broadcast from the satellites (although it is possible to receive a signal through a window depending on the topography outside).