At startup and by default, wireless devices passively scan for and select the least-congested channel. The
channel settings on wireless devices correspond to the frequencies available in your regulatory domain.
For example, in the European Telecommunications Standards Institute (ETSI) domain, the regulatory
agencies do not allow the channel to be set on 5.0 GHz (802.11a/h) radios by the users. However, channel
groups can be blocked on wireless devices running ETSI images. When a wireless device boots from an
ETSI image, it automatically selects the least congested channel where radar is not detected by using
Dynamic Frequency Selection (DFS).
Transmission Power Control (TPC) is used to automatically adjust the transmission power level on
5.0-GHz radios, also to avoid interfering with radar.
Understanding Dynamic Frequency Selection
TPC is used to automatically adjust the transmission power level on 5.0-GHz radios, also to avoid
interfering with radar. 5.0 GHz (802.11a/h) radios in wireless devices running Cisco IOS version
12.4(6)T and later shipped to Europe and Japan are required to use DFS to detect and avoid interfering
with radar signals to comply with those regulatory domains.
DFS is the process of detecting radar signals that must be protected against interference from 5.0 GHz
(802.11a/h) radios, and upon detection switching the operating frequency of the 5.0 GHz (802.11a/h)
radio to one that is not interfering with the radar systems. TPC is used to adapt the transmission power
of a radio based on regulatory requirements and range information.
The wireless device automatically sets the frequency on a DFS-enabled 5.0 GHz (802.11a/h) radio
operating in a regulatory domain where DFS is required for compliance with the rules of that regulatory
agency.
For the ETSI regulatory domain, 5.0-GHz radios support the DFS 1.3.1 compliance requirement.
DFS Actions
DFS-enabled radios monitor the operating frequency for radar signals. If radar signals are detected on
the channel, the wireless device takes these steps:
• Blocks new transmissions on the channel.
• Flushes the power-save client queues.1
• Broadcasts an 802.11h channel-switch announcement.
• Disassociates remaining client devices.
• Randomly selects a different channel:
– If the wireless device does not select a DFS-required channel, it enables beacons and accepts
client associations.
– If the wireless device selects a DFS-required channel, it scans the new channel for radar signals
for 60 seconds. If there are no radar signals on the new channel, the wireless device enables
beacons and accepts client associations. If a radar signal is detected, the wireless device selects
a different channel.
If a preferred channel is configurable and available, it is selected first.
Dynamic Frequency Selection Channels
When a DFS–enabled radio is operating on one of the following channels, the wireless device uses DFS
to monitor the operating frequency and switch to another frequency or reduce power as necessary:
• 52 (5260 MHz)
• 56 (5280 MHz)
• 60 (5300 MHz)
• 64 (5320 MHz)
• 100 (5500 MHz)
• 104 (5520 MHz)
• 108 (5540 MHz)
• 112 (5560 MHz)
• 116 (5580 MHz)
• 120 (5600 MHz)
• 124 (5620 MHz)
• 128 (5640 MHz)
• 132 (5660 MHz)
• 136 (5680 MHz)
• 140 (5700 MHz)
1. 52, (5260 MHz), 64 (5320 MHz), 120 (5600 MHz), and 128 ( 5640) are not supported on the Cisco 3200 Series
WMIC.
3 The maximum legal transmit power is greater for some 5 GHz channels than for others. When the
wireless device randomly selects a 5 GHz channel on which power is restricted, the wireless device
automatically reduces transmit power to comply with power limits for that channel in that regulatory
domain.
If the access point is set to use DFS and it is deployed in a regulatory domain that does not allow or does
not require the use of DFS, disable DFS by using the no dfs band block interface configuration
command.
