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