Best practices for OSI common sources of wireless issues

Not all WiFi problems are related to signal strength, interference, or misconfiguration. The following Open System Interconnection (OSI) model identifies some of the more common issues per layer.

Best practices for troubleshooting vary depending on the affected layer. See the following illustration.

Common sources of wireless issues

Best practices for Layer 1

Common physical layer issues include:

• weak received signal
• WiFi capability: 802.11b, 1x1, 2x2
• co-channel WiFi interference
• side band WiFi interference
• non 802.11 noise (such as microwave ovens)

To avoid physical layer issues:

• Determine the RST (Receiver Sensitivity Threshold) for your device, or use -70 dBm as a rule of thumb.
• Match the AP TX output power to the client TX output power.
• Use DFS (Dynamic Frequency Selection) for high performance data 20/40 MHz.
• Use 5 GHz UNII-1 & 3 (Non-DFS) bands with static channel assignment for latency-sensitive applications.
• Do not use 40 MHz channels in 2.4 GHz band. (FortiOS does not allow channel bonding.)

Best practices for Layer 2

Common data link (MAC) layer issues include:

• too many clients on a single channel (CSMA/CA) backoff
• too many high-priority traffic clients (WMM)
• incorrect password or encryption settings
• too many beacons (in high-density installations)

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.
• 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 and high-capacity installations, use lower transmit power to create smaller cells (set FortiPlanner at 10 dBm TX power), but bear in mind that this setting requires 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 neighboring APs.

The interference zone can be twice the radius of the signal, and the signal at its edge can be -67 dBm.

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 or unwanted traffic, or both.

To resolve issues at the TCP/IP layer and above, you can:

• 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.
  • Use the traffic shaping on a policy to rate-limit this traffic.

You perform these configurations directly on the FortiGate.