DFS Channels

DFS Channels

Dynamic Frequency Selection (DFS) is a regulatory requirement for operating on certain 5 GHz WiFi channels. This document explains how DFS works on MikroTik devices, common issues, and deployment strategies.

Overview

DFS protects radar systems (weather radar, military, air traffic control) from WiFi interference. When a MikroTik AP detects radar on a DFS channel, it must:

1. Vacate immediately - Switch to a different channel within 10 seconds
2. Block the channel - Cannot use that frequency for 30 minutes (per FCC/ETSI regulations)
3. Resume after period - May use the channel again after the non-occupancy period

Affected Channels

Region	DFS Channels	Non-DFS Channels
US (FCC)	52-64, 100-144	36-48, 149-165
EU (ETSI)	52-144	36-48
Most regions	52-144	36-48, 149-165

Channel Ranges

• UNII-1 (5150-5250 MHz): Channels 36-48 - Non-DFS, indoor use
• UNII-2 (5250-5330 MHz): Channels 52-64 - DFS required
• UNII-2 Extended (5470-5725 MHz): Channels 100-144 - DFS required
• UNII-3 (5725-5850 MHz): Channels 149-165 - Non-DFS, higher power allowed

Regulatory Requirements

When using DFS channels, MikroTik devices must comply with:

• CAC (Channel Availability Check): Scans for radar before using a DFS channel (typically 60 seconds)
• In-Service Monitoring: Continuous radar detection during operation
• Channel Move Time: Must vacate within 10 seconds of radar detection
• Non-Occupancy Period: Cannot use the channel for 30 minutes after radar detection

MikroTik Configuration

Legacy Wireless (RouterOS 6.x)

# Enable DFS with static mode
/interface wireless set wlan1 dfs-mode=static

# Use DFS channels (default)
/interface wireless set wlan1 skip-dfs-channels=no

# Skip all DFS channels
/interface wireless set wlan1 skip-dfs-channels=yes

# Check DFS status
/interface wireless monitor wlan1

WiFi (RouterOS 7.x)

# Create channel with DFS enabled (default)
/interface wifi channel
add frequency=5500,5700 name=DFS-Channels width=20/40/80mhz

# Skip DFS channels entirely
/interface wifi channel
add frequency=5180,5260 name=Non-DFS width=20/40/80mhz skip-dfs-channels=all

# Check current channel and DFS status
/interface wifi monitor [find]

Configuration Options (WiFi)

Option	Description
skip-dfs-channels=no	Use all supported channels including DFS (default)
skip-dfs-channels=10-percentage	Skip 10% of DFS channels
skip-dfs-channels=all	Skip all DFS channels

# Mixed approach - primary non-DFS, secondary DFS
/interface wifi channel
add frequency=5180,5260,5500 name=5G-Primary width=20/40/80mhz skip-dfs-channels=all

/interface wifi channel
add frequency=5600,5700 name=5G-Secondary width=20/40/80mhz

Common Issues

Radar False Positives

DFS can trigger on non-radar sources:

• Weather phenomena: Rain, snow, heavy clouds (especially in 5 GHz)
• High-power nearby transmitters: Non-WiFi devices operating nearby
• Neighboring networks: Interference from nearby wireless deployments

Impact on Deployments

When DFS triggers:

1. AP detects “radar” event
2. Logs the event (check with /log print)
3. Switches to a non-DFS or alternate channel
4. Channel blocked for 30 minutes

Community feedback: “DFS issues… radar false positives, 30-minute lockouts ruining deployments”

The 30-minute lockout is the primary concern - once blocked, significant 5 GHz spectrum becomes unavailable.

Symptoms

• Unplanned channel changes, especially during rain/snow
• “radar detected” messages in logs
• Clients disconnecting unexpectedly
• Reduced available channels during certain weather conditions

Deployment Strategies

Strategy 1: Non-DFS Only

For maximum reliability, skip DFS channels entirely:

# RouterOS 7.x - Non-DFS only
/interface wifi channel
add frequency=5180,5260,5500,5765,5825 name=5G-NoDFS width=20/40/80mhz skip-dfs-channels=all

Pros: No unexpected channel changes, no 30-minute lockouts
Cons: Fewer available channels, may be congested

Strategy 2: Mixed Approach

Use both DFS and non-DFS with fallback:

# Primary - non-DFS for reliability
/interface wifi channel
add frequency=5180,5260,5500 name=5G-Primary width=20/40/80mhz skip-dfs-channels=all

# Secondary - DFS for additional capacity
/interface wifi channel
add frequency=5600,5700 name=5G-Secondary width=20/40/80mhz

Pros: Access to more spectrum when stable
Cons: More complex, risk of disruptions

Strategy 3: Include All Channels

Let the AP select from all available channels:

/interface wifi channel
add frequency=5180,5200,5220,5240,5260,5280,5300,5320,5500,5520,5540,5560,5580,5600,5620,5640,5660,5680,5700 \
    name=5G-Auto width=20/40/80mhz

Pros: Maximum flexibility
Cons: DFS events may cause disruptions

Monitoring and Troubleshooting

Check Logs for DFS Events

/log print where message~"dfs"

Or look for messages containing:

• radar detected on channel
• switching to channel
• DFS: channel blocked

Check Current Status

# RouterOS 6.x
/interface wireless monitor wlan1

# RouterOS 7.x
/interface wifi monitor [find]

# Check registration table
/interface wifi registration-table print

Reduce False Positives

If experiencing frequent false positives:

1. Reduce TX power - Lower power reduces false detection
2. Use narrower channel width - 20 MHz has less false positive risk
3. Position antennas carefully - Avoid pointing at potential radar sources

Missing Channels

If channels are unavailable:

1. Check country setting (/ip cloud or /system identity)
2. Verify DFS configuration
3. Confirm regulatory domain settings

Regional Considerations

United States (FCC)

• Non-DFS: 36-48 (U-NII-1), 149-165 (U-NII-3)
• DFS: 52-64 (U-NII-2), 100-144 (U-NII-2 Extended)
• Max power varies by channel

European Union (ETSI)

• Non-DFS: 36-48 only
• DFS: 52-144 (more restrictive than FCC)
• Typically 30-minute non-occupancy

Other Regions

Check local telecommunications regulations. Many countries follow FCC or ETSI rules.

Recommendations

For Most Deployments

1. Start with Non-DFS: 36-48 (and 149-165 in US)
2. Test DFS: Enable and monitor for issues
3. Enable if stable: If no false positives, use both

For High-Reliability

• Avoid DFS entirely
• Accept reduced channel options
• Use 80 MHz channel width for more capacity

For High-Density

• Use all available channels (both DFS and non-DFS)
• Monitor actively for DFS events
• Have fallback plans when DFS triggers

Summary

Approach	Pros	Cons
Non-DFS Only	Reliable, no blocking	Fewer channels
DFS Enabled	More spectrum available	Risk of 30-minute block
Mixed	Best of both worlds	More complex configuration

Recommendation: Start with non-DFS channels. Add DFS only after testing and monitoring in your specific environment.

Related Commands

/interface wifi channel     # Channel definitions
/interface wifi monitor     # Current status
/interface wifi configuration  # Channel assignment
/log print                 # Check for DFS events

See Also

• Frequency Bands - Channel selection overview
• WiFi Configuration - General WiFi setup
• CAPsMAN - Centralized AP management