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Table Of Contents
Radio Channels and Transmit Frequencies
Configure the Radio Channel Number or Frequency
Configure the Radio Channel Spacing
Configuring a Preferred Channel
Wireless Device Channels and Frequencies
2.4 GHz (802.11b/g) Channels and Frequencies
4.9 GHz (public safety) Channels and Frequencies
5.0 GHz (802.11a/h) Channels and Frequencies
Radio Channels and Transmit Frequencies
This configuration note provides the information on determining the radio type and configuring the radio channel frequency and spacing. It also describes Dynamic Frequency Selection and lists Wireless Device Channels and Frequencies for most regulatory domains.
By default, the channel selected by Cisco wireless devices is the one that is least congested. 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.
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). Blocking a channel group forces the wireless device to skip a block of channels.
Note
The ability to block a channel group is not available on the Cisco 3200 Series WMICs.
Additional Information
For general information on channel selection and transmit power, see the "FCC Regulations Update For 2004" white paper at:
http://www.cisco.com/en/US/products/hw/wireless/ps4555/products_white_paper0900aecd801c4a88.shtml
Additional information on DFS and TPC can be found in the Cisco "Dynamic Frequency Selection and IEEE 802.11h Transmit Power Control" document available at:
http://www.cisco.com/en/US/products/ps6441/products_feature_guide09186a008060f7c2.html
For additional information on the 4.9 GHz (public safety) band, see the "Cisco Support for 4.9 GHz Public Safety Broadband Spectrum in the US" white paper at:
http://www.cisco.com/en/US/products/hw/routers/ps272/prod_brochure0900aecd802d816e.html
Setting a Channel
To set the channel, the following steps are described in detail in this section:
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Note
Determine the Radio Type
Determine the radio type to establish the frequency range of the radio. Use the show controllers dot11Radio command to show the radio type, frequency, and current channel for the wireless device. For example:
wd>enablewd>password ! If promptedwd#show controllers dot11Radio interfacenumberinterface Dot11Radio0!Radio AIR-AP1242GA, Base Address 0014.1b58.08f ! AIR-AP1242GA radio type shownVersion 5.80.12Serial number: GAM09200992Number of supported simultaneous BSSID on Dot1Carrier Set: Americas (US) ! DomainDFS Required: No ! DFS requirement!Current Frequency: 2412 MHz Channel 1 ! FrequencyConfigure the Radio Channel Number or Frequency
To set the radio channel, use the following command, beginning in privileged EXEC mode:
channel {channel_number | frequency | least_congested}
To automatically search for the least congested channel on startup and use this channel for the wireless device radio, use the least-congested parameter.
When specifying a frequency, enter the center frequency for the radio channel. The valid center frequencies and channel numbers depend on the channels allowed in the regulatory domain.
If the radio is operating in an area where it is known that radar is in use, blocking a specific frequency group can save processing time.
Note
To prevent a wireless device from selecting a group of frequencies, use the dfs band command, beginning in privileged EXEC mode.
dfs band frequency-group block !
The frequency-group value can be 1, 2, 3, or 4:
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Configure the Radio Channel Spacing
To set the radio channel spacing, use the following command:
spacing {5 | 10 | 20} channel
To let the DFS mechanism determine which channel to use, see the "Configuring a Preferred Channel" section.
Note
Dynamic Frequency Selection
Transmission Power Control (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 Dynamic Frequency Selection (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. Transmission Power Control (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. A specific channel cannot be configured for DFS-enabled radios; the channel command is disabled.
Note
If the wireless device 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 command in interface configuration mode.
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:
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If a prefered channel is configurable and available, it is selected first.
Radar Detection By Clients
Note
By default, radar detection is disabled in the workgroup bridge and non-root bridge modes (client modes). However, you can use the radar-detection-enabled command in the workgroup bridge or non-root bridge mode to enable radar detection in the client mode.
After a wireless device in client mode associates with a root device, it checks whether radar detection is enabled and whether the current uplink channel is a DFS channel. If radar detection is enabled and the current uplink channel is a DFS channel, the wireless device in client mode enables radar detection for this channel.
When the wireless device detects a radar signal, it immediately notifies the root device, brings down its interface, and restarts mixed uplink scanning, as described in Mixed Uplink Scanning Mode.
When the root device receives radar detection notification from a client, it check the association status:
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In addition, the root device sends an SNMP trap to the network administrator.
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Mixed Uplink Scanning Mode
Note
The wireless device supports a mixed scanning mode in the client mode for wireless clients to comply with DFS rules, which require clients to perform in the way of "listen before you talk."
The wireless client always starts with passive scanning. During a passive scan, the client marks the channels on which it detects beacons from any root device. If the client cannot find a matched root device after a full round of passive scanning, the client switches to the active scanning mode, but broadcasts probe request only on the channels marked by the previous passive scanning.
If the wireless device in client mode still cannot find a matched root device after a full round active scanning, it resets the channel marks and goes back to passive scanning. This process repeats until the wireless device in client mode finds a matched root device.
After the wireless device successfully associates to a root device, the scanning mode switches to active mode to avoid beacon processing.
Note
Configuring a Preferred Channel
Note
By default, a wireless device in the root mode randomly selects its operating channel to meet the uniform channel distribution requirement for standalone deployments. However, the random selection of the operating channel can result in adjacent and co-channel interference in deployments with multiple overlapping access points.
To minimize this type of interference, specify the channel of operation (prefer channel) of each access point. The user is then responsible for ensuring that the selected channels are uniformly distributed across the network.
To configure a prefered channel, use the following command:
spacing {5 | 10 | 20} channel prefered_channel [return-time <1-48>]
This command lets you specify a prefered channel for the wireless device to use as its operating channel. The command marks the specified channel with the highest priority to be chosen, but not the only channel that can be used.
After the prefered channel is configured, the wireless device resets its radio and selects the configured prefered channel to go through the channel availability check (60 seconds of scanning for radar signals) prior to enabling transmission on the channel.
However, if a radar signal is detected on the prefered channel, a new operating channel is randomly selected, which again can result in interchannel interference.
To resolve this issue, you can use the return-time option to specify a timeout period. This forces the wireless device to periodically try to return to the configured prefer channel after the timeout period expires, provided no radar is operating on the channel. If radar is operating on the prefer channel, the wireless device tries again after another timeout period expires. The wireless device keeps trying until it finds no radar on the prefer channel, at which point the wireless device switches back to the prefer channel.
The return-time parameter pecifies in 30-minute time units how long to wait before trying to return to the prefer channel after radar has been detected on the operating prefer channel. The minimum value is 1 time unit (30 minutes). The maximum value is 48 time units (1,440 minutes or 24 hours). The default value is 1 (30 minutes).
The wireless device sends an SNMP management trap whenever a channel is switched due either to radar detection or a return to the prefer channel.
Dynamic Frequency Selection Channels
When a Dynamic Frequency Selection (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:
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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.
Wireless Device Channels and Frequencies
This section lists the channels and frequencies for most regulatory domains.
2.4 GHz (802.11b/g) Channels and Frequencies
802.11b and 802.11g radios operate on 11 channels from 2412 MHz to 2462 MHz. Although the center frequency of the channels are 5 MHz apart, each channel is 22 MHz wide and overlaps the adjacent channels. For best performance, use channels that are not adjacent (such as channel 1, channel 6, and channel 11) for wireless devices that are physically close enough to each other that their signals overlap.
The channel identifiers, channel center frequencies, and regulatory domains of each IEEE 802.11b and 802.11g channel are shown in Table 1.
Table 1 Channels for IEEE 802.11b and 802.11g
1
2412
X
X
X
X
-
-
X
X
2
2417
X
X
X
X
-
-
X
X
3
2422
X
X
X
X
-
-
X
X
4
2427
X
X
X
X
-
-
X
X
5
2432
X
X
X
X
X
X
X
X
6
2437
X
X
X
X
X
X
X
X
7
2442
X
X
X
X
X
X
X
X
8
2447
X
X
X
X
X
X
X
X
9
2452
X
X
X
X
-
-
X
X
10
2457
X
X
X
X
-
-
X
X
11
2462
X
X
X
X
-
-
X
X
12
2467
-
-
X
X
-
-
X
X
13
2472
-
-
X
X
-
-
X
X
14
2484
-
-
-
-
-
-
X
-
1 Complementary Code Keying
2 Orthogonal Frequency Division Modulation
Note
4.9 GHz (public safety) Channels and Frequencies
This band is available only in the U.S. The radio operates on 5 MHz wide, 10 MHz wide, or 20 MHz wide channels between 4940 MHz and 4990 MHz for the licensed public safety community.
The channel identifiers, channel center frequencies, and channel width for the 4.90GHz band are shown in Table 2.
5.0 GHz (802.11a/h) Channels and Frequencies
The channel identifiers, channel center frequencies, and regulatory domains of each IEEE 5.0 GHz (802.11a/h) channel are shown in Table 3.
Table 3 5 GHz Radio Channels
34
5170
X
36
5180
X
X
X
X
40
5200
X
X
X
X
44
5220
X
X
X
X
48
5240
X
X
X
X
52
5260
X
X
X
X
56
5280
X
X
X
X
60
5300
X
X
X
X
64
5320
X
X
X
X
100
5500
-
X
-
104
5520
-
X
-
108
5540
-
X
-
112
5560
-
X
-
116
5580
-
X
-
120
5600
-
X
-
124
5620
-
X
-
128
5640
-
X
-
132
5660
-
X
-
136
5680
-
X
-
140
5700
-
X
-
149
5745
X
-
-
X
153
5765
X
-
-
X
157
5785
X
-
-
X
161
5805
X
-
-
X
1 The Cisco 3200 Series WMIC has not been tested for this domain.
Note
5.0-GHz Support
The 5.0 GHz (802.11a/h) radio supports the ETSI regulatory domains listed in Table 4.
Table 4 802.11a Channelization
5.250 to 5.350 GHz:
5260 MHz (52)
5280 MHz (56)
5300 MHz (60)
5320 MHz (64)5.470 to 5.725 GHz:
5500 MHz (100)
5520 MHz (104)
5540 MHz (108)
5560 MHz (112)
5580 MHz (116)
5600 MHz (120)
5620 MHz (124)
5640 MHz (128)
5660 MHz (132)
5680 MHz (136)
5700 MHz (140)
1 Channels 52 through 140 are ETSI outdoor.
Any Internet Protocol (IP) addresses used in this document are not intended to be actual addresses. Any examples, command display output, and figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses in illustrative content is unintentional and coincidental.
©2007 Cisco Systems, Inc. All rights reserved.
Printed in the USA on recycled paper containing 10% postconsumer waste.
1 Not supported on the Cisco 3200 Series WMIC
Posted: Wed Oct 24 12:06:55 PDT 2007
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