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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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  • 3
    Why do you have this limit of 5 bytes total (including any transport layer overhead)
    – hardillb
    Nov 13, 2018 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, 2018 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, 2018 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, 2018 at 13:57
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    Withhold your number when sending? ;-) Btw, if you can't cram all your data into 5 bytes, use the 5 bytes to send an index into a look-up table that the recipient can use to retrieve the data. One thing about PITAs like your instructor is that they make you think. When you have finished this, you will have learned a lot (especially to take no more of their courses).
    – Mawg says reinstate Monica
    Nov 14, 2018 at 13:58

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If the environment is suitable I suggest you to try data-over-sound and shock your teacher :) This will transmit really 5 bytes. You just need a cheap speaker and microphone.

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