Wireless Troubleshooting

Troubleshooting WiFi

Systematic WiFi troubleshooting methodology covering physical layer through connectivity issues

Troubleshooting WiFi

WiFi Connection Issues Overview

  • WiFi problems typically fall into four categories: authentication, association, DHCP, and connectivity
  • Use systematic approach: verify physical layer first, then work up OSI model
  • Most issues occur at Layer 1 (RF problems) or Layer 2 (802.11 frame issues)

Common WiFi Problem Categories

Problem Type Layer Symptoms Primary Causes
RF Issues Layer 1 Weak signal, frequent disconnects Distance, interference, obstacles
Authentication Layer 2 Cannot connect to SSID Wrong PSK, certificate issues
Association Layer 2 Connects but no data AP overloaded, incompatible settings
DHCP Issues Layer 3 Connected but no IP DHCP pool exhausted, VLAN issues

Step-by-Step Troubleshooting Process

1. Physical Layer Verification

  • Check signal strength: minimum -70 dBm for basic connectivity, -50 dBm for optimal performance
  • Verify channel utilization (should be <50% for good performance)
  • Identify interference sources: microwaves (2.4GHz), Bluetooth, other APs
  • Test at different distances from AP to isolate range issues

2. Authentication Problems

  • Verify Pre-Shared Key (PSK) matches exactly (case-sensitive)
  • Check security method compatibility (WPA2/WPA3, TKIP/AES)
  • For enterprise: validate certificate chain and RADIUS server connectivity
  • Common issue: mixed WPA2/WPA3 environments causing compatibility problems

3. Association Issues

  • Verify SSID broadcast is enabled (or client knows hidden SSID)
  • Check AP client limit (typically 50-100 concurrent users per radio)
  • Validate band steering settings aren’t forcing incompatible bands
  • Ensure client supports AP’s 802.11 standard (a/b/g/n/ac/ax)

4. DHCP and IP Connectivity

  • Verify DHCP pool has available addresses
  • Check VLAN assignment matches client expectations
  • Test static IP assignment to isolate DHCP vs routing issues
  • Validate default gateway and DNS server reachability

Advanced Troubleshooting Techniques

Wireless Controller Diagnostics

  • Use controller CLI for real-time client debugging
  • Monitor roaming events and sticky client issues
  • Check for proper load balancing between APs
  • Verify tunnel establishment (CAPWAP) between AP and controller

Spectrum Analysis

  • Identify non-WiFi interference using spectrum analyzer
  • Check for radar detection on DFS channels (5GHz)
  • Monitor channel utilization across all available channels
  • Look for hidden node problems in dense deployments

Common Enterprise Issues

  • Sticky clients: devices not roaming to stronger APs (adjust power levels)
  • Co-channel interference: multiple APs on same channel causing collisions
  • Band steering problems: forcing 5GHz when client has poor 5GHz reception
  • Fast roaming issues: 802.11r/k/v misconfiguration causing slow handoffs

Vocabulary

RSSI (Received Signal Strength Indicator): Measurement of RF power received by client, typically expressed in dBm (negative values, closer to 0 is stronger)

SNR (Signal-to-Noise Ratio): Difference between signal strength and background noise floor, minimum 20dB recommended for reliable connectivity

Channel Utilization: Percentage of time channel is busy with any RF activity, includes WiFi and non-WiFi sources

Sticky Client: Device that remains connected to distant AP instead of roaming to closer, stronger AP

DFS (Dynamic Frequency Selection): Requirement for 5GHz channels to detect and avoid radar, can cause temporary channel changes

CAPWAP (Control and Provisioning of Wireless Access Points): Protocol used between lightweight APs and wireless controllers for management and data tunneling

Notes

Practical Troubleshooting Tips

  • Always check basics first: power, cables, and RF coverage before diving into advanced diagnostics
  • Use WiFi analyzer apps on mobile devices for quick signal strength verification
  • Document baseline performance metrics during normal operation for comparison during issues
  • Consider environmental changes: new construction, moved furniture, or added equipment can impact RF

Common Pitfalls

  • Don’t assume authentication success means good connectivity - association and DHCP can still fail
  • 2.4GHz has better range but more interference; 5GHz has less interference but shorter range
  • Channel width settings (20/40/80/160 MHz) affect both performance and interference - wider isn’t always better
  • Guest networks often use different VLANs/subnets, requiring separate troubleshooting approach

Exam-Relevant Details

  • Know dBm values: -30 dBm (excellent), -50 dBm (good), -70 dBm (minimum), -90 dBm (unusable)
  • Understand 802.11 standards and backward compatibility requirements
  • Remember that WiFi is half-duplex - only one device can transmit at a time per channel
  • CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is fundamental to 802.11 operation