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Monitoring RF Interference Sources on One Radio

 

Because the 2.4-GHz band includes radio transmissions from devices other than wireless networks, interference is a common problem. Network Director detects, classifies, and displays radio interference in several monitors. This topic describes monitoring the interference of one radio displayed in a pie chart.

Note

You can also monitor interference by Monitoring RF Interference Sources on Wireless Devices and Monitoring RF Interference Sources For Radios on One Access Point.

Monitoring RF Radio Interference Sources

To view a radio’s RF interference sources in a pie chart over a fixed period of time:

  1. Select Monitor Mode in the Network Director banner.
  2. Select any view from the View pane.
  3. Expand the wireless list in the View pane, then select a radio.

    The monitor mode RF tab becomes available when you select a radio.

  4. In the Tasks pane on the right, click Interference Sources.

    A pie chart is displayed with a breakdown of the interference sources detected on the selected radio.

  5. Optionally, change the timeframe covered by the monitor by selecting a different time from the list.
  6. Click Help (?) for information about the radio interference chart or see RF Interference Sources Pie Chart for a Radio.
Note

To change the polling interval for monitors, see Setting Up User and System Preferences.

RF Interference Sources Pie Chart for a Radio

The RF Interference Sources pie chart for a single radio reflects all devices that have interfered with the traffic of the radio selected in the View pane. Network Director tracks and monitors interference from these sources:

  • Microwave ovens—Most domestic microwave ovens use 2.45 GHz, and can interfere with Wi-Fi channels from 8 to 10 (or even 7 to 11). Interference varies depending on the model of the oven—for example, commercial restaurant microwave ovens sweep over a wider spectrum and have a higher duty cycle.

  • Continuous wave devices continuously transmit at a particular frequency without attempting to share the radio frequency medium with other devices. Devices that use continuous wave technology in the same frequency bands as wireless LAN networks will interfere with wireless communications, reducing performance or totally preventing communication. Several examples of devices that use continuous wave transmission that interferes with WiFi are video surveillance cameras and baby monitors.

  • Bluetooth devices

  • Phone FHSS from cordless phones

  • Unknown devices

To track these interference devices, Network Director polls the access point’s controller at the standard interval. The categories with the largest sections of the pie cause the most radio interference.

You can perform the following actions on the pie chart:

  • Change the time period over which to display interference by selecting a time period from the list in the upper right corner.

  • Display a numeric value for interference objects by mousing over a section of the chart.

  • Click the monitor’s title to see a list of interference incidents along with the information listed in Table 1.

    Table 1: Information on RF Interference Sources for a Radio

    Information

    Description

    Last Seen

    Date and time the interference was last detected.

    Transmitter ID

    If the interference is caused by an object with a MAC address, the MAC address is displayed. If the object has no MAC address, MSS calculates a MAC address, using the characteristics of the object. This way, you can correlate interference events over time.

    Listener MAC

    MAC address of the access point that detected the interference.

    AP

    Name of the access point that detected the interference.

    Controller

    Name of the controller that reported the interference.

    Channel

    Channel the interference affected.

    RSSI

    Received signal strength indication (RSSI), in decibels referred to 1 milliwatt (dBm). A higher value indicates a stronger signal.

    Duty Cycle

    Reported fraction of time that the source is emitting RF.

    Source Type

    Possible sources of interference include Bluetooth, Continuous Wave, Microwave Oven, Unknown, and Phone FHSS.

    CIM (%)

    Estimated severity of interference on this channel caused by the source.

Interference is frequently not a problem on wireless networks with light traffic, but as traffic becomes heavier, throughput and capacity decrease and other problems become apparent. RF interference can cause packet retransmission (see Monitoring the Percentage of RF Packet Retransmissions). Interference is also a security concern because jamming can bring down the network .

Ideally, interference retransmission does not cause more than 10% of the total number of packets sent. If your retransmission percentage is higher, you can try to lower it by:

  • Locating and eliminating offending devices. If the item cannot be removed, you can add electromagnetic interference (EMI) shielding such as grounded mesh, foils, insulating foams, or insulating paint. This will limit the interference to a small area.

  • Moving the affected access point.

  • Moving clients to channels with less interference. Keep in mind, however, that Bluetooth devices, cordless phones, 802.11FH devices, and jamming emissions are broadband, so it's not possible to change channels away from them—they are everywhere in the band. For more information, see Understanding Wireless Radio Channels and Understanding Adaptive Channel Planner.

For more information about wireless interference, see Understanding Wireless Interference .