# Why are precise frequency standards important in the telecommunications industry? [closed]

I understand why having a precise definition of time can be very useful for navigation purposes. The GPS system works using the simple equation $d=ct$, where $c$ is the speed of light, so the uncertainty in position reflects the uncertainty in time.

I also have always heard the example of the telecommunications industry relying on the existence of precise frequency and time standards.

Why does the telecommunications industry rely so much on this standards?

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 Is this question off-topic? – Qmechanic♦ Apr 7 at 0:02 Probably more suited for the EE SE. – mng Apr 7 at 5:49

## closed as off topic by Ϛѓăʑɏ βµԂԃϔ, Qmechanic♦May 7 at 18:14

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Modern techniques for accurate time measurement use atomic clocks, in which the frequency of EM radiation is measured as the basis reference for time measurement. This can be seen by the SI Unit definition of the 'second' (unit of time):

'The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

It follows that the hyperfine splitting in the ground state of the caesium 133 atom is exactly 9 192 631 770 hertz, (hfs Cs) = 9 192 631 770 Hz.'

Since the definition of the SI unit for time (the second) is based on the frequency measurement of radiation emitted from casein 133, it follows that accurate measurement of frequency is crucial for precise measurement of time intervals using atomic clocks.

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The telecommunications industry aren't the only ones using the electromagnetic spectrum. You have telecom (cellphones, etc), radio stations, TV stations, defense communications, shortwave, and radio astronomers competing in that space (Just to name the ones I can think of off the top of my head.)

Cellphones themselves have a variety of bands that they use depending on the specific technology (look at this article), so you can see that this is getting very crowded very quickly.

Each industry/group of people do have a specific band that is 'protected' for their use, which is why the telecommunications industry really have to use their allocated spectrum wisely. They do have to handle and route all their traffic through those limited frequency bands.

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In some wavelength-division multiplexing systems for fiber-optic communications, you have optical signals with frequency around 200 terahertz, and the different channels are separated by as little as 25 gigahertz. Let's say the transmitter and receiver need to be matched to within 5% of the channel separation (I'm not sure the actual number). If that's the case, then the calibration process would need to reproducibly measure frequencies at 1-part-per-200,000 accuracy. In other words, you need to be able to say "The frequency of this laser is 234.567 terahertz ... not 234.566, not 234.568!" And it has to be a universal standard so that two different manufacturers on two different continents can calibrate their lasers and receivers separately to 234.567 THz, and they will find that the components are compatible with each other.

Maybe there are other examples in telecommunications where even more digits of accuracy are required. I don't know.

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