Not all WiFi problems are related to signal strength, interference, or misconfiguration. The following OSI model identifies some of the more common issues per layer.
Best practices for troubleshooting vary depending on the affected layer (see below).
Common sources of wireless issues
Best practices for Layer 1
Common physical layer issues include:
- Weak received signal, l WiFi capability: 802.11b, 1×1, 2×2, l Co-channel WiFi interference, General problems
- Side band WiFi interference, l Non 802.11 noise (microwave ovens…).
To avoid physical layer issues:
- Determine RST (Receiver Sensitivity Threshold) for your device, or use -70dBm as a rule of thumb.
- Match AP TX output power to the client TX output power. l Note: iPhone TX power is only 10dBm.
- Use DFS (Dynamic Frequency Selection) for high performance data 20/40 MHz. l Use 5GHz UNII-1 & 3 (Non-DFS) bands with static channel assignment for latency-sensitive applications. l Do not use 40MHz channels in 2.4 GHz band (channel bonding is not allowed in FortiOS).
Best practices for Layer 2
Common data link (MAC) layer issues include:
- Too many clients on a single channel (CSMA/CA) backoff, l Too many high-priority traffic clients (WMM), l Incorrect password or encryption settings, l Too many beacons (in dense installs).
To avoid data link layer issues:
- Only use CCMP/AES (WPA2) encryption (not TKIP).
- In high density deployments, turn off SSID broadcast or turn down SSID rates. Review and possibly reduce the beacon interval. l Determine the best cell size for applications:
- For few users and low bandwidth latency sensitive applications, use high transmit power to create larger cells.
- For high performance/high capacity installations, use lower transmit power to create smaller cells (set FortiPlanner at 10dBm TX power), but bear in mind that this will require more roaming.
Cells and co-channel interference
In high density deployments, multiple APs are used, and each one services an area called a cell. However, these cells can cause interference with each other. This is a common problem. The radio signal from one AP interferes with, or cancels out, the radio signal from another AP.
In the following diagram, note the interference zone created by one radio, causing interference on its neighbouring APs.
The interference zone can be twice the radius of the signal, and the signal at its edge can be -67dBm.
Reducing co-channel interference
For best results, use a ‘honeycomb’ pattern as a deployment strategy. The idea is to stagger repeated channels furthest from each other to avoid interference.
Best practices for Layer 3 and above
For TCP/IP layers and above, a common source of latency, or slowness in the wireless traffic, is too many broadcasts or multicasts. These types of issues can result from non-business and/or unwanted traffic.
To resolve issues at the TCP/IP layer and above:
- Identify business-critical applications.
- Use Application Control, Web Filtering, Traffic Shaping, and QoS to prioritize applications.
- Identify unwanted traffic, high-bandwidth web-related traffic, and use Security Profiles. l Use the traffic shaper on a policy to rate-limit this traffic.
These configurations are performed directly on the FortiGate.
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