Notes IoT provider. Introduction

Hello, dear readers of Habr. My name is Oleg Plotnikov, I am the director of the Industrial Internet Center of one of the IT companies.

With this article I would like to start a whole cycle of notes about the Internet of Things and specifically about the LoRa technology. About her already wrote more than once on Habré, they wrote interestingly and from different sides. However, our Center has built and has been operating the LoRaWAN network for almost a year now and we have gained a lot of experience in the practical operation of this technology. Let's agree, I do not set as my goal a PR company in which I work. But I will periodically turn to our projects when I need to illustrate something. I would like to focus on my notes not on theory, but in practice. I will try to infect you with the idea that the Internet of Things is interesting and the tortoise-like LoRa will become an important brick in the common foundation.

Start will have all the same from the beginning.

So, LoRa. It stands for Long Range, a technology for highly secure transmission of small amounts of information. Developed by the French company Semtech.
Actually, the term LoRa itself is a type of modulation. L1 by OSI model.

LoRaWAN is already a stack over physics, the next level. Most often, saying LoRa, imply a combination of modulation and stack.
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There are solutions on LoRa, but without LoRaWAN, as a rule, proprietary and highly specialized. I will talk about them in the future, while we assume that by “Laura” we understand the classic view: modulation plus stack.

So, stop. What is LoRa, what are you talking about? Why constantly invent something? Want a wireless future? Internet of Things? We have excellent technology: Wi-Fi or LTE. What they do not suit you?

Not satisfied. Wi-Fi (like LTE) works under the concept of Internet People. Give a limited number of users the widest possible Internet channel. How many users can hang on a single Wi-Fi point? 20, 30, 100? And on the base station sector? About the same numbers.

But each user has a tolerable communication channel. Ten users can count on several LTE-megabits / sec. And a few dozen Mb / s, if we are talking about Wi-Fi. For this they will pay a wide spectrum, with their battery and shouts of “return the connection!” On the square in the New Year.

The part of the Internet of Things that is responsible for telemetry requires a different approach.
Suppose we have a microdistrict, where all the counters in the apartments transmit their data via radio. They do it not often, not more than once per hour. And even once a day. The size of the package they also calculated in bytes. But there are many, several thousand. They stand in iron shields, in tile-laid openings, in “dark corners” and basements. They are not always possible to supply power, because some of them live on batteries. There is no requirement for them to transfer packets in milliseconds, but these packets must pass.

Do you feel we have a slightly different concept? Wi-Fi and LTE are redundant for us. They will land the battery, score a place on the access point (or sector), do not accelerate to serious speed. This is where special standards created for the Internet of Things come to our aid. At the moment there are several dozen. After careful analysis, we chose LoRa.

What can she do?

Let's slowly get used to the local terminology and introduce the first important concept - SF (Spreading Factor). Literally - the coefficient of expansion of the spectrum. If on fingers, then SF is an integer from 7 to 12, to which a number of parameters are attached, such as maximum transmission speed or packet size. The greater the SF, the slower the transmission rate, but the higher the noise immunity. Conversely, less SF, higher speed, but less resistance to interference. This parameter is also linked to the time spent on the air.

Spreading Factor and some of the values ​​that depend on it

So, the advantages of LoRa:

1. High noise immunity. In the city, LoRa will penetrate a kilometer or even two at a transmitter power of 25 mW. True, this is on the highest SF = 12. For this, we will pay the packet size (51 bytes of the payload) and the transmission rate (292 bits / sec). A simple principle - the more energy in a bit of information, the further this bit will fly away.
   These volumes are unthinkable for the Internet of the People, but the apartment electric meter will be quite suitable.
2. Low power consumption. The standard spelled out three classes of devices for energy consumption. The A-class is the most economical, it lives on batteries for several years. True, such a device cannot be called up on-line, it wakes up on a schedule and exchanges information with the server, and then goes to sleep on. If you need to be constantly in touch - this is class C, it does not provide for battery power. There is also B, something in between, but he somehow does not take root. For our electric meter, the A-Class is the most.
3. Short time spent on the air. In the process of activation, the device will exchange a pair of packets with the server (join_request? Join_access!) And after that it will only contact you on business. No registrations in the network, a minimum of official traffic. In A-class in general: woke up, sent a packet, received confirmation (if provided) and fell asleep further. If everything is bad and SF = 12, then the packet will take 2 seconds. At SF = 7 - 0.1 seconds. Now imagine how many subscribers can we hang on a single LoRa base station if they go out once an hour? Account for hundreds. And if once a day? Then for thousands.

Bonus! LoRa is an open standard. The ball is still ruled by the developer (Semtech), he has a monopoly on chips for chips. However, the chips themselves are already being made by several companies, and the equipment will soon be riveted in the basements. Those. we have a large selection of independent vendors. And if they honestly support the LoRaWAN specification, then they will be compatible with each other. This is very important and in the future I will talk a lot about it.

Bonus2! LoRa operates in the 868 MHz band. In Russia, “as much” one and a half megahertz in this frequency band does not require licenses. These are 864.0-865.0 MHz and 868.7-869.2 MHz. There is a power limit (no more than 25 mW) and a number of “no”, but we are passing everywhere. One problem, the use of 250 and 500 kHz channels in such a situation is simply waste, which is why we have not taken root for these variations. Well, okay, and turn around at 125 kHz.

Unlicensed frequency 868 MHz in the Russian Federation and their limitations

As you can see, LoRa, seemingly slow and unprepossessing, fits perfectly into the concept of telemetry.

In this article, we made the first insight into technology and imbued with the idea. So far everything is beautiful. In the second part we will start building a network. You will learn how to get a rake on the forehead, if you don’t read the small print in the instructions and why you need to file the base stations with a file.