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Cisco 3900 Series, Cisco 2900 Series, and Cisco 1900 Series Integrated Services Routers Generation 2 Software Configuration Guide
Chapter Configuring Radio Settings
Configuring Radio Channel Settings
• Randomly selects a different 5-GHz channel.
• If the channel selected is one of the channels in Table 2, scans the new channel for radar signals for
60 seconds.
• If there are no radar signals on the new channel, enables beacons and accepts client associations.
• If participating in WDS, sends a DFS notification of its new operating frequency to the active WDS
device.
Note You cannot manually select a channel for DFS-enabled 5-GHz radios in Europe and Singapore. The
access points randomly selects a channel. However, in Japan, you can manually select a channel if a radar
has not been detected on it for the previous 30 minutes. If you attempt to select a channel that is
unavailable due to radar detection, the CLI displays a message stating the channel is unavailable.
The full list of channels that require DFS is shown in Table 2.
For autonomous operation, DFS requires random channel selection among the channels listed in Table 2.
The user interface prevents you from manually configuring these channels. The channels that are not
listed in Table 2 do not require random selection and may be manually configured.
Prior to transmitting on any channels listed in Table 2, the access point radio performs a Channel
Availability Check (CAC). The CAC is a 60 second scan for the presence of radar signals on the channel.
The following sample messages are displayed on the access point console showing the beginning and
end of the CAC scan:
*Mar 6 07:37:30.423: %DOT11-6-DFS_SCAN_START: DFS: Scanning frequency 5500 MHz for
60 seconds
*Mar 6 07:37:30.385: %DOT11-6-DFS_SCAN_COMPLETE: DFS scan complete on frequency
5500 MHz
When operating on any of the DFS channels listed in Table 2, in addition to performing the CAC, the
access point constantly monitors the channel for radar. If radar is detected, the access point stops
forwarding data packets within 200 ms and broadcasts five beacons that include an 802.11h channel
switch announcement, indicating the channel number that the access point begins using. The following
example message displays on the access point console when radar is detected:
*Mar 6 12:35:09.750: %DOT11-6-DFS_TRIGGERED: DFS: triggered on frequency 5500 MHz
When radar is detected on a channel, that channel may not be used for 30 minutes. The access point
maintains a flag in non-volatile storage for each channel that it detects radar on in the last 30 minutes.
After 30 minutes, the flag is cleared for the corresponding channel. If the access point is rebooted before
a flag is cleared, the non-occupancy time is reset to 30 minutes when the channel initializes.
Table 2 DFS Channel List
Channel Frequency Channel Frequency Channel Frequency
56 5280 MHz 108 5520 MHz 128 5640 MHz
60 5300 MHz 112 5560 MHz 132 5660 MHz
64 5320 MHz 116 5580 MHz 136 5680 MHz
100 5500 MHz 120 5600 MHz 140 5700 MHz
104 5500 MHz 124 5620 MHz — —
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