0
1
00:00:00,240 --> 00:00:05,130
In a previous video, we saw how we could optimize the 868 MHz band.
1

2
00:00:05,580 --> 00:00:11,490
The first idea we had was to separate this band with channels and we were able to get 8 end devices
2

3
00:00:11,490 --> 00:00:13,560
transmitting with 8 different channels.
3

4
00:00:13,740 --> 00:00:19,710
Then, the second idea was to have several end devices talking on the same channel, and we saw that with
4

5
00:00:19,710 --> 00:00:24,240
6 different colors, we could have 6 transmission on each channel.
5

6
00:00:24,480 --> 00:00:30,780
And then knowing that we had 8 channels overall, we had 48 data transmissions at the same time.
6

7
00:00:31,500 --> 00:00:36,660
Having 48 devices is not enough, as we said, and we expect to have much more than that.
7

8
00:00:37,710 --> 00:00:43,250
So we're going to use a third method, and this third method will limit the transmission time of each
8

9
00:00:43,260 --> 00:00:44,130
end device.
9

10
00:00:44,670 --> 00:00:49,370
And for that we need to understand a transmission time of a LoRaWAN packet.
10

11
00:00:49,400 --> 00:00:54,390
Each LoRaWAN packet can last between approximately 50 milliseconds to 2 seconds.
11

12
00:00:54,830 --> 00:00:58,830
We will use an average of 1 second per message to make an easy calculation.
12

13
00:00:59,310 --> 00:01:07,440
And so, if each end devices transmits during 1 second, the European specification on the 868 MHz band
13

14
00:01:07,770 --> 00:01:13,980
defines that when you speak for 1 second, then you must remain silence for 99 seconds.
14

15
00:01:14,640 --> 00:01:20,610
And if we take the example of the temperature sensor we saw in the first video, it transmits one message
15

16
00:01:20,610 --> 00:01:21,720
every 10 minutes.
16

17
00:01:21,930 --> 00:01:24,930
So in that case, we have a very comfortable margin.
17

18
00:01:25,440 --> 00:01:32,130
This limit is called the one percent duty cycle and we can simply say that an end device shouldn't transmit
18

19
00:01:32,130 --> 00:01:33,990
more than 1% of the time.
19

20
00:01:34,710 --> 00:01:37,680
Then an easy calculation : in one hour,
20

21
00:01:37,680 --> 00:01:43,380
so 3600 seconds, an end device transmits less than 36 seconds.
21

22
00:01:44,700 --> 00:01:51,570
So finally, if an end device is authorized to speak only 1% of the time, it means that we can multiply
22

23
00:01:51,570 --> 00:01:53,910
by 100 the number of end devices.
23

24
00:01:54,570 --> 00:02:04,980
So during 100 seconds we will have 48 end devices times 100, and that gives 4800 end device
24

25
00:02:05,280 --> 00:02:07,560
which can transmit during this period of time.
25

26
00:02:08,700 --> 00:02:11,460
They obviously can't talk together at the same time.
26

27
00:02:11,940 --> 00:02:13,440
Otherwise there would be collisions.
27

28
00:02:13,950 --> 00:02:19,290
But during this period there will still be thousands of them capable to transmit their messages to the
28

29
00:02:19,320 --> 00:02:19,770
gateway.
29

30
00:02:20,370 --> 00:02:26,100
In the previous example, I used a 1 second packet transmission, but obviously all LoRaWAN messages are
30

31
00:02:26,100 --> 00:02:31,260
not that long and obviously some LoRaWAN end device will transmit only a few packets.
31

32
00:02:31,980 --> 00:02:37,770
So, we can assume that I used the worst case, and at the end of the day, that will allow many more devices
32

33
00:02:37,770 --> 00:02:38,640
to communicate.