1. Introduction
As 802.11 devices operate in unlicensed 2.4GHz and 5GHz bands, managing WLAN networks bring additional challenges compared to wired networks as these devices experience noise and interference from other neighboring wireless networks operating in the same channel. In addition, other wireless devices like cordless telephones, wireless cameras, Bluetooth, weather radars, microwave ovens, etc. operate in same frequency band in which WLAN networks operate.
As a result, the presence of any of these application devices in the vicinity of 802.11 networks will have a profound impact on the reliability and throughput performance of these networks. So, organizations need IT staff with special RF skills and tools to detect interference and manage RF spectrum in which WLANs operate.
The Spectrum Analysis (SA) module in the AirDefense Service Platform (ADSP) provides exactly what is needed for organizations to “look-into” the air for potential noise and interference sources for troubleshooting physical (PHY) layer issues from a centralized remote location.
2. Spectrum Analysis
802.11 signaling exists only at layer-1(PHY) and layer-2(MAC) of the OSI model. If you only examine layer-2 frame captures, you will miss important information about the operation of your devices. 802.11 devices share the same unlicensed medium not only with each other in their WLAN network, but also with non-802.11 based devices configured to operate in the same frequency bands. When 802.11 devices conduct their clear channel assessment (CCA) prior to transmitting a frame and do not hear any 802.11 frame transmissions, they assume that the wireless medium is idle. If a source of non-802.11 noise exists in the basic service set area, 802.11 devices will not be able to detect the noise as an interference source. So, they assume that the medium is not busy and start transmitting the frames. As a result, the signal will collide with the noise and it will not be delivered to the intended receiver. The transmitting station will not receive an acknowledgement frame and begin the retransmission process. This will directly impact network performance. Common sources of non-802.11 interference include microwaves, cordless phones, Bluetooth devices and x.10 cameras as well as many other things.
Spectrum Analysis will enable you to detect and classify the source of interference; thus allowing you to remove it or work around it. This is a valuable tool in troubleshooting and resolving performance issues which are prevalent in WLAN networks.
Spectrum Analysis is an add-on module that gives you a tool to perform spectrum scan across 2.4GHz and 5GHz band, and identify interference sources on your wireless network.
The analysis is conducted using radio share enabled APs or Sensors. You must possess a valid Spectrum Analysis license for each Sensor or radio share enabled APs on which you want to perform Spectrum Analysis.
2.1 System Diagram
The following picture depicts system components involved in carrying out Spectrum Analysis in wireless networks. In order to use the Spectrum Analysis module, the deployment should have an AirDefense appliance with one or more Sensors, or radio share enabled Access Points.
When a Sensor or radio share enabled AP is configured for Spectrum Analysis, they monitor radio spectrum, analyze and classify potential interference sources, and communicate this information to the centralized AirDefense Services Platform. ADSP uses this information to generate alarms when the system detects noisy channel conditions or any interference sources in the vicinity.
ADSP provides the most advanced wireless LAN troubleshooting with a distributed architecture of remote Sensors or radio share enabled APs that communicate with a centralized server.
ADSP System Architecture
3. Licensing Spectrum Analysis
Spectrum Analysis is part of the Network Assurance Module and requires a license for unlocking this functionality in the ADSP appliance. Each device needs its own Spectrum Analysis license, where the device can be a Sensor or a radio share enabled AP. Note that, the Sensor can be a dedicated Sensor device or a dual-radio AP where one radio is configured for a Sensor and other for WLAN mode.
A radio share based Spectrum Analysis license can be used in environments where there are no dedicated Sensors. The following table provides different licensing options for the Spectrum Analysis module.
License Options for Spectrum Analysis
The above licenses can be applied on an ADSP appliance running ADSP version 9.x from Configuration > Appliance Platform > Appliance Licensing in the ADSP User Interface (UI).
NOTE If the selected Sensor is not licensed for Spectrum Analysis, you get the following error.
License Error
4. Spectrum Analysis
Spectrum Analysis can be run in the following modes:
• Background (In-line) Spectrum Analysis
• Dedicated Spectrum Analysis
4.1 Background Spectrum Analysis
In this mode, each Sensor continuously carries out background scan for interference and noise sources in those channels for which the Sensor is configured, and reports the information back to the AirDefense appliance. In a radio share based deployment, each AP performs background spectrum scan, but only in its operating channel.
ADSP generates alarms when the Sensor or radio share enabled AP detects and reports any noise or interference.
You can select the scan pattern, and enable the background spectrum from ADSP UI. Go to Configuration > Operational Management > Sensor Operation and select Scan Settings tab, as shown below:
Background Spectrum Analysis Settings
Select Enable radio button for Background SA Scan, as shown above.
You can customize the spectrum scan pattern of the channels, by selecting Custom Scan from the Scan Mode list box, as show in the above picture.
The ASA In-Line Settings tab is used to configure Sensor settings for background Advanced Spectrum Analysis (ASA), as shown below.
ASA In-line Settings
Threshold (dBm)—This is the master level control for ASA scanning. During scanning, any signal levels below this threshold will be dropped. Only those signal levels greater than this threshold will be admitted for further processing.
Duty Cycle (dBm)—The duty cycle is a measure of % utilization for each frequency. 100% duty cycle for a channel or frequency indicates that the channel is busy all the time. On the other hand, 0% duty cycle indicates that the channel is not used. The Duty Cycle controls the threshold level for duty cycle measurement. Only signal levels greater than the Duty Cycle threshold are counted in the duty cycle measurement.
There are four settings: two for 2.4 GHz band and two for 5GHz band. The user is not expected to change the default settings for normal use.
4.2 Dedicated Spectrum Analysis
The on-demand dedicated Spectrum Analysis disables the background Spectrum Analysis configured for a Sensor and performs a full spectrum scan or interference scan for a chosen scan period based on user selection.
WARNING! When you run dedicated Spectrum Analysis, the Sensor stops its current operation (for example, WIPS) and performs only Spectrum Analysis.
4.2.1 Performing Spectrum Analysis
To initiate Spectrum Analysis, go to Network tab in the ADSP GUI, and select Network Devices in the Show list box for any desired scope in the network hierarchy. This lists all Sensors and APs in that scope.
Select the target Sensor or radio share enabled AP which you want to use for spectrum scan. Right click on the device, and choose the Spectrum Analysis option as shown below to open the Spectrum View window.
Depending on the Sensor capabilities, you will see Spectrum View window with either first-generation Spectrum Analysis or Advanced Spectrum Analysis options. Refer to the following sections for more details on the dedicated Spectrum Analysis settings and spectrum charts.
Invoking Spectrum Analysis
NOTE The Spectrum View window can only be accessed if the selected Sensor is licensed and it supports Spectrum Analysis. If the Sensor does not support Spectrum Analysis, the following error popup is displayed.
Sensor SA Support Error
WARNING! If you attempt to run Spectrum Analysis on an AP configured for client access (device configured as AP and Sensor), the following error popup may display. This usually happens if you only have one radio turned on. If you continue, your wireless application may be disrupted but Spectrum Analysis will run.
Spectrum Analysis Warning
5. First-Generation SA
First-generation Spectrum Analysis is supported on the following devices:
• AP510/AP520
• AP7131/AP7161/AP7181
• AP650/AP6532.
Unlike Advanced Spectrum Analysis, the Spectrum Analysis on these devices supports only Spectrogram and Duty Cycle charts, and detection of limited Interference Sources.
Dedicated Spectrum Analysis can be performed at any time on a Sensor or a radio share AP as mentioned in Section 4.2.1. The dedicated spectrum scan starts automatically when the Spectrum View window is opened, as shown below.
First-Generation Spectrum View Window
You can stop a scan by clicking the Stop button– or by selecting Stop Scan from the Scan menu. Similarly, the spectrum scan can be restarted by clicking the Start button– or by selecting Start Scan from the Scan menu.
NOTE The default scanning time is 10 minutes and a timeout occurs if you leave the spectrum scan running for more time, as shown below. Scanning time can be adjusted by selecting Menu > Appliance Manager > System > Settings > Spectrum Scan Timeout in the ADSP GUI.
Spectrum Timeout Warning
Click OK to close the popup.
5.1 Spectrum Scan Settings
The default pulse definition for the spectrum scan can be changed. You can adjust the settings by selecting File > Settings.
Spectrum Analysis Pulse Settings
There are two fields that define the pulse: Threshold and Width. You can adjust the pulse threshold (in dBm) and the pulse width (in microseconds) by typing new values.
Click OK to set the new values and close the window.
5.2 Spectrum Scanning Modes
There are two scanning modes:
• Full Scan
• Interference Scan.
Full Scan scans the entire 2.4GHz spectrum in 5MHz steps and 5GHz spectrum in 20MHz steps with a short dwell time of around 50 ms. Full Scan supports limited classification of interference sources, as it spends less time on each channel.
Interference Scan scans only three channels in the 2.4GHz band and three channels in the 5GHz band with a longer dwell time of around 500 ms in each channel. Interference Scan supports classification for all interference sources.
By default, Interference Scan is enabled. But, you can change the scanning mode by selecting the Full scan radio button or by going to the Scan menu in the Spectrum View window.
5.3 Spectogram & Duty Cycle
Spectrogram is a time sweep plot of the spectrum that shows how the RF power of the selected channels varies with time. X-axis denotes time period and Y-axis denotes frequency.
This chart displays spectral power observed across 2.4 and 5GHz channels. It indicates whether the spectrum is busy or not based on the transmit power seen from both 802.11 and non-802.11 sources.
The duty cycle chart displays how busy a particular frequency is in real-time. X-axis represents duty cycle, while Y-axis represents frequency. A 100% duty cycle for a frequency indicates it is continuously occupied, and 0% indicates that the frequency is quiet.
Spectrogram and Duty Cycle Charts
When a scan starts, data starts displaying in the right side of the Spectrogram. As new data is scanned, the older data moves to the left. Once the graph is full, a horizontal scrollbar becomes visible, as show in above picture.
You can adjust the size of the Spectrogram and Duty Cycle chart by clicking and dragging the divider (left or right) between them.
Click on the Data Point Details button on the Spectrum View window. It displays, frequency, duty cycle, and average, minimum, and maximum power level, as shown in the next figure.
SA – Data Point Details
5.4 Interference Sources
The first generation Spectrum Analysis module detects and classifies the following interferences sources:
• Microwave Oven
• Frequency Hopping
• Continuous Wave
• Bluetooth.
When the system detects any of these interference sources, it highlights them in the Interference Sources window. Each time a scan is started; the interference source table is cleared and gets updated as the system detects these interference sources.
5.5 Closing Spectrum View Window
Select File > Close to exit the Spectrum View window.
You will be prompted to save the spectrum scan data to an ADSA file. This file can be opened in Live View or in AirDefense Mobile by navigating to File > Open > Open Spectrum Data.
6. Advanced Spectrum Analysis
This section describes the next generation Advanced Spectrum Analysis implementation of the Motorola AirDefense Solutions. ASA is different from the previous implementation of Spectrum Analysis in that, it takes advantage of the Fast Fourier Transform (FFT) engine in the hardware to capture spectrum samples that provide advanced detection capability to classify various types of RF interference commonly found in the 802.11 spectrum. By using FFT technique in the radio, the ASA runs in parallel with the AP and other Sensor software.
Note that, ASA will only run on devices with the MB92 or newer chipsets. Currently, the following Motorola products support this advanced Spectrum Analysis:
• AP6511
• AP621
• AP6521
• AP622
• AP8132.
The on-demand Advanced Spectrum Analysis can be invoked in a similar way to that of Spectrum Analysis, as mentioned in Section 4.2.1. But, if the chosen AP/Sensor supports ASA, you see the window shown in the next figure.
ASA – Spectrum View Window
ASA Window – Field Description
6.1 ASA Charts
Unlike the first-generation SA, the ASA supports the following seven charts.
• Spectrogram
• Real Time FFT
• Spectral Density
• Interference Scan
• Device Count
• RF Quality Index
• Duty Cycle.
6.1.1 Spectrogram
Spectrogram is a time sweep plot of the spectrum that shows how the RF power of the selected channels varies with time. This chart displays spectral power observed across 2.4 and 5GHz channels for which Advanced Spectrum Analysis is configured. It indicates whether the spectrum is busy or not based on the transmit power seen from both 802.11 and non-802.11 sources. This is a color coded chart.
Select the Spectrogram option from the Chart dropdown menu on the right side of the window to display the chart. The legend for different power levels is also shown in the chart. Adjust the sliders on 2.4GHz and 5GHz bands to view Spectrogram on desired channels in these bands, as shown below. You can place the cursor at any place in the window to see the channel and power level at any given time during the spectrum scan.
ASA Spectrogram Chart
6.1.2 Spectral Density
This chart plots the snapshot of the density of power observed on each channel during the Spectrum Analysis scan. The intensity of the color indicates the power density for the frequencies. The amplitude of the curve indicates a measure of the density of the observed energy during the scan. The higher the amplitude of the curve, the busier is the spectrum.
Unlike the Spectrogram which provides a historic view of the spectral power, this chart represents instantaneous power, and it provides a quick measure of which channels are busy and which are relatively quieter.
Select Spectral Density option from the Chart dropdown menu on the right side of the window to display this chart. The legend for different power levels is also shown in the chart.
As shown in the following figure, adjust the sliders on 2.4GHz and 5GHz bands to view density of the power on desired channels in these bands. You can place the cursor at any place in the window to see the channel and power density.
ASA Spectral Density Chart
6.1.3 Real-Time FFT
The real-time Fast Fourier Transformation (FFT) shows the power spectrum for the current FFT sample in terms of the average, minimum and maximum power values. In addition, it also shows the minimum and maximum power values out of all FFT samples since the Spectrum View GUI has started.
Select Real Time FFT option from the Chart dropdown menu on the right side of the window to display this chart. It plots the following power values, and you can enable or disable any of these power levels if you do not want to see it on the FFT plot:
Average Power—Average RF power of the collected samples during the scan period
Max-Hold—Maximum RF power of all the collected samples since the ASA was started
Maximum Power—This is similar to average RF Power, but it displays the maximum RF power of the collected samples instead of averaging
Min-Hold—Minimum RF power of the collected samples since the ASA was started
Minimum Power—This is similar to average RF Power but it displays the minimum RF power of the collected samples instead of averaging.
ASA Real Time FFT Chart
6.1.4 Duty Cycle
The duty cycle chart displays how busy a particular frequency is in real-time. A 100% duty cycle for a frequency indicates it is continuously occupied and 0% indicates that the frequency is quiet.
Select Duty Cycle option from the Chart dropdown menu on the right side of the window to display this chart. The legend for Average and Current power levels are shown in the chart.
Adjust the sliders on 2.4GHz and 5GHz bands to view Duty Cycle plot on desired channels in these bands, as shown in the next figure. You can place the cursor at any place in the window to see the following Duty Cycle, Power levels and spectrum utilization for any channel.
ASA Duty Cycle Chart
6.1.5 RF Quality Index
The RF Quality Index is a measure of the “air” quality of the RF spectrum. The index is represented on the Y-axis, where an index of 0 for a particular channel indicates that the channel is quite bad (busy) and index of 10 indicates that the channel quite good (idle).
The RF Quality Index is calculated from various factors including:
• Channel utilization
• Number of retries and Cyclic Redundancy Check (CRC) errors observed in the channel
• Channel noise
• Total number of transmitted frames in the channel
Select RF Quality Index option from the Chart dropdown menu on the right side of the window to display this chart. The legend for Average and Current power levels are shown in the chart.
Adjust the sliders on 2.4GHz and 5GHz bands to view RF Quality Index plot on desired channels in these bands, as shown below.
ASA RF Quality Index Chart
6.1.6 Interference
This chart plots all interference sources observed across all channels during a spectrum scan. During the interference scan, the system detects and classifies various interference sources commonly found in the shared 2.4 and 5GHz ISM bands. The following interference sources are supported by ASA:
• Microwave
• Wireless Camera
• 802.11 OFDM Activity
• 802.11 CCK Activity
• Bluetooth low data rate—Single slot Bluetooth
• Bluetooth high data rate—Multi Slot Bluetooth
• Cordless Phone
• Jammer—CW Source over more than 100MHz contiguous frequency range
• Continuous Transmitter—CW Source less than 100MHz contagious frequency range.
Select Interference option from the Chart dropdown menu on the right side of the window to display this chart. Adjust the sliders on 2.4GHz and 5GHz bands to scan and detect interferences on desired channels in these bands. You will see a red ball on the Status column against an Interference source when the system detects that particular interference category during an on-demand spectrum scan.
ASA Interference Chart
6.1.7 Device Count
This chart indicates total devices found during spectrum scan and plots these devices based on the following classification:
• Sanctioned
• Neighboring
• Unsanctioned.
This chart provides a measurement of wireless activity and interference from neighboring and unsanctioned devices operating on sanctioned devices operating in each of the channel.
To launch this chart, select Device Count option in the Chart dropdown menu, as shown in the next figure.
By default, the system plots Basic Service Set (BSS) found in each channel and you can select whether the 802.11 client devices need to be included in the analysis or not.
ASA Device Count Chart
You can select the red, green, or gray color on any of the channels to display the number of Unsanctioned, Sanctioned and Neighboring devices respectively. For example, the above picture shows total unsanctioned BSSs discovered in channel 1.
7. Scheduled Spectrum Analysis
Spectrum Analysis can be scheduled to perform a dedicated scan to detect and notify you of noise and interference issues affecting the network. Scheduled Spectrum Analysis generates alarms when it detects any RF or interference issues so that corrective action can be taken. You can configure the system to automatically perform one-off on-demand Spectrum Analysis at a given time in the day, or schedule the system to run SA daily, weekly, or monthly.
The scheduled SA can be configured from Spectrum View window, as mentioned in Section 5.
Go to File menu in Spectrum View window, and select Schedule Spectrum Analysis. You will see the following window.
Scheduled Spectrum Analysis
You can search the Sensor or radio share enabled AP that you want to use for scheduled Spectrum Analysis using the search icon on the top right corner in the windows.
You can configure scheduled SA to One Time Schedule, Intra-day, Daily, Weekly, and Monthly, by selecting the date, time, and day, as appropriate.
The Time Limit specifies the duration for which the scheduled on-demand Spectrum Analysis will run.
The File Name Prefix indicates the file name into which the Spectrum Analysis data is stored and can be opened later with AirDefense Mobile or Live View tools.
The Spectrum Settings and Advanced Spectrum Settings tabs allow you to configure the scan type, threshold, and duty cycle parameters, as mentioned in Section 4.1.
Click OK to schedule Spectrum Analysis.
8. Interpreting Spectrum Analysis Alarms
When a scheduled Spectrum Analysis test is carried out on a Sensor, the system generates alarms if it detects any interference sources on the configured channels. These alarms can be viewed from Alarms tab in the main GUI.
To find the description on all alarms generated by Spectrum Analysis module, you need to perform the steps below.
1. Go to Configuration > Operational Management > Alarm Configuration.
2. Select Performance > RF Spectrum Analysis.
3. Select the desired alarm, as shown in the following figure, and press View Expert Help to get details on when this alarm would be generated and how to mitigate this failure condition.
Spectrum Analysis Alarms
9. Supported Devices/Versions
The following table lists the firmware support for Sensor and radio share based Spectrum Analysis for various models:
AP/Sensor Matrix for Spectrum Analysis
*** Source: Best practices deployment WIPS - Extreme Network
