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We saw in the previous video that the real bitrate, when sending one byte LoRa payload is 309.3 bits per
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second at SF7 and 9.6 bit per second at SF12.
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So we're far from the documentation values.  The temperature used, as the LoRa payload, is called the phy payload in the
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specification.
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That's what the gateway is interested in. The gateway takes the phy payload and sends it to the server.
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So if you are using a point to point LoRa transmission, everything is fine and you can keep a one byte
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payload.
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But what we want to do is not a LoRa transmission.
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It's a LoRaWAN transmission.
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And there is a big difference.
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LoRaWAN will set up new services for the device.
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For example, there will be special fields relative to its security, to its identification or to its application
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parameters.
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This means that there will be a LoRaWAN header.
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This LoRaWAN header will increase the number of bit transmitted through the network. And the user
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data,
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our temperature, will be encapsulated into the LoRa Header and the LoRaWAN Header.
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Obviously the extra data in the LoRaWAN Header will necessarily increase the transmission time of
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the whole frame. The previous phy payload is now the user data, our temperature, and the LoRaWAN header.
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So now to calculate the new bitrate, we need to know the new Time On Air of the entire LoRaWAN frame.
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For that, I could dive into the specifications and find out the size of the LoRaWAN Header, but we
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haven't spoken much about
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the LoRaWAN specifications so far. So let's do it in another way.
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I'm going to send a LoRaWAN frame with one byte user payload for my device. One time with SF7, 125 kHz
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and a second time with SF12.
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And on my gateway log, I'm going to take the Time On Air.
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Let's do that.
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I go on my gateway log and I send a frame.
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We can see here that the first frame uses SF7, 125 kHz bandwidth and four over five coding
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rate.
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And the second one, SF 12, 125 kHz bandwidth and four over five coding rate.
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And what do we see?
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We see that the airtime,
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that's just another word for Time On Air,
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is 46.3 milliseconds, when we use SF 7 and 1155.1 milliseconds when we use SF
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12.
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These are the two time I will use to calculate my LoRaWAN bitrate right after. When I transmit it with
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my LoRaWAN device,
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I transmitted only one byte as we agreed, so you can be very surprised to see that the payload size
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is 14 bytes.
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What do these 14 bytes include?
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These 14 bytes are the fee payload.
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The include my one byte temperature and of course, the LoRaWAN Header.
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We can therefore said that the LoRaWAN Header has in that case a size of 13 bytes.
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In the LoRaWAN Header, there will be the device address, integraty message, the application port and
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so on, but no stress,
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we'll see that soon.
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So same calculation as before,
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but we take into consideration the new frame duration. Here for SF 7, I transmitted 8 bits divided
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by 46.3 millisecond and that gives 172.7 bits per second.
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And when we use SF 12, I still have 8 bits transmitted, but a duration of 1155.1 milliseconds,
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and that gives a result of 6.9 bits per second.
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So again, the bitrate has been significantly reduced.
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Unfortunately, it's not over yet because the European standard on the 868 MHz band has another
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restriction which will reduce even more the final bitrate. That will be the last step, and we're
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going to see that in the next video.