Technology “hidden” by life: WIFI

Nowadays, we use WIFI so many times every day that we can't even remember it. In this era of highly popular wireless networks, we are surrounded by various wireless signals anytime and anywhere. Today we will talk about one of these wireless signals: Wi-Fi.

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The Chinese name of Wi-Fi is "Wireless Fidelity", which is a brand owned by the Wi-Fi Alliance. This alliance has nothing else to do but to solve the connectivity issues between products that comply with the IEEE 802.11 standard. Therefore, the technology corresponding to Wi-Fi is the IEEE 802.11 standard. The evolution of the IEEE 802.11 standard can be seen in the figure below:

The 802.11ac standard in the above picture is what we call 5G network in daily life. Its theoretical transmission speed can reach 6.93 Gbps. The nominal transmission speed of 5G routers on the market is also around 1Gbps. However, due to the higher frequency band of 5G network, its "wall penetration" ability is slightly poor.

A wireless network card and a wireless access point (Wireless Access Point) can form a Wi-Fi system. The wireless router we use at home is a router with wireless AP function. In a Wi-Fi network, before the wireless network card and the wireless access point establish a data connection, two steps must be taken: SCAN (scan) and Authentication (authentication).

Take our home router as an example. When we configure "Enable SSID Broadcast" in the router's settings interface, devices that need to access the Wi-Fi network (such as mobile phones, etc.) can identify the wireless network created by the router. There are two ways to identify: active scanning and passive scanning.

Active scanning: The access device sends a broadcast to all Wi-Fi channels it supports, asking whether there is a wireless access point on the corresponding channel. When the wireless router receives the broadcast of the access device, it will respond to the access device and inform the other party of its SSID, supported authentication methods, encryption algorithms and other basic information.

Passive scanning: The wireless router periodically sends broadcasts to its own channel. The content of the broadcasts is roughly the same as the responses in active scanning. When the access device receives the broadcasts, it "discovers" the Wi-Fi network established by the wireless router.

When the access device decides to access the router's Wi-Fi network, it enters the 802.11 standard authentication process.

Even if the Wi-Fi network does not have a password, it will still go through an authentication process called "open system authentication." The entire authentication process is roughly like this: the access device says "Hello!", and the wireless router says "Come in!" (You invite everyone into your home?)

For a Wi-Fi network with a password set, the authentication process will be slightly more complicated, as shown below:

Careful old friends may feel that the whole process is familiar. Yes, the whole process is similar to a simplified version of the symmetric key transmission process during the HTTPS connection, so I will not go into details here.

After successful authentication, the access device can freely swim in the Wi-Fi ocean.