18.1 What Time Is It?

     

I still find it amazing that we spend so much time (pardon the pun) and effort in establishing what the correct time is. From a philosophical perspective, time doesn't actually exist. Time is a notional construct for which man has devised many weird and wonderful formulae for calculating its current value (http://www.greenwich2000.com/channels/time/home.htm). In the end, what we are measuring is a relationship between one event happening and another event happening. In science and business, this is crucial in order to map and log when transactions occur. A transaction can be anything from a quantum particle passing a detector to a customer withdrawing money from an ATM. Many applications require that we know and log the time of these transactions.

Every server has its own internal clock. We can rely on it to tell us the time. The accuracy of that time will vary from machine to machine, but one thing is certain: none of the machines in our network can reliably maintain the correct time for extended periods. A reasonable tolerance for the quartz crystal inside your server (this is the thing that gives the time) is 50 parts per million (ppm). This sounds pretty good, but if you think about it 1 day = 86400 seconds, which means that within a single day we may lose (86400/1000000)*50 = 4 seconds per day. Now, this may seem like a great way to get home quicker day by day, but our client-server applications are going to become very confused when different machines have different interpretations of what the correct time is. For some applications, knowing the correct time is crucial; in fact, it can literally be the difference between life and death; administering drugs to a patient 4 seconds too late is not acceptable, is it? Our first job in configuring NTP is to decide how critical it is for our applications to know the correct time. Worldwide, the correct time is maintained by a number of military, government-sponsored, and scientific organizations (approximately 200). Table 18-1 shows some of the key players.

Table 18-1. Worldwide Timekeepers

Organization	URL or Transmitter Frequency	Source
U.S. Naval Observatory	http://tycho.usno.navy.mil	Cluster of 28 Block II/IIA/IIR GPS satellites
U.S. National Institute of Standards and Technology	http://www.bldrdoc.gov/timefreq	NIST-F1 cesium fountain atomic clock with an accuracy of approximately 1 second in 20 million years .
	WWVB terrestrial longwave (60kHz) radio transmission
	WWV terrestrial shortwave (2.5MHz, 5MHz, 10MHz, 15MHz, 20Mhz) radio transmission
	WWVH terrestrial shortwave (2.5MHz, 5MHz, 10MHz, 15MHz) radio transmission
International Earth Rotation Service	http://hpiers.obspm.fr/eop-pc/	Advise BIPM when leap seconds need to be inserted into UTC
National Physical Laboratory (UK)	http://www.npl.co.uk	Cesium fountain atomic clock.
	MSF terrestrial longwave (60kHz)
Royal Greenwich Observatory	http://www.greewich2000.com	In October 1884, the International Meridian Conference established Greenwich, England, as the Prime Meridian .
Bureau International des Poids et Mesures (International Bureau of Weights and Measures)	http://www.bipm.com	Calculates International Atomic Time (TAI) from the +200 atomic clocks around the world.

The BIPM's mandate is to provide the basis for a single, coherent system of measurements throughout the world, traceable to the International System of Units (SI). This task takes many forms, from direct dissemination of units (as in the case of mass and time) to coordination through international comparisons of national measurement standards (as in length, electricity, radiometry, and ionizing radiation). A key measurement they provide is the correct time from a worldwide perspective. The most accurate time we currently use is derived from the perturbations of a cesium atom; a second is now defined as " the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom." This is why we could say that the correct time is actually the International Atomic Time (TAI). The correct time calculated from over 200 clocks around the world is a stable time standard because it is based on the highly predictable behavior of the cesium atom. The rotation of the Earth is less than predictable. In fact, there are only about four days a year that are precisely 24 hours long. Having a time standard that is more in tune with our day-to-day earthly existence is more practical and acceptable to everyone. Such a time standard is known as Coordinated Universal Time (UTC). UTC is a calibrated version of TAI so that over the course of a year, on average, the sun crosses over the Prime Meridian (yep, this is still situated in Greenwich, England) at noon to within 0.9 seconds. The calibration to TAI is by having leap seconds added to or subtracted from (there's never been a subtraction) TAI. Leap seconds are introduced under the guidance of the International Earth Rotation Service. Because UTC and GMT have the same meridian, some people regard UTC and GMT to be the same thing. Technically, this is not accurate; but for most day-to-day implementations , it is a generalization that won't harm anyone . So we now have some idea of what the correct time is. Before setting up a timeserver, we need to choose which source of time we will use.