Wifi/Wireless Diagnostics

Finding issues in wireless networks can be hard , however there are some tools you can use before you get the Spectrum Analyser in! 


Download and install inSSIDer Home

Great way to visualise SSID strength and channels, just to note when you run this , your Pings will go up!


Great Heatmapping software and paid for software for scanning

For Home or small office WiFi network tuning/refining, we can simply use some of following software to help us:


How to check to DeAuths 

Once you identify the channel, launch https://www.wireshark.org/ on that channel and listen for a minute or two.

First, apply this filter:

wlan.fc.type_subtype == 0xc

This will show you all the deauthentication frames that have been sent out.

Deauth Flood

Apply this filter next:

wlan.fc.type_subtype == 0x8 && wlan.sa == <BSSID of the SSID you are inspecting>

This will display beacon frames from your AP. Check the signal strength. In this case, we’ve got a good strong signal because we’re right next to the AP (right around -40 dBm on average).

Our Beacons

Next, apply this filter:

wlan.fc.type_subtype == 0xc && wlan.sa == <BSSID of the SSID you are inspecting>

This shows deauthentication frames from your AP. Note the signal strength on the far right…

Spoofed Deauths

The deauthentication frames are coming in much weaker than the valid beacon frames. This indicates strongly that another AP is spoofing your system.



2 What is SNR or S/N or Signal-to-noise ratio

SNR is the difference between the received wireless signal and the noise floor. The noise floor is simply erroneous background transmissions that are emitted from either other devices that are too far away for the signal to be intelligible, or by devices that are inadvertently creating interference on the same frequency. [1]

e.g. A client device’s radio receives a signal at -75 dBm, and the noise floor is -90 dBm, then the effective SNR is 15 dB. This would then reflect as a signal strength of 15 dB for this wireless connection.

3 Why does SNR matter for WiFi

To achieve certain speed, certain SNR is required, e.g. to achieve 1300Mbps, 32dB or SNR is required (Note: SNR is not the only requirement, other conditions like number of spatial streams, WiFi generation etc. are also required to achieve it)

4 What decides WiFi roaming between/among different WiFi AP (Access Points)?

It’s not as simple as we thought, e.g. this WiFi Access Point is close to use, our device (Mobile, computer/laptop etc.) will then switch/connect to it directly, WiFi roaming does not work like that, but usually it is the optimal case which we want to achieve.

Usually by default, WiFi clients are “sticky” which means they tends to keep associated/connected to the already connected/associated AP. They will only switch/roam if it is impossible or almost impossible to transfer data (e.g. Signal strength dropped significantly) [This behaviour can be altered by changing setting on the operating system/driver e.g. WiFi roaming aggressiveness or by modifying some configuration on the WiFi system/Access Point, e.g. force to disconnect the client which has certain low signal strength, changing AP radio power level etc.]
Note: If a client is too aggressive, always switch to AP with stronger signal instantly, the user’s browsing/VOIP experience etc. will be negatively and heavily impacted. It can make the internet/network unstable or even unusable.

So, usually factors which will impact WiFi client roaming aggressiveness are WiFi signal strength, vendor of client device, operating system of the client device, configuration of the client device, AP/WLAN (Wireless LAN) system configuration etc.

5 How to eliminate or reduce sticky clients/cases? (How to make clients roam between APs as desired)

Usually it is the client that decides when to roam/switch to another AP.

There are many ways to reduce/eliminate sticky client issue.

At the Client side

  • If we are using computer/laptop, usually there are settings to control wireless roaming aggressiveness, we can change it to medium or high if necessary (Test should be done before deciding which option is the best for our specific needs/situation)
  • Manually disconnect wait 5-10 seconds and reconnect to the WiFi (Usually the client will associate/connect the one with strongest signal in this case)

At the AP/WLAN side

Although in the 802.11 standards, there is not much standards on roaming, many AP vendors/systems still provides some control over client roaming.

  • Place APs far enough from each other physically, so that WiFi signal from each AP does not overlap/cover same spot/area
  • Reduce AP radio output power level, so that WiFi signal from each AP does not overlap/cover same spot/area
  • Some AP vendors/systems provide features like actively disconnect clients which has weak signal under certain/specified level (Usually this is not good if we want clients to have fast roaming experience among different APs)
  • Setting minimum RSSI may or may not force client to roam (Using minimum RSSI may not be a good idea, lower the radio output power maybe better)

6 How to Design/Plan AP positioning (How to properly design deploy APs for enterprise/business, education, office, home etc.)

WiFi/WLAN deployment can be tricky sometimes, especially when designing a network that has multiple APs. Obscures in the radio wave path can block or even reflect radio wave. Different material will have different level of impact on signal and its strength, some may block the signal completely while some will only have slight impact, as mentioned, some will reflect the signal.

To properly design and deploy the WiFi/WLAN network. We may need to do some testing and site survey, e.g. using Ekahau. There are also some vendors provide WiFi/WLAN designer, e.g. UniFi Design Center, Ruckus Wi-Fi planner, Ekahau etc.

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