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Monitoring the RF Spectrum of a Radio

 

WLAN radios are continually scanning for potential clients. In addition to finding clients, these scans detect other electronic objects, such as other access points and various non-802.11 equipment. (For more information about scanning, see Understanding Wireless Scanning.) You can view some of the data collected by the Network Director scanning function by looking at a radio spectrogram in Network Director.

The Network Director radio spectrogram consists of two charts, the Channel Spectrogram on the top half of the screen, and the Spectrum Sweep on the bottom half of the screen. The Channel Spectrogram displays the selected radio’s channel activity, while the Spectrum Sweep displays objects detected in the selected radio’s scanned area.

Note

A spectrogram times out after five minutes. During this time, all clients are dropped.

This topic describes:

Procedure for Viewing the Radio Spectrogram

To view the status of the spectrum:

  1. Select Monitor Mode in the Network Director banner.
  2. Select Logical View from the View pane.
  3. Expand the list in the View pane, and then select a radio.Tip

    The Monitor mode RF tab becomes available when you select either an access point or a radio. The RF Spectrogram monitor becomes available only when you select a radio.

  4. Click the RF tab.
  5. Click Spectrogram from the Tasks pane. Two windows open, the RF 2.4-GHz Spectrogram window and the Swept Spectrum and Duty Cycle window.
  6. Configure the sweep criteria for the four lines on this chart by adding or deleting the plotted lines. Select or clear the check boxes for Max, Max Hold, Duty Cycle, or Channels. (For an explanation of these values, see Table 1.)
  7. Click Start to begin the sweep using the criteria that you defined.Warning

    Your clients are dropped from the selected radio during a sweep.

  8. Sweeps take place in spectral scan mode, which drops all client connections during the sweeps. You are asked if you want to continue. Click Yes to drop clients and continue the configured sweeps.
  9. The upper graph displays the real-time state of the radio’s channels, while the lower graph displays the electronic state of the radio’s spectrum. Click Help (?) for more information about the graphs or see Spectrogram Charts.
  10. When you have seen the desired information, click Stop.

    The radio returns to beaconing for potential clients.

Spectrogram Charts

Use the following information to interpret the spectrogram.

The Network Director radio spectrogram consists of charts representing signal strength. On the top half of the screen, the channel power spectrogram for either the 2.4-GHz channel or the 5-GHz channel is displayed. On the lower half of the screen, two tabs display the ongoing spectrum power sweep and the duty cycle sweep.

Warning

Your clients are dropped during a spectrum sweep.

Channel Spectrogram Chart

The channel in use (2.4-GHz or 5-GHz) by the selected radio in the View pane is displayed as a grid with these parameters:

  • Maximum power

  • Average power

  • Duty cycle

  • Maximum hold

The 2.4-GHz channel has only one tab and all data is plotted on that tab. The larger 5-GHz channel is broken down into tabs for channels: CH-36 to CH-48, CH-52 to CH-64, CH-100 to CH-140, and CH-149 to CH-165. Click each 5-GHz tab to view the individual charts.

Four lines can be plotted on each chart, as listed in Table 1:

Table 1: Spectrum Sweep Results

Line

Description

Max Power

(red)

Indicates maximum power usage for this radio, plotted at the configured polling interval.

Avg Power

(green)

Indicates average power usage for this radio, plotted at the configured polling interval.

Duty Cycle

(yellow)

Indicates the amount of RF energy present in the spectrum as a result of an object emitting RF.

Max Hold

(pink)

Peak power level that was seen across multiple samples.

Note

The polling interval can be reconfigured by Setting Up User and System Preferences.

You can perform the following actions on this chart:

  • Highlight a line in the graph by mousing over the line’s legend.

  • Display a numeric value by placing the cursor where a vertical grid line bisects a data line.

  • Remove any or all of the four lines on this chart by clearing the check boxes for Max, Max Hold, Duty Cycle, or Channels.

  • Click Start to drop clients and begin the sweep using the criteria that you defined. Click Stop to stop the spectrum sweep and return the radios to normal operation.

    Warning

    Your clients are dropped during a spectrum sweep.

Swept Spectrum and Duty Cycle Charts

The chart on the bottom half of the window is dynamic, with new sweep results added to the top of the chart after each polling period. The power detected in the sweep is indicated by color, with blue representing the lowest power and red representing the highest power. Typically, the sweep results are blue and green, with occasional yellow segments. Any red that appears in the sweep indicates a problem.

This chart has two tabs, one for average power used in the spectrum called Swept Spectrum, and one for the percentage of the Duty Cycle used. When you are scanning for either result, all other traffic is dropped. For this reason, you must click Start to view spectrum results or see the duty cycle plotted. When you click Stop, normal traffic is resumed.

Click the Swept Spectrum tab to view the plotted average power spectrum for the radio that you selected in the View pane. The parameters used for this chart are time and average power.

Click the Duty Cycle tab to view the duty cycle data for the radio that you selected in the View pane. The parameters used for this chart are time and duty cycle percentage.

This data in these charts is available only for radios.