Hiding the hidden node in the site on the Wireless Network

In this article, we will talk about what to do if your wireless network conflicts.

IT professionals should focus on managing unlicensed Wi-Fi spectrum to avoid network conflicts and optimize the performance of WLAN network applications. This kind of management involves many aspects. Conflicts can be produced by the collision of signals in overlapping channels. In addition, if another operator's 802.11 devices compete for the spectrum with your device, or some non-802.11 devices run in a certain spectrum, or environmental factors that prevent or weaken signals may lead to conflicts.

The biggest cause of the conflict is hidden nodes. Hidden nodes are basically client devices that are located within the range of WLAN Access Point signals, but are located within the range of other nodes. See the circle in:

You can think of this circle as a clock. In the figure, assume that client A is located at nine o'clock, 50 meters away from the AP Access Node. Client B is located at three o'clock, 50 meters away from the AP Access Node. The two clients are 100 m apart, that is, 328 feet ).

You may already know that the 802.11b/a/g series standard uses a media access control mechanism (MAC Mechanism), known as "Multi-Channel Access and anti-air collision by carrier" CSMA/CA) method. After the client nodes are more than 300 feet apart, they are unlikely to "listen" to each other's data transmission, thus avoiding conflicts. The two nodes transmitted on a public channel will immediately cause conflicts, which will cause mutual interference and reduce throughput and response time.

Tool for detecting problems

Some Wi-Fi wireless device monitoring and management products can detect hidden nodes. For example, AirMagnet's portable wireless network analyzer is a good choice. The new version of its Laptop Analyzer product is called Laptop Analyzer 7.0 PRO, and the latest version has been upgraded to 8.0. If the performance of the wireless network is seriously degraded, one thing you need to do is to find the crux of the problem. Therefore, this feature is a good tool.

What the user should do

Having a high-density AP deployment helps solve the problem, because the smaller the overlapping "circle" section, that is, the smaller the circle diameter or the client distance, the easier it is for nodes to listen to each other. The test shows that if the device uses the 2.4Gz band. Although your circle is so small as the three channels that do not overlap each other, it also interferes with the access nodes that recently used the same bandwidth.

Once the problem is detected, one of the measures that can be used is to increase the power level of client nodes so that they can "see" each other. However, this method may fail. If there are obstacles, such as the walls of reinforced concrete structures, the signal transmission will be blocked. This is obviously a problem you need to consider in the planning of the site in the early stage.

You may think of using an omnidirectional radiation antenna or a non-oriented antenna instead of a targeted antenna. This will indeed allow your antenna to listen at 180 degrees rather than 360 degrees, which will increase the client's chances of obtaining signals from other nodes. If you do this, you must be consistent with your security policy. After all, you are not willing to let your WLAN signal pass through the walls and leak it out in all directions.

Note that 802.11 requires the help of the "send"/"Clear send", that is, the RTS/CTS) protocol. In the process of coordinated transmission, these protocols require other nodes to wait for a period of time while sending signals that are allowed to be transmitted to some clients. However, considering the dynamic characteristics of the wireless network and the increase in the number of clients, we think this is not very safe.

In addition, the 802.11e QoS standard also includes the protocol for access control. This is a more robust communication coordination function, mainly for the VoIP QoS service. It is also possible to use this mechanism to further coordinate the client-to-access point communication and other data transmission.

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