We've seen that we use the 868 MHz band. 1
But in reality the 868 band is not only one frequency, it's a frequency band that goes from 867 MHz 2
to 869 MHz. 3
In this band, we'll have to find a way for several end devices to communicate at the same time. 4
The first idea is to divide this frequency band into different channels. 5
In LoRaWAN, we have eight channels used for uplink, and downlink. Uplink is the capability to transmit 6
data from the end device to the server. 7
And downlink means that the data comes from the user in the direction of the end device and there is a specific 8
channel only for downlink. 9
The channels we are interested in are actually more uplink ones. 10
Why is that? 11
Because in the world of the Internet of Things, we have many more sensors than actuators. 12
This means that we are more interested in getting data from the end device, than providing some to it. 13
And we will see later that a LoRaWAN infrastructure, has indeed more capabilities in uplink than downlink. 14
8 channels allow us to have 8 end devices transmitting at the same time. 15
That is a really low number compared to what we could expect for an IoT transmission, where we should 16
speak about thousands of devices which would be able to reach a gateway. 17
So we'll have to find other techniques. 18
And the next one we're going to explain is called spread spectrum. 19
Spread spectrum is the capability to have several devices talking in the same channel at the same time. 20
For example, in the first channel that I've presented here, 867.1 MHz. 21
The first device transmits some information, so it chooses the channel. 22
Then the second, third, fourth, and so on. 23
They all can communicate using the same channel at the same time. 24
But there is only one condition for that. 25
They must use a different color. 26
This color in the LoRaWAN modulation is called a spreading factor. 27
For this slide, we speak about colors and we leave for later the explanation of what a spreading factor is, 28
and how it will influence the number of devices that can talk to the gateway at the same time. 29
In fact, the number of spreading factors, 30
so, the number of available colors we've got, is six. 31
So with six different colors, this means that, for example, in the first channel, we have six devices 32
that can talk at the same time. 33
Knowing that we have eight different channels, we have in fact, 48 end devices 34
that can speak at the same time. 35
Obviously, that only happens if we assume that on a specific channel they all have a different color 36
during the transmission. If there are two similar colors in the same channel, 37
is it serious? 38
Well, there will be a collision. 39
This means that a retransmission would be necessary. In LoRaWAN, 40
we don't target real time application, so it's not so bad to have a collision. 41
But on the other hand, it's necessary to know if both the end device and the user wants to be notified of 42
the fact that the message has arrived or not. 43
To do so we'll have the possibility of having message acknowledgement, but we will see that in another 44
video. 45
So the best we can do so far is to have 48 end devices talking at the same time, which is far less than 46
we expected. 47
So we'll need another technique to increase this number, and that's what we will see next.