I need to transfer wirelessly a message of maximum 5 bytes of data (including everything such as start bits, stop bits etc etc).

What I tried:
Empty Ethernet package is at least 56 bytes according to this post
Empty SMS PDU mode is bigger than 5 bytes according to this link

Are there any other options?
Considering that I'm pretty much a beginner into wireless communications:
Is it even possible to do it without reinventing the wheel?
Are there any devices that can send only as few bytes as I need?

Distance: in the order of kilometers, but a few meters is ok too for now

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    Why do you have this limit of 5 bytes total (including any transport layer overhead) – hardillb Nov 13 '18 at 21:36
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    I need to send a message from a moving train to the control dispatch and the message must not be bigger in size than 5 bytes. This is for a school project and I think this requirement is just way to hard to implement for a student. @Mawg I still have to find a way of transmitting the data (3g/4g/RF it doesn't matter) – Codrut Nov 14 '18 at 11:30
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    @Mawg The teacher is a dickhead as I already developed the circuit which includes a IC and GSM modem and developed the source code to send the SMS and receive it to the other end. After I did everything he tells me that the transmitted message size is bigger because the phone number is attached to my 5 byte message. – Codrut Nov 14 '18 at 13:50
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    Now the only different option that I'm thinking of is to build some kind of RF circuit that sends a radio square wave signal and another one that receives that radio signal. But as I was saying this seems too hard because the message transmitting and receiving is only a small part of the whole project. – Codrut Nov 14 '18 at 13:57
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    I'm actually the first who got this far with the project. I will ask for some tips and hopefully I'll not receive some abstract answers. Thank you for your help. – Codrut Nov 14 '18 at 14:50

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