Daily Basic Knowledge Record

What is the difference between WLAN and Wi-Fi?

   Wi-Fi refers to the trademark of the Wi-Fi Alliance, and is also a wireless network communication technology based on the IEEE 802.11 standard, with the purpose of improving the interoperability between wireless network products based on the IEEE 802.11 standard.
   The full name of WLAN is Wireless Local Area Network. The narrow definition of WLAN is based on the IEEE 802.11 series of standards, using high-frequency wireless radio frequency (such as wireless electromagnetic waves in the 2.4GHz or 5GHz frequency band) as a wireless local area network as the transmission medium. The WLAN in our daily life refers to the narrow definition of WLAN. During the evolution and development of WLAN, there are many technical standards for its implementation, such as Bluetooth, Wi-Fi, and HyperLAN2. Due to its relatively simple implementation, reliable communication, high flexibility, and relatively low implementation cost, Wi-Fi technology has become the mainstream technical standard of WLAN, and Wi-Fi technology has gradually become synonymous with WLAN technical standards.
   Simply put, WLAN是一个网络系统，而Wi-Fi是这个网络系统中的一种技术. Therefore, there is an inclusion relationship between WLAN and Wi-Fi, and WLAN includes Wi-Fi.

What do you think of Wi-Fi 7? Will it replace Wi-Fi 6 anytime soon?

See Zhihu for specific answers

The difference between WAN port and LAN port

WAN port: external network interface, used to connect to all external networks such as the Internet (this interface cannot be used to connect to a computer), and the external network cable is connected to this port

LAN port: internal network interface, used to connect computers or other routers and other equipment (this interface cannot be connected to MODEN (telephone line dial-up Internet access), optical modem, etc.)

Domain name and IP address

The difference between domain name and ip address

Bluetooth working frequency band

There are 79 channels in the Bluetooth bandwidth of the United States, Europe and other countries, and each channel occupies a bandwidth of 1MHz. Japan, Spain and France use 23 channels.

Wireless signal frequency band division

radio frequency

   In the theory of electronics, when current flows through a conductor, a magnetic field will be formed around the conductor; when an alternating current passes through a conductor, an alternating electromagnetic field will be formed around the conductor, which is called electromagnetic wave. When the electromagnetic wave frequency is lower than 100kHz, the electromagnetic wave will be absorbed by the surface and cannot form effective transmission, but when the electromagnetic wave frequency is higher than 100kHz, the electromagnetic wave can propagate in the air and be reflected by the ionosphere at the outer edge of the atmosphere to form a long-distance transmission capability . We call high-frequency electromagnetic waves with long-distance transmission capabilities radio frequency.
   Radio frequency (RF) is the abbreviation of Radio Frequency, which means the electromagnetic frequency that can radiate into space, and the frequency range is from 300kHz to 300GHz

Active Radar and Passive Radar

When conventional radar detects targets, it needs to continuously emit radio waves, so it is called active radar. The advantage of active radar is that it can search for targets independently, because it receives electromagnetic waves emitted by itself, so it has high sensitivity and good resolution. But this kind of radar is vulnerable to electromagnetic interference from the target, and it is also easy to expose itself to the attack of anti-radiation missiles.

Passive radar has no transmitting device, only a receiving antenna, which can receive the electromagnetic waves radiated by the target itself, so as to judge and determine the identity, orientation and distance of the target. This kind of radar that does not emit electromagnetic waves is not easy to expose itself and is not easy to be counter-reconnaissance by the enemy. It not only has good concealment, strong anti-interference ability, high security, but also small size and long detection distance.

multicomponent signal

Definition 1: In a narrow sense, a multi-component signal refers to a mixed signal formed by the superposition of signals from different sources. In a broad sense, it can also include complex signals from the same source that contain multiple components to be analyzed. Definition 2: From different
sources Signals, clutter and noise often overlap in the time-frequency domain to form multi-component signals

confusion matrix

Confusion Matrix

regression problem

   Regression is an important problem in supervised learning.
   Regression is used to predict the relationship between input variables (independent variables) and output variables (dependent variables), especially when the value of the input variable changes, the value of the output variable follows changes that occur.
   A regression model is exactly a function representing the relationship between input variables and output variables 映射.
   Learning for regression problems is equivalent to function fitting: choose a function curve that fits known data well and predicts unknown data well