Signal strength issues
This section includes information to help you identify and troubleshoot poor signal strength issues.
Asymmetric power issue
Asymmetric power issues are a typical problem in wireless communications. Access points (AP) can have a high transmit power which means that a signal can travel a long distance. However, clients may not have a transmit power strong enough for the APs to detect their signal.
Measuring signal strength in both directions
To solve an asymmetric power issue, measure the signal strength in both directions. APs usually have enough power to transmit long distances, but sometimes battery-powered clients have a reply signal that has less power, and therefore the AP cannot detect their signal.
It is recommended that you match the transmission power of the AP to the least powerful wireless client—around 10 decibels per milliwatt (dBm) for iPhones and 14 dBm for most laptops.
Even if the signal is strong enough, other devices may also emit radiation and cause interference. To identify the difference, read the client Rx strength from the Signal Strength widget (under Dashboard > WiFi) or CLI.
The Signal Strength/Noise value provides the received signal strength indicator (RSSI) of the wireless client. For example, a value of -85 dBm to -95 dBm is equal to about 10 dB levels; this is not a desirable signal strength. In the following screenshot, one of the clients is at 18 dB, which is getting close to the perimeter of its range.
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The recommended Signal Strength/Noise value from and to the FortiAP by clients is in the range of -20 dBm to -65 dBm. |
You can also confirm the transmission (Tx) power of the controller on the AP profile (wtp-profile) and the FortiAP (iwconfig), and check the power management (auto-Tx) options.
Controller configured transmitting power - CLI:
config wireless-controller wtp-profile
config <radio>
show
(the following output is limited to power levels)
auto-power-level : enable
auto-power-high : 17
auto-power-low : 10
Actual FortiAP transmitting power - CLI:
iwconfig wlan00
Result:
wlan00 IEEE 802.11ng ESSID:"signal-check"
Mode:Master Frequency:2.412 GHz Access Point:<MAC add>
Bit Rate:130 Mb/s Tx-Power=28 dBm
Perform a site survey
The most thorough method to solve signal strength issues is to perform a site survey.
A site survey helps with the optimal placement for your APs based on the variables in your environment. You must provide the site survey detailed information such as a floor plan (to scale) and structural materials. You can then overlay the floor plan with a Wi-Fi heat map, allowing you to map the APs and adjust the radio bands and power levels while providing you with visual wireless coverage.
Sample depiction of a site survey using Hamina Wireless Network Planner
The following includes mechanisms for gathering further information on the client for Rx strength. The goal is to see how well the client is receiving the signal from the AP. You can also verify FortiAP signal strength on the client using WiFi client utilities, or third-party utilities such as InSSIDer or MetaGeek Chanalyzer.
- Professional Site Survey software (Hamina Wireless Network Planner, Ekahau, AirMagnet Survey Pro, and etc.)
- InSSIDer
- On Windows: "netsh wlan show networks mode=bssid" (look for the BSSID, it's in % not in dBm)
- On MacOS: Use the "airport" command:
/System/Library/PrivateFrameworks/Apple80211.framework/Versions/A/Resources/airport" airport –s | grep <the_bssid> (live scan each time)
- On Android: WiFiFoFum
Frequency interference
If the wireless signal seems to be strong but then periodically drops, this may be a symptom of frequency interference. Frequency interference is when another device also emits radio frequency using the same channel, co-channel, or adjacent channel, thereby overpowering or corrupting your signal. This is a common problem on a 2.4 GHz network.
There are two types of interference: coherent and non-coherent.
- Coherent interference is a result of another device using the same channel as your AP, or poor planning of a wireless infrastructure. Perhaps the other nearby APs are using the same channel or the signal strength is too high.
- Non-coherent interference is a result of other radio signals such as Bluetooth, microwave, cordless phone, or x-ray machines (as in medical environments).
The most common and simple solution for frequency interference is to change your operation channel. Typically, the channel can be set from 1 to 11 for the broadcast frequency, although it is recommended to use channels 1, 6, and 11 on the 2.4 GHz band.
Another solution, if it is appropriate for your location, is to use the 5 GHz band instead.
MetaGeek Chanalyzer
You can perform a site survey using spectrum analysis at various points in your environment to locate sources of interference. MetaGeek Chanalyzer is an example of a third-party utility used for spectrum analysis of complex WiFi networks.
Fortinet wireless adapters ignore signals of -95 dBm or less.
