Diagnosing and Fixing Intermittent High Latency Spikes in Linux Wi-Fi Network Connections


2 views

When dealing with intermittent high ping times (20,000ms spikes) to a Netgear WNDR4000 router across multiple Linux machines (Debian Jessie/Arch), we observe a distinctive pattern:

  • Simultaneous latency spikes across 3 machines (while 2 remain unaffected)
  • Severity varies by physical location (10m non-line-of-sight vs 3m direct line)
  • Complete resolution when moving affected device closer to router

# First establish baseline measurements
ping -c 100 192.168.1.1 | tee ping_baseline.log

# Monitor wireless interface statistics (run simultaneously in another terminal)
watch -n 1 "iwconfig wlan0 | grep -E 'Quality|Bit Rate'"

# Check for packet loss and jitter
mtr --report --report-cycles 100 192.168.1.1

1. Channel Congestion Analysis


# Install scanning tools (Debian/Arch)
sudo apt-get install iw wireless-tools   # Debian
sudo pacman -S iw wireless_tools         # Arch

# Scan surrounding networks
sudo iwlist wlan0 scan | grep -E 'ESSID|Frequency|Quality'

2. Driver and Firmware Verification


# Check current driver version
lsmod | grep -i wireless
dmesg | grep -i firmware

# Compare against latest available
modinfo $(lsmod | grep -i wireless | awk '{print $1}')

Create a new NetworkManager connection profile with optimized settings:


[connection]
id=Optimized_WiFi
uuid=... (generate new)
type=wifi
interface-name=wlan0

[wifi]
mode=infrastructure
ssid=Your_SSID

[wifi-security]
key-mgmt=wpa-psk
psk=YourPassword

[ipv4]
method=auto

[ipv6]
method=auto

# Critical performance settings
[connection]
autoconnect-priority=100
timestamp=...

[802-11-wireless]
powersave=0  # Disable power saving

For deeper analysis, capture wireless frames with tcpdump:


sudo tcpdump -i wlan0 -w wireless.pcap -s 0 \
    '((type mgt subtype beacon) || \
     (type ctl) || \
     (type data and dir fromds))'

Analyze with Wireshark looking for:

  • Excessive retransmissions
  • Beacon frame intervals
  • Channel switch announcements

Recommended Netgear WNDR4000 settings:


Wireless Mode: 802.11n only (disable legacy modes)
Channel Width: 20MHz (avoid 40MHz in congested areas)
Security: WPA2-PSK [AES] (no TKIP)
Transmit Power: 100% (if heat permits)
Beacon Interval: 100 (default is fine)
DTIM Period: 3 (default)

When analyzing wireless network performance issues, consistent patterns in latency spikes often reveal underlying problems. In this case, we observe:

# Sample ping output during spike (purple machine)
64 bytes from 192.168.1.1: icmp_seq=102 ttl=64 time=21.4 ms
64 bytes from 192.168.1.1: icmp_seq=103 ttl=64 time=18456.2 ms
64 bytes from 192.168.1.1: icmp_seq=104 ttl=64 time=20321.8 ms

Before diving into software troubleshooting, let's verify physical conditions:

# Check current signal quality (run on affected machines)
iwconfig wlan0 | grep -E "Signal|Rate"

Expected output for healthy connection:

Bit Rate=144.4 Mb/s   Tx-Power=20 dBm   
          Retry short limit:7   RTS thr:off   Fragment thr:off
          Link Quality=70/70  Signal level=-32 dBm

Simultaneous degradation across multiple devices suggests spectrum contention:

# Scan surrounding networks (requires iw)
sudo iw dev wlan0 scan | grep -E "SSID|freq|signal" | awk '/SSID/{print ""} {printf "%s ",$0} END{print ""}'

Different Linux distributions may use varying wireless drivers. Check current driver:

# Identify wireless driver in use
lspci -knn | grep Net -A3
lsmod | grep iwl

For debugging kernel-level issues:

# Enable verbose logging (temporary)
sudo dmesg -wH &
sudo iw wlan0 set debug 0xffff

When ping shows spikes but TCP connections seem fine:

# Simultaneous ping and TCP connection test
ping 192.168.1.1 | tee ping.log &
sudo tcpdump -i wlan0 -w wlan_debug.pcap &
curl --connect-timeout 5 http://192.168.1.1

For Netgear WNDR4000 specifically, these settings often help:

# Recommended settings (via router admin page):
- Disable "Auto Channel Selection"
- Set channel width to 20MHz
- Disable WMM (QoS) temporarily for testing
- Enable "Afterburner" if available

For long-term observation, create a monitoring script:

#!/bin/bash
LOG_FILE="/var/log/wlan_quality.log"
while true; do
    TIMESTAMP=$(date +"%Y-%m-%d %T")
    PING_RESULT=$(ping -c 3 192.168.1.1 | tail -1)
    SIGNAL_STR=$(iwconfig wlan0 | grep Quality)
    echo "$TIMESTAMP | $PING_RESULT | $SIGNAL_STR" >> $LOG_FILE
    sleep 30
done