0 1 00:00:01,290 --> 00:00:04,050 The bitrate of a transmission is an easy calculation. 1 2 00:00:04,830 --> 00:00:07,590 As its name mentioned, it's the number of bits per seconds. 2 3 00:00:08,190 --> 00:00:13,650 So when we calculate the bitrate, we simply divide the number of bits by the transmission time. 3 4 00:00:14,460 --> 00:00:16,530 That's exactly what we've done so far. 4 5 00:00:17,550 --> 00:00:23,430 But when we use a protocol, there is usually only a little part of the frame which is useful for the user. 5 6 00:00:24,240 --> 00:00:25,990 Let's take an example of a LoRa packet 6 7 00:00:26,910 --> 00:00:33,840 We assume that a user wants to transmit one byte for let's say a temperature. We won't send right away the byte 7 8 00:00:33,840 --> 00:00:34,350 in the air. 8 9 00:00:34,710 --> 00:00:38,430 We need to use a specific format in respect to the LoRa specification. 9 10 00:00:39,090 --> 00:00:42,060 So let's see how a complete LoRa frame looks like. 10 11 00:00:43,140 --> 00:00:50,130 In a LoRa frame, there is a Preamble and a Header following by the Payload, 11 12 00:00:50,130 --> 00:00:51,310 here our temperature. 12 13 00:00:51,930 --> 00:00:54,720 And finally, a CRC for error detection. 13 14 00:00:55,740 --> 00:01:00,990 The previous bitrate was calculated using the Payload size and the time to transmit this Payload. 14 15 00:01:01,590 --> 00:01:06,660 But the real bitrate should be the Payload size divided by the overall Frame Time. 15 16 00:01:06,660 --> 00:01:10,860 And a Frame Time is what we call the Time On Air. 16 17 00:01:12,000 --> 00:01:18,030 If we make an analogy, we can imagine a car journey between two cities 100 kilometers apart. 17 18 00:01:18,630 --> 00:01:23,580 You can say that if you drive at 100 kilometers per hour, you will take one hour to get there. 18 19 00:01:24,000 --> 00:01:27,180 This is what your car dealer told you, and everyone believes it. 19 20 00:01:27,570 --> 00:01:31,470 But this is never true because you will have to get out of the city, 20 21 00:01:31,830 --> 00:01:33,120 you will have few stops, 21 22 00:01:33,120 --> 00:01:37,830 and again, you will never be able to drive at 100 kilometers until the final destination. 22 23 00:01:38,700 --> 00:01:43,920 So, we must make the difference between the car speed capacity and the average speed. 23 24 00:01:44,310 --> 00:01:50,370 And the average speed is in that case, the number of kilometers divided by the real duration of your 24 25 00:01:50,370 --> 00:01:50,850 journey. 25 26 00:01:51,780 --> 00:01:57,180 This is exactly the same thing here, and it is why we need to know the overall Time On Air. 26 27 00:01:57,660 --> 00:02:03,300 In the book provided with these courses, there is a formula that you can use but here will use a simulator 27 28 00:02:03,300 --> 00:02:04,740 called LoRa Calculator. 28 29 00:02:05,190 --> 00:02:12,240 This simulator depends on the transceiver used and the one used for this video is the SX1261 transceiver. 29 30 00:02:12,900 --> 00:02:16,800 I attached this little software with this video so you can download and try it. 30 31 00:02:17,970 --> 00:02:23,640 So we're going to set the same configuration corresponding to our two cases and check the result in the 31 32 00:02:23,640 --> 00:02:31,650 Time On Air field. The settings for the first case were SF7, 125 kHz bandwidth with a 32 33 00:02:31,650 --> 00:02:33,390 coding rate of 4/5. 33 34 00:02:34,380 --> 00:02:37,170 And we have several choices for the packet configuration. 34 35 00:02:37,860 --> 00:02:40,830 We could use anyone for a point to point LoRa transmission. 35 36 00:02:41,100 --> 00:02:47,700 Of course, as long as the transmitter and the receiver has the same configuration. But we're going 36 37 00:02:47,700 --> 00:02:48,180 to prepare 37 38 00:02:48,180 --> 00:02:50,880 the next step where we'll use only LoRaWAN. 38 39 00:02:51,660 --> 00:02:58,500 So we are not building a LoRaWAN frame here, but the LoRaWAN protocol will need these specific settings later. 39 40 00:02:58,980 --> 00:03:02,160 So that's why we apply these ones instead of choosing other ones. 40 41 00:03:03,240 --> 00:03:09,390 There is a Preamble length of eight symbols, Header mode enable and CRC enable. 41 42 00:03:11,580 --> 00:03:17,940 Finally, the Payload length is our simulated temperature of one byte, which gives a calculated Time 42 43 00:03:17,940 --> 00:03:21,330 On Air of 25.85 milliseconds. 43 44 00:03:22,560 --> 00:03:31,950 And we can do the same thing for the second case with a SF of 12, which gives a Time On Air of 827.39 milliseconds. 44 45 00:03:33,150 --> 00:03:38,010 So we can use these two durations to calculate the real bitrate of our LoRa transmission. 45 46 00:03:39,240 --> 00:03:45,660 Once again, the bitrate is the number of bid divided by the time, 8 bits divided by the Time On Air. 46 47 00:03:45,660 --> 00:03:53,910 In the first case, which was 25.85 milliseconds, that gives a result of 309.3 bits per 47 48 00:03:53,910 --> 00:03:54,090 second. 48 49 00:03:55,320 --> 00:03:56,520 When we use a SF of 12, 49 50 00:03:56,610 --> 00:04:03,300 we still have 8 bits, but this time it's divided by 827.39 milliseconds. 50 51 00:04:03,930 --> 00:04:07,590 That gives a result of 9.6 bits per second. 51 52 00:04:08,700 --> 00:04:15,000 You can see that we now reached a very low bitrate, far from the one given in the documentation. 52 53 00:04:15,810 --> 00:04:21,360 And unfortunately, we're not going to stop here since there is still two other things to take into 53 54 00:04:21,360 --> 00:04:21,810 account. 54 55 00:04:22,260 --> 00:04:24,120 This is what we will see in the next video.