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    Understanding Wireless Radio Channels

    Each wireless radio operates on a configured radio frequency (RF) channel identified by numbers. A radio assigned to a particular channel both transmits and receives all traffic on that channel.

    Depending upon the network configuration, some channels might have less interference than others. Choosing the right channel lets you optimize performance.

    This topic describes:

    What WLAN Channels Are Available?

    There are 14 channels designated for wireless networks in the 2.4-GHz frequency band and 42 channels in the 5-GHz frequency band.

    The 14 channels in the 2.4-GHz band are spaced 5 MHz apart. The protocol requires 25 MHz of channel separation, which means that it is possible for adjacent channels to overlap and then interfere with each other. For this reason, only channels 1, 6, 11 are typically used in the US to avoid interference. In the rest of the world, the four channels 1, 5, 9, 13 are typically recommended. The 2.4-GHz frequency band is heavily used because most devices can operate on that band.

    The 5-GHz band is actually four frequency bands: 5.1-GHz, 5.3-GHz, 5.4-GHz, and 5.8-GHz. The 5-GHz band has a total of 24 channels with 20 MHz bandwidth available. Unlike the 2.4-GHz band, the channels are non-overlapping, therefore all channels have the potential to be used in a single wireless system. Formerly, only 802.11a devices used this band, but now this band is used for the newest 802.11ac technology.

    What Channels Are not Available?

    Because each country has different regulatory requirements, the country code determines which channels you can configure on the radios. When you specify the country of operation for an access point, the radios are restricted to using the valid channels for that country.

    The FCC (United States) requires that devices operating the 5-GHz band must employ dynamic frequency selection (DFS) and transmit power control (TPC) capabilities. This is to avoid interference with weather-radar and military applications. Additional channels in the 5-GHz band are restricted to avoid interference with Terminal Doppler Weather Radar (TDWR) systems. This eliminates the use of channels 120, 124, and 128. Channels 116 and 132 can be used if they are separated by more than 30 MHz (center-to-center) from a TDWR located within 35 km of the device.

    How Do I Know Which Channels I Should Use?

    For best performance, choose a channel at least 5 channels apart from your neighbors' networks. Determine this by completing a site survey—a site survey includes a test for RF interference.

    Try to use non-overlapping channels (1, 6, 11 typically in the US), or minimize overlap of signals by using channels as far apart as possible from other networks in range.

    You also need to know what channel your clients are capable of using, so that you can provide connection for that channel. For example, you can use Microsoft Windows 7 Device Manager to find the channel number at which your Microsoft Windows client can operate by following these steps:

    1. Click Start > Control Panel.
    2. Click Device Manager > Network Adapters.
    3. Right-click the link describing Wi-Fi and select Properties from the menu.
    4. Click the Advanced tab.

    Two columns are displayed: Property and Value. The list under Property tells you the wireless capabilities of the computer. The current operating properties are highlighted—this tells you the current mode of operation.

    Once you know the networking transmission standards used by clients (802.11n, 802.11g, 802.11b, or 802.11a) you can determine which channels the device can use—see Table 1 for details.

    Table 1: Channels a Device Can Use

    Device Capability

    Band and Channel Width used:

    Typical Channel Use

    802.11b

    2.4-GHz band, 20 MHz channel width

    1, 6, 11 (US) or 1, 5, 9, 13

    802.11g

    2.4-GHz band, 20 MHz channel width

    1, 6, 11 (US) or 1, 5, 9, 13

    802.11n

    2.4-GHz band, 20 MHz channel width

    5-GHz band, 40 MHz channel width

    1, 6, 11 (US) or 1, 5, 9, 13

    (36,1) (40,-1) (44,1) (48,-1) (52, 1) (56,-1) (60,1) (64,-1) (100,1) (104,-1) (108,1) (112,-1) (116,1) (120,-1) (124,1) (128,-1) (132,1) (136,1) (149,1) (153,-1) (157,1) (161,-1)

    802.11a

    5-GHz band, 40 MHz channel width

    3, 11

    How Do I Avoid Co-Channel Interference?

    Prevent interference and signal overlapping by doing an initial site survey of your wireless spectrum before deploying a wireless network. Once you discover all nearby signals, you can choose an optimum wireless channel that will provide the best performance with the least signal interference.

    To automatically avoid co-channel interference, enable automatic channel tuning in a Radio profile—all radios using that Radio profile will then use algorithms to switch to optimum channels. See Creating and Managing a Radio Profile to enable automatic channel tuning.

    DFS Channels

    In countries where Dynamic Frequency Selection (DFS) is required, Juniper Networks devices perform the appropriate check for radar on the 802.11a band. If radar is detected on a channel, the access point radio stops using the channel for the amount of time specified in the country’s regulations. Log messages are generated when this occurs. To enable DFS, see Creating and Managing a Radio Profile.

    802.11n Channels can be Wider and Work on Both Bands

    802.11n and 802.11ac devices work on the 5-GHz radio band as well as the more-populated 2.4-GHz radio band. On the 5-GHz band, 802.11n and 802.11ac channels can be either 80 MHz, 40 MHz or 20 MHz wide. This is one reason that 802.11n and 802.11ac devices are faster than 802.11a/b/g/n devices.

    Note: 40 MHz channels work only with 802.11n and 802.11ac on the 5-GHz band—40 MHz channels cannot not be configured on a 2.4-GHz radio.

    802.11n and 802.11ac radios configured for the 5-GHz band have a primary channel and a secondary channel. The primary channel is listed using the channel number, and the secondary channel adds another 20 MHz to make the channel 40 MHz. Therefore, the secondary channel is either the channel above (1) or the channel below (-1) the primary channel. This notation keeps you from configuring non-contiguous channels. For example, if the primary channel is 36 and the secondary channel is 40, the combination would be (36,1). If the primary channel is 44 and the secondary channel is 40, the notation would be (44,-1).

    The following channels, listed as (primary channel, secondary channel) are supported for 802.11n at 5-GHz with 40 MHz bandwidth: (36,1) (40,-1) (44,1) (48,-1) (52, 1) (56,-1) (60,1) (64,-1) (100,1) (104,-1) (108,1) (112,-1) (116,1) (120,-1) (124,1) (128,-1) (132,1) (136,1) (149,1) (153,-1) (157,1) (161,-1)

    How Are Channel Numbers Assigned?

    Juniper Networks access point radios use channel auto-tuning by default. You can change the channel tuning interval, channel tuning range, or the channel tuning holddown settings. You can also turn off auto-tuning. For more information, see Understanding Adaptive Channel Planner and to turn off channel auto-tuning, see Creating and Managing a Radio Profile.

    How Do I Know What Channel an Access Point Is Using?

    If the access point is already installed and operating, use the Equipment tab of the monitor mode in Network Director to view the access point channel numbers.

    Modified: 2017-04-20