IoT communication technology, things you don’t know

Abstract: communication technology is the foundation of the Internet of Things. If the Internet of Things is likened to a logistics system, then the communication technology is equivalent to various modes of express delivery, such as air, water and land transportation. In communication technology, it can be roughly divided into two categories, one is wireless communication technology, and the other is wired communication technology.

This article is shared from HUAWEI CLOUD COMMUNITY " Things You Don't Know About Wired Communication Technology ", the original author: cat who loves bread.

The communication technology of the network layer is equivalent to the medium connecting the perception layer and the platform layer. Communication technology is the foundation of the Internet of Things. If the Internet of Things is compared to a logistics system, then communication technology is equivalent to various modes of express delivery, such as air, water, and land transportation. In communication technology, it can be roughly divided into two categories, one is wireless communication technology, and the other is wired communication technology. First of all, let's first look at what types of wired communication technologies are there, and what are the differences between them?

Ethernet

Ethernet (ETH) is simply a network cable network used by users. Ethernet is the current TCP/IP main local area network technology, and it is also the most common communication protocol standard adopted by existing local area networks. In the field of Internet of Things, in addition to wired access in office scenarios, Ethernet is mainly used in industry. Because Ethernet is low in cost and is a general standard of IEEE, it is improved Become an industrial Ethernet.

The core technology of Ethernet is the use of CSMA/CD (Carrier Sense Multiple Access/Conflict Detection) communication control mechanism. The CSMA protocol requires stations to monitor the channel before sending data. If the channel is idle, the station can send data; if the channel is busy, the station cannot send data. However, if both stations detect that the channel is free and start transmitting data at the same time, this will almost immediately cause a conflict. In addition, when the station is monitoring the channel, it is heard that the channel is idle, but this does not mean that the channel is really idle, because the data of other stations may be transmitted on the channel at this time, but due to the propagation delay, the signal has not arrived. Site, which leads to wrong judgments of the channel status. In the early CSMA transmission mode, due to the channel propagation delay, even if the stations of the two communication parties did not detect the carrier signal, collisions may still occur when sending data. Because they may send data at the same time when they detect that the medium is free, causing conflicts to occur.

Although CSMA can detect conflicts, it does not have the conflict detection and prevention functions of the prophet, resulting in frequent conflicts. Therefore, the CSMA protocol can be further improved, so that the sending station continues to listen to the medium during the transmission process to detect whether there is a conflict. If both stations detect that the channel is free at a certain time and start to transmit data at the same time, they will almost immediately detect that there is a conflict. If a conflict occurs, electromagnetic waves that exceed the amplitude of the carrier signal sent by the sending site can be detected on the channel, and the existence of the conflict can be judged from this. Once a conflict is detected, the sending station immediately stops sending and sends a series of blocking signals to the bus to notify the other station communicating on the bus to quickly terminate the corrupted frame, which can save time and bandwidth. The station is sending data. Conflict detection is performed in the process, and once the conflict is detected, the data transmission is stopped immediately. Such a protocol is called a carrier sense multiple access protocol with collision detection.

RS-232 and RS-485

Readers who have studied embedded development may have a better understanding of RS232. Figure 4-1 is a schematic diagram of the RS-232 interface. Many readers will feel familiar with it because there is such an interface behind the desktop computer. The characteristic of RS232 is that it mainly supports one-to-one communication and the communication distance is relatively short, which can only be no more than 20 meters. RS485 is equivalent to an improved version of RS232. When it comes to RS485, it supports one-to-many transmission. A maximum of 128 transceivers are allowed on the bus. At the same time, the transmission rate and communication distance have also been greatly improved.

Comparison of RS-232 and RS-485

Table 4-1 is the difference between RS-232 and RS-485. Simply put, the difference between the two lies in three points: The first point is that the transmission method is different. RS-232 adopts an unbalanced transmission method, the so-called single End communication. RS-485 uses balanced transmission, that is, differential transmission. The second point is that the transmission distance is different. RS-232 is suitable for communication between local devices, and the transmission distance is generally no more than 20m. The transmission distance of RS-485 is tens of meters to thousands of meters. The third point is that the number of communications is different. RS232 only allows one-to-one communications, while the RS-485 interface allows up to 128 transceivers to be connected on the bus.

Communication serial bus

In serial communication, in addition to RS232 and RS485. There is also USB, which is also called Universal Serial Bus, which is a serial bus standard for connecting computers and other external devices. Before the emergence of the USB interface, the computer interface was in the Spring and Autumn Period and the Warring States Period. Multi-party data such as serial and parallel ports, such as keyboard, mouse, modem, printer, scanner, etc., must be connected to different interfaces. One interface can only be connected to one device, but It is impossible for a computer to support so many interfaces, so the expansion capability is insufficient and the speed is limited. USB was born to solve the speed, scalability, and ease of use.

Just because it is very common in life, the Internet of Things, a technology that is in line with life, will also widely use USB for data transmission. One thing that needs to be emphasized is that USB is divided into different types according to the interface, among which the more common are the four types in Figure 4-2, Type-A, Type-B, Micro-B and Type-C.

M-Bus technology

M-Bus, also called MeterBus. It is a bus specially used for remote meter reading services. For example, it is widely used in electric meters, water meters, and gas meters. This technology is not common in domestic meter reading services, but it is used in Europe. widely used. The characteristic of this kind of bus technology is that it can supply power to the device remotely, and does not need to lay a power cord, so if the power is cut off at home, it will not affect the instrument.

Power line carrier PLC technology

PLC is also called PowerLineCommunication. This technology means to transmit data by attaching data to the wires that are usually used. So how does it work? Firstly, the high-frequency signal carrying information needs to be loaded on the current, and then transmitted through the wire, and then the high-frequency signal is separated from the current with an adapter at the other end, and then transmitted to the computer Use this to achieve communication. But in fact, the PLC technology has shortcomings. It can only be used in near-end scenarios where the voltage does not change. This is because the principle of this technology is to load high-frequency signals on the wires, but when the voltage on the wires changes, the high-frequency signals on the wires will disappear. Therefore, this technology can only be applied to near-end scenarios where the voltage does not change. In the meter reading business, PLC technology is mainly applied from the meter reading terminal to the management terminal, because when the data is transmitted upwards, it will go through the steps of power transformation and transmission, so the data will disappear after the voltage changes, so it cannot be The upper layer continues to use PLC technology. The data will be first loaded on the wire and uploaded to the management terminal, and then the management terminal is connected to the base station and the data can be uploaded to the database for operation through the switch and server. This is the main process of using PLC power meter reading.

Table 4-2 is a brief comparison of the above-mentioned wired communication technologies. Among the wired communication technologies, these technologies are basically used in industry and public utilities. Because in the field of the Internet of Things, devices are relatively mobile, so there will be fewer scenarios for wired communication applications, and more will use wireless communication for data transmission.

Four short-range wireless communication characteristics and application scenarios

Next, we introduce the common wireless communication technologies of IoT. Among them, the wireless technology can be subdivided into many different parts, such as the cellular network used by operators, and a series of short-range communication technologies such as Bluetooth.

BluetoothBluetooth, namely Bluetooth

This technology is very common in life, and Bluetooth is already an essential technology in mobile phones, computers, tablets and other devices. Bluetooth technology was originally created by the telecommunications giant Ericsson in 1994 as an alternative to the RS232 data line. Bluetooth can connect multiple devices, overcoming the problem of data synchronization. In the Internet of Things, such as sports bracelets and smart electronic scales, Bluetooth technology is required. Compared with other wireless communication technologies, the transmission distance of the old version of Bluetooth technology is relatively short, with a range of only 10 cm to 10 meters. But its transmission rate is relatively fast, up to 1Mbps.

But now Bluetooth technology has developed to the version of Bluetooth 5.0. Although it is still a short-range wireless communication technology, its transmission distance can reach very far. Bluetooth 5.0 supports a transmission rate of up to 3Mbps and a transmission distance of up to 300 meters. At the same time, Bluetooth technology is divided into two types of technology in the later stage of development, one is BR/DER, and the other is LE. Among them, what needs to be focused on is the LE type, because the LE type of Bluetooth technology is very suitable for use in the Internet of Things. The Bluetooth technology that readers may be familiar with still communicates in a point-to-point manner, but the LE type of Bluetooth technology can support multiple forms of network topologies such as point-to-point, broadcast, and Mesh, which is very suitable for multi-device connection in IoT scenarios. Data transfer is in progress.

Wi-Fi

Most people will definitely use Wi-Fi for daily Internet access in scenarios such as home or office. So the application of Wi-Fi is very extensive. Wi-Fi is usually used in 2.4G and 5G frequency bands. Through these two different frequency bands, it can provide different services for different devices. Compared with the previous version of Bluetooth, the Wi-Fi communication distance is relatively far, and supports one-to-many connections. At the same time, its transmission rate is also very fast. However, the shortcomings of Wi-Fi are also very obvious. First of all, its security is not good and its stability is very poor. For example, when watching a video, you may find that the video is half stuck. Also, when the user is playing a computer game, the experience will be very obvious. If you use Wi-Fi to play the game online, the delay will change very greatly, sometimes it is 20 to 30 milliseconds, and sometimes it changes directly. It's one or two hundred milliseconds. Therefore, the stability of Wi-Fi is relatively poor, and the power consumption of Wi-Fi is relatively high. Like Bluetooth, Wi-Fi has also developed to a new generation of Wi-Fi6, which supports a transmission rate of 9.6Gbps and a delay as low as 20ms.

ZigBee

Compared to Wi-Fi and Bluetooth, ZigBee and the readers of Z-Wave described below may know little about it. ZigBee is also a short-distance and low-power wireless technology. Figure 4-5 is a schematic diagram of the working mode of ZigBee devices. Compared with the working schematic diagram of Wi-Fi devices in Figure 4-4, you can actually find the characteristics of this technology. Wi-Fi devices can only be connected to APs or main concentrators when they are connected, but ZigBee is different, and its data can also be transferred between devices. This represents the easy networking feature of the ZigBee technology. If the intermediate access point of the Wi-Fi device is broken, the entire network will be paralyzed. But ZigBee is different, because each device of ZigBee can act as a relay. If one of the devices breaks down, the other devices can reorganize the network, and find another device that can act as a relay to rebuild a network. The characteristics of ZigBee technology are actually very similar to its name. ZigBee is also known as the Zigbee Protocol, because it comes from the horoscope dance of bees, because bees rely on the "dance" of flying and "buzzing" their wings to communicate with their companions. The transmission of pollen location information means that bees rely on this method to form a communication network in the swarm, which is very similar to the flexible networking characteristics of ZigBee.

In addition, the cost of ZigBee module is very low, only about 2 dollars, and compared with Wi-Fi, its rate is very low, only 20 to 250kbps. At the same time, its shortcomings are poor compatibility and difficult to maintain.

Z-Wave

In addition to ZigBee, there is also a short-range wireless technology called Z-Wave. Z-Wave is actually similar to ZigBee, but their difference is that Z-Wave is relatively more reliable, but its protocol standards are not open, and Z-Wave chips can only be obtained through SigmaDesigns. Z-Wave technology was positioned in the field of smart home wireless control when it was initially designed. Using small data format transmission, the transmission rate of 40kb/s is enough to deal with, and the transmission rate of 9.6kb/s was even used in the early days. Compared with other similar wireless technologies, it has a relatively low transmission frequency, a relatively long transmission distance and a certain price advantage.

Short-range wireless technology comparison

A simple comparison is made for the above four IoT short-range wireless technologies. The main difference is that the transmission rate of Bluetooth and Wi-Fi is relatively high, but the old version of Bluetooth can only connect one-to-one, and Wi-Fi can be one-to-one. So, Bluetooth is mainly used in devices such as mice, earphones, and mobile phones, while Wi-Fi is mainly used in homes or other indoor high-speed Internet. At the same time, based on the low speed and many connection nodes of ZigBee and Z-Wave devices, it is basically unlikely to be applied in other fields except the Internet of Things, because their transmission rate is too low. Therefore, ZigBee and Z-Wave are mainly used in home automation, smart home, smart buildings and other fields.

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