WLAN Anti-interference technologies - Huawei Enterprise Support Community

Dear All,

Today we are going to learn about WLAN Anti-interference technologies.

Interference From Non-WLAN Devices

The operating frequency bands of the WLAN include 2.4 GHz and 5 GHz.
The 2.4 GHz Industrial Medicine (ISM) is a universal wireless frequency band. Therefore, all wireless products can use this frequency band, and microwave ovens, cordless phones, and Bluetooth devices may cause frequency interference to WLAN networks.
Compared with the 2.4 GHz frequency band, the 5 GHz frequency band has less interference. Currently, devices that use the 5 GHz frequency band are mainly radars, wireless sensors, digital satellites, wireless ATMs, and software-defined radio.

If there are non-WLAN devices that work at the ISM frequency band, such as Bluetooth devices and microwave ovens, the WLAN communication will be greatly interfered, and the WLAN performance will deteriorate sharply. Therefore, in a WLAN environment, these interferences should be eliminated first, and at least ensure that there is no similar device that works with the WLAN for a long time.

Spectrum Analysis.

The spectrum analysis function enables WLAN devices to detect different types of interference sources on a wireless network. The spectrum analysis server analyzes the characteristics of collected wireless signals to identify and locate non-Wi-Fi interference devices to eliminate the interference on the WLAN network.

The 802.11 wireless technology has been widely used in homes, SOHOs, and enterprises. Users can easily access the network through these WLANs. However, the 802.11 wireless technology uses public spectrum resources, and many wireless devices, such as Bluetooth devices and cordless phones, use these frequency bands as well. As a result, radio signal conflicts and interference are severe in a wireless network environment, causing poor user experience.
Spectrum analysis can detect and identify these interference sources and present them to users for troubleshooting and interference migration, improving user experience.

Hidden Node

The boundary of a wireless network is not clear. Sometimes, not all nodes can directly communicate with other nodes, causing conflicts between nodes. A hidden node is a node that is within the communication range of the receiver but not within the communication range of the sender.
As shown in the following figure, the AP can directly communicate with PC1 and PC2. However, the wireless signals generated by PC1 and PC2 cannot reach each other. Therefore, PC1 and PC2 are hidden nodes of each other. When PC1 sends data to the AP, PC2 may send data to the AP at the same time because PC2 cannot detect that the current channel is not idle. As a result, a conflict occurs.

Hidden Node - RTS/CTS

The conflict caused by hidden nodes may be solved by reserving a channel using a RTS (Request to Send)/CTS (Clear to Send) frame before sending a data frame.

Exposed Node

An exposed node is a node that is within the communication range of the sender but outside the communication range of the receiver. In this example, PC1 and PC2 are mutually exposed nodes.
AP1 and AP2 work on the same channel. When PC1 is sending data to AP1, if PC2 wants to send data to AP2, PC2 will detect that PC1 is occupying channel resources according to the CSMA/CA mechanism. Therefore, PC2 mistakenly considers that it cannot send data to AP2. But actually, its sending of the packet does not affect the receiving of AP1.

Exposed Node - RTS/CTS

Before data frames are sent, RTS/CTS frames can be used to ensure that no channel conflict occurs.
Before sending data, PC1 sends an RTS message. AP1 receives it and returns a CTS message to PC1. If PC2 receives the CTS sent by AP1, it performs backoff and cannot transmit data. If only the RTS message is received but the CTS message is not received, data can be transmitted. Therefore, PC2 does not receive the CTS sent by AP1 and can send data to AP2.

CCA

In high-density Wi-Fi environments, such as spacious conference halls, student dormitories, and libraries, APs are densely deployed. As a result, APs on the same channel may be visible to and interfere with each other. The channel multiplexing technology can further reduce the coverage area of the AP, thereby reducing interference. Channel multiplexing increases the CCA threshold of the AP.
The Clear Channel Assessment (CCA) indicates that a WLAN chip needs to evaluate whether a channel is idle before
transmitting a signal to an air interface. If the channel is idle, the packet can be sent after the conflict backoff algorithm is executed. Otherwise, it needs to wait.

BSS Color (1)

Before 802.11ax is used, an STA can determine whether a received data frame is sent to itself only after parsing the data frame.
AP1 and AP2 send data to the STA. For example, the STA associated to AP1 first receives the data sent by AP2 and finds that the data is not sent to itself after parsing. However, now the STA has missed the data sent by AP1.

BSS Color (2)

802.11ax introduces the BSS color technology. If an STA detects that the BSS color is different from that of the AP it associated to, the STA considers that the data is from another AP and directly performs backoff.
AP1 and AP2 send data to the STA. The STA under AP1 receives the data sent by AP2 first. The STA detects that the BSS Color of the data is inconsistent with that of AP1 and discards the data. In this case, the STA timely receives the data from AP1 and demodulates the data.