In a previous video, we saw how we could optimize the 868 MHz band. 1
The first idea we had was to separate this band with channels and we were able to get 8 end devices 2
transmitting with 8 different channels. 3
Then, the second idea was to have several end devices talking on the same channel, and we saw that with 4
6 different colors, we could have 6 transmission on each channel. 5
And then knowing that we had 8 channels overall, we had 48 data transmissions at the same time. 6
Having 48 devices is not enough, as we said, and we expect to have much more than that. 7
So we're going to use a third method, and this third method will limit the transmission time of each 8
end device. 9
And for that we need to understand a transmission time of a LoRaWAN packet. 10
Each LoRaWAN packet can last between approximately 50 milliseconds to 2 seconds. 11
We will use an average of 1 second per message to make an easy calculation. 12
And so, if each end devices transmits during 1 second, the European specification on the 868 MHz band 13
defines that when you speak for 1 second, then you must remain silence for 99 seconds. 14
And if we take the example of the temperature sensor we saw in the first video, it transmits one message 15
every 10 minutes. 16
So in that case, we have a very comfortable margin. 17
This limit is called the one percent duty cycle and we can simply say that an end device shouldn't transmit 18
more than 1% of the time. 19
Then an easy calculation : in one hour, 20
so 3600 seconds, an end device transmits less than 36 seconds. 21
So finally, if an end device is authorized to speak only 1% of the time, it means that we can multiply 22
by 100 the number of end devices. 23
So during 100 seconds we will have 48 end devices times 100, and that gives 4800 end device 24
which can transmit during this period of time. 25
They obviously can't talk together at the same time. 26
Otherwise there would be collisions. 27
But during this period there will still be thousands of them capable to transmit their messages to the 28
gateway. 29
In the previous example, I used a 1 second packet transmission, but obviously all LoRaWAN messages are 30
not that long and obviously some LoRaWAN end device will transmit only a few packets. 31
So, we can assume that I used the worst case, and at the end of the day, that will allow many more devices 32
to communicate.