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# Stratum: How we know what time is it?

Posted on: Nov, 2nd 2010
Have asked yourself sometime if exists an universal reference clock?. In reality doesn't exists, because the time is relative, but a lot of computer applications, specially for the programs that make operations in the stock exchange or bank transactions, is needed have the clocks in the earth synchronized.

We can synchronize the clock of our computer with a time server, that provides the time using the communication protocol NTP (Network Time Protocol).

There are several kind of servers of this kind. The ones that have a higher level of precision are the stratum-0, that are devices that include atomic clocks. These clocks are based in the oscillation of the atom of cesium 133, measuring a second every 9.192.631.770 oscillations and have a drift of one second every 300.000 years (the drift is the time that the clock differs from the perfect clock), therefore these are the reference clocks in the world.

Inside of this group of servers stratum-0 there are some of them that have a precision level slightly lower, because it receive the time from the satellites (using a GPS) or radio stations, and correct it taking into account the transmission time and the theory of relativity of Einstein. In the transmission by radio the error can go from the 0.1 to the 10 milliseconds, while by satellite, the error is around of one microsecond.

Any computer can't be connected, using the network, to the stratum-0 servers and these send the time to the following kind of server, the stratum-1, through a direct cable.

We can connect to the stratum-1 through the network and are the primary time servers in Internet.

Next we have the secondary servers or stratum-2 that, using the network, are synchronized with the primary servers using algorithms specially thought for this task. These algorithms, taking into account the delays of the network, allow a precision of tens of milliseconds.

Next we have the stratum-3 that are synchronized with the stratum-2 and so on, so that, unless all the stratum-0 and all the stratum-1 go offline, always we will have a server to ask the time very adjusted to the real time.

Although is certain that to the human isn't so useful have the same time precision that have the machines, also this doesn't would serve to nothing because we don't know the delay that our brain make us perceive the reality in the head of every one of us. But if you are interested in have the perfect time here you have a list of servers stratum-1 that you will can use with NTP clients. In the majority of the operating systems we can get the time accessing to the properties of the clock in the taskbar and using one of this servers.

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