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In the LoRaWAN frame,
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The Frame Payload is the encrypted user data.
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So, for example, if the device sends a one-byte temperature, then the Frame Payload is also on one
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byte, but encrypted with the Application Session Key.
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Of course, the only way to decrypt the payload is to use the same Application Session Key that the
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one used during encryption.
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This is the role of the application server. And it also works the other way around:
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So when the user wants to send the data from the application, just before passing it to the LoRa MAC layer,
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the LoRaWAN protocol encrypts the data to be transmitted.
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When the device receives the frame payload, it will decrypt it with the same Application Session Key.
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Okay.
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Now that we have the overall LoRaWAN frame, let's check how many bytes are in there.
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Of course, for that, we need to dive into the LoRaWAN specification.
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But I will do it for you for this little example.
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The MAC Header is an 1 byte, plus 4byte for the Device Address, plus 1 byte for the Frame Control,
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plus 2 for the frame counter.
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We'll consider a frame without options, plus 1 byte for the frame port, 1 byte for a Frame Payload,
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which in our case is the temperature.
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And finally, 4 bytes for the Message Integrity Control.
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That gives it a total of 14 bytes.
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Okay.
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That's the theory.
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Now, I would like to show you that it can be verified with demonstration because it's exactly what
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happens in reality.
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So I'll show you a very small demonstration of a LoRaWAN transmission of 1 bite of temperature.
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This demonstration uses a TTN version 2 Network Server that is no longer available on the Web.
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But the logs were so clear that I would like to use it for the sake of clarity.
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Of course, this demonstration can be done with any other Network Server.
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For this demonstration, I've got on the left
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a window with the gateway traffic.
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And on the right, the decrypted frame when they arrived on the Application Server.
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So I start my end device.
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And right after the first transmitted frame, I pause the live data event on the Gateway.
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And I also stop it on the Application Server.
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What do we see ?
On the gateway,
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the payload size is 14 bytes.
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Yes, that's exactly what we've calculated before.
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14 byte is what we call the raw payload of physical payload.
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There are many things in there: the devadrr, the Message Integrity Control, the Frame Counter
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and so on...
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But there is also the encrypted user payload.
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Great.
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Now, what do we see on the right on the application server side?
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There is only one byte.
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That's my temperature data.
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I don't see here the 14 bytes, because among these 14 bytes, only one corresponded to the user payload.
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It was the frame payload.
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It's been decrypted and reported here in the console.
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Right.
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So we've seen here in details the content of the LoRa modulation and the LoRaWAN  frame.
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Now let's have a look at the transmission between the gateway and the network server.