What is the reason why the wireless signal is blocked by the wall of the house? What does wireless signal penetration capability mean? Such questions are likely to be asked by those new to wireless networking. The ability of wireless signal to pass through the wall The reason why there are the concepts of "through wall ability" and "wall blocking" in WLAN technology is determined by the wireless frequency band used by WLAN technology. Since the radio frequency of the wireless local area network uses the ISM (industrial, scientific, medical) wireless frequency band, the 802.11b and 802.11g standards use the 2.4-2.4835GHz frequency, and the 802.11a standard uses the 5.8GHz frequency. These frequencies are all microwaves. The characteristics of microwave are high frequency, short wavelength, and straight line propagation . In the direction of back propagation, it can hardly avoid obstacles , which is not like medium wave and short wave in radio stations. Radio waves can be classified and named by frequency or wavelength. We call electromagnetic waves with frequencies higher than 300MHz as microwaves. Since the propagation characteristics of each band are different, they can be used in different communication systems. For example, medium waves mainly travel along the ground and have strong diffraction capabilities , making them suitable for broadcasting and maritime communications. The short wave has strong ionospheric reflection ability , which is suitable for global communication. Ultrashort waves and microwaves have poor diffraction ability due to their short wavelengths, and can be used as line-of-sight or beyond-line-of-sight relay communications. Therefore, the ability of wireless signals to pass through the wall refers to whether the wireless signals sent between wireless LAN devices can penetrate the wall that is blocked in the middle, and if they can penetrate, to what extent the signal is attenuated, and whether wireless devices can still be wireless. connect.
The biggest feature of microwave is that it propagates almost in a straight line, and the diffraction ability is very weak, so the wireless receiving device behind the obstacle will be blocked by the obstacle. Therefore, for the wireless microwave signal propagating in a straight line, it can only "penetrate" the obstacle to reach the wireless device behind the obstacle . The wireless signal that "penetrates" the obstacle will be attenuated into a weaker signal. As for how strong the signal is, this is the penetrating ability or simply the "wall penetration ability". Because electromagnetic waves have wave-particle duality , electromagnetic wave wavelength λ=C/f (C is the speed of light, λ is the wavelength, f is the frequency of the electromagnetic wave), and the electromagnetic wave energy E=hv (E is the energy, h is the Planck constant, v is the frequency wavelength Inversely proportional to the photon energy) , when the wavelength is shorter, the photon energy is larger, and the penetration is stronger (here refers to the penetration rate) . For example, high-energy X-rays can penetrate almost all non-metallic objects, even thin aluminum. While gamma rays penetrate most metals, only heavy metals (such as very thick lead) can block them. At the same transmission power, different wavelengths, and the same medium in the same environment, the penetration ability of the shorter wavelength must be stronger (here refers to the penetration value), but its penetration loss is longer than the wavelength. big . This is a question of penetration rate and penetration value. Why do people have different opinions? It is because these two concepts are used when introducing penetration in different materials. In general, we discuss penetration by penetration rate. In the microscopic world, many things can be explained by the principle of resonance. Resonance means that when two individuals have similar radii, they will vibrate together. energy transfer. From this it is easy to explain the diffracting and penetrating power of frequencies.First of all, energy is not lost in vacuum. The lower the frequency, the longer the wavelength band, so it is less likely to lose energy. The specific performance is that the diffraction ability is stronger; the higher the frequency, the smaller the wavelength band, and the closer to the atomic radius of the molecule, the easier energy transmission. The specific performance is penetration The stronger the force. Therefore, the higher the frequency, the stronger the energy of the electromagnetic wave, the stronger the penetrating ability, the more opportunities it has to interact with matter, and the greater the loss when penetrating. The lower the frequency, the weaker the energy of the electromagnetic wave, the weaker the penetration ability, the less chance of interacting with the material, and the smaller the loss when penetrating.When we use a dual-band wireless router, when the transmission power of the wireless signal in the 2.4GHz band is the same as the transmission power of the wireless signal in the 5GHz band, and the wireless signal penetrates the same wall, the received wireless signal strength in the 2.4GHz band is higher than The signal strength of the 5GHz band is because the attenuation of the wireless signal in the 2.4GHz band is smaller than that of the wireless signal in the 5GHz band when penetrating the same wall, that is, the penetration value of the wireless signal in the 2.4GHz band is greater than the penetration value of the wireless signal in the 5GHz band. An important indicator of the "wall penetration capability" of wireless devices When using a wireless router, everyone hopes that the wireless signal can penetrate at least the walls and floors of the house. There are various materials for the walls, such as wooden walls, glass walls, brick walls, concrete walls, etc. The floors are generally reinforced concrete. Every time a separation wall is penetrated, the wireless reception signal is more or less attenuated, and the building structures above are attenuated in turn from low to high. Passing through a layer of wood, the received signal will be attenuated by 4dB; passing through a brick wall, the received signal will be attenuated by 8dB-15dB; passing through a reinforced concrete wall, at least 15dB-30dB. Once a wireless device with too low transmitting power, insufficient receiving sensitivity, and insufficient antenna gain is selected, the wireless signal will be attenuated very much, the transmission rate will drop rapidly, and even wireless blind spots will easily appear in the home. will be disconnected. The transmitting power, receiving sensitivity (this is bidirectional), antenna gain, and effective transmission distance of wireless devices are directly related to the ability to cut off penetration, whether the connection is stable, and the final actual transmission rate. . To improve the ability of wireless signals to penetrate the partition wall, the following technical indicators must be guaranteed. (1) The maximum transmit power of wireless LAN equipment specified by IEEE 802.11 is 20dBm (100mW), and generally larger products should reach 17dBm. (2) The current optimum receiving sensitivity is -105dB. Passing through a layer of wooden boards, the received signal will be attenuated by 4dB; passing through a brick wall, the received signal will be attenuated by 8-15dB; passing through a reinforced concrete wall, at least 15-30dB. Wireless devices with a transmission sensitivity of up to 105dB have strong wall penetration; they can continuously penetrate three sides up to a thickness of 1. 2 meters of reinforced concrete walls with a total interval of 30 meters without any relay equipment. (3) The antenna gain is preferably 3-5dBi. The antenna gain of a general wireless LAN device is 2dBi. According to experience, the antenna signal with a gain of 2dbi can penetrate two walls. If there are too many rooms and there are many partition walls, it is best to have a detachable antenna, so as to configure a high-gain antenna, such as replacing a 5dBi omnidirectional antenna to enhance it. Obstacles of metal objects not only block microwave wireless signals, but also absorb electromagnetic energy and generate weak currents to leak out. Therefore, the largest obstacle of metal objects without wireless signals in the home environment is steel bars. The floor of the net, the signal in this direction is almost impossible to penetrate. To be able to penetrate, the signal is also very weak. With such a large-scale obstacle, the diffraction of microwaves is even more impossible. If the wireless device is placed in the center of the house, the wireless signal can only be transmitted straight from the window from the open passage. In order to achieve the desired effect of wireless devices, it is necessary to choose an optimal placement location. The requirements are as follows: (1) The position should be higher, so as to radiate downward in a higher place, reduce the blocking of obstacles, and minimize the signal blind area; (2) The position should make the signal pass through the number of partitions as little as possible, preferably Wireless clients in the room are visible to and from wireless devices. Therefore, the best place to place the wireless device at home should be in the living room, and it can best be visible to the computer host in the room, or can be "invisibly visible through the wooden door".