Time Synchronisation

Atomic clocks are incredibly expensive and generally they are normally only to be found in large scale physics laboratories such as MIT (Massachusetts Institute of Technology), NIST (National Institute of Standards and Technology (Colorado) or the National Physical Laboratory in the UK.

Fortunately many national laboratories broadcast the UTC (Coordinated Universal Time) time from their atomic clocks via a radio broadcast.

In the UK the national timing broadcast is called MSF and is broadcast by NPL (National Physical Laboratory) in Cumbria. The MSF broadcast is used by throughout the UK and parts of Europe to synchronise consumer electronic products like wall clocks, clock radios, and wristwatches. In addition, MSF is used for high-level applications such as network time synchronisation utilising NTP.

The time code contains the year, day of year, hour, minute, second, and flags that indicate the status of Daylight Saving Time, leap years, and leap seconds.

MSF operates on a frequency of 60 kHz and carries a time and date code that can be received and decoded by a wide range of readily available radio-controlled clocks and provides a received accuracy should be less than 10 milliseconds (1/100 of a second).

While many NTP servers now use GPS to receive a timing reference, the advantage of using a radio transmission is that a signal can be received indoors (a GPS antenna needs a good view of the sky).

However, the radio signal has a finite range and can be blocked by skyscrapers, mountains and dense conurbations. 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.

Similar national timing transmissions are broadcast from other countries in the US the signal is referred to as WWVB and is broadcast by the NIST (National Institute for Standards and Technology) in Fort Collins, Colorado, other systems are broadcast in Frankfurt, Germany (DCF-77), Japan (JJY) and France (TDF).

The atomic clock is the culmination of mankind’s ability to keep time that has spanned several millennia. Humans have always been preoccupied with keeping track of time ever since early man noticed the regularity of the celestial bodies.

The sun, moon, stars and planets soon became the basis for out timescales with periods of time such as years, months, days and hours based solely on the regulation of the Earth’s rotation.

This worked for thousands of years as a reliable guide to how much time has past but over the last few centuries humans have strode to find even more reliable methods for keeping track of time. Whilst the Sun and celestial bodies were an affective way sundials didn’t work on cloudy days and as the days and night s altered during the year only noon (when the sun is at its highest) could be reasonably relied upon.

The first foray into an accurate timepiece that was not reliant on celestial bodies and was not a simple time (such as a candle taper or water clock) but actually told time over a prolonged period was the mechanical clock.

These first devices dating as far back as the twelfth century were crude mechanisms using a verge and foliot escapement (a gear and lever) to control the ticks of the clock. After a few centuries and a myriad of designs the mechanical clock took its next step forward with the pendulum. The pendulum gave clocks their first true accuracy as it controlled with more precision the ticks of the clock.

However, it wasn’t until the twentieth century when clocks entered the electronic age did they become truly accurate. The digital and electronic clock had its ticks controlled by using the oscillation of a quartz crystal (its changed energy state when a current is based through) which proved so accurate that rarely a second a week was lost.

The development of atomic clocks in the 1950’s used the oscillation of a single atom which generates over 9 billion ticks a second and can maintain precise time for millions of years without losing a second. These clocks now form the basis of our timescales with the entire world synchronised to them using NTP servers, ensuring wholly accurate and reliable time.

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.

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.

Accurate time using Atomic Clocks is available across North America using the WWVB Atomic Clock time signal transmitted from Fort Collins, Colorado; it provides the ability to synchronize the time on computers and other electrical equipment.

The North American WWVB signal is operated by NIST – the National Institute of Standards and Technology. WWVB has high transmitter power (50,000 watts), a very efficient antenna and an extremely low frequency (60,000 Hz). For comparison, a typical AM radio station broadcasts at a frequency of 1,000,000 Hz. The combination of high power and low frequency gives the radio waves from WWVB a lot of bounce, and this single station can therefore cover the entire continental United States plus much of Canada and Central America.

The time codes are sent from WWVB using one of the simplest systems possible, and at a very low data rate of one bit per second. The 60,000 Hz signal is always transmitted, but every second it is significantly reduced in power for a period of 0.2, 0.5 or 0.8 seconds: • 0.2 seconds of reduced power means a binary zero • 0.5 seconds of reduced power is a binary one. • 0.8 seconds of reduced power is a separator. The time code is sent in BCD (Binary Coded Decimal) and indicates minutes, hours, day of the year and year, along with information about daylight savings time and leap years.

The time is transmitted using 53 bits and 7 separators, and therefore takes 60 seconds to transmit. A clock or watch can contain an extremely small and relatively simple antenna and receiver to decode the information in the signal and set the clock’s time accurately. All that you have to do is set the time zone, and the atomic clock will display the correct time.

Dedicated NTP time servers that are tuned to receive the WWVB time signal are available. These devices connect o a computer network like any other server only these receive the timing signal and distribute it to other machines on the network using NTP (Network Time Protocol).

NTP – Network Time Protocol
SNTP – Simple Network Time Protocol

MSF – Radio Time Signal for United Kingdom
WWVB – Radio Time Signal for American
DCF – Radio Time Signal for Germany
TDF – Radio Time Signal for France
CHU – Radio Time Signal for Canada
JJY – Radio Time Signal for Japan
HBG – Radio Time Signal for Switzerland
GPS – Global Positioning System

UTC – Coordinated Universal Time
GMT – Greenwich Meantime – also known as UT1
TAI – International Atomic Time

LAN – Local Area Network
WAN – Wider Area Network
UDP – User Datagram Protocol
TCP – Transmission Control Protocol
IP – Internet Protocol
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
BIPM – Bureau International des Poids et Mesures (weights and measures)

The WWVB time signal is a dedicated radio broadcast providing an accurate and reliable source of United States civil time, based on the global time scale UTC (Coordinated Universal Time), the WWVB signal is broadcast and maintained by the United States’ NIST laboratory (National Institute for Standards and Time).

The WWVB time signal can be utilised by anyone requiring accurate timing information although its main use 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 WWVB signal is broadcast from Fort Collins, Colorado. It is available 24 hours a day across most of the USA and Canada, although the signal is vulnerable to interference and local topography. Users of the WWVB 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 WWVB 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 NIST

The signal’s field strength exceeds 100 µV/m (microvolts a meter) at a distance of 1000 km from Colorado – covering much of the USA.

The WWVB signal is in the form of a simple binary code containing time and date information The WWVB  time and date code includes the following information: year, month, day of month,  day of week,  hour, minute, Summer Time (in effect or imminent).