CCNA Wireless Summary Notes: Dealing with Wireless Interference

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Interference – 802.11 that originates form a source other than the expected APs

Noise – Signals that originate from a source that is not 802.11

Common non-802.11 devices that can interfere with a WLAN

Bluetooth

  • Has low power consumption
  • Grouped in 3 classes (class 1 – 1mW, class 2 – 2.5 mW, class 3 – up to 100mW and is less common)
  • Operates in 2.4-GHz ISM band but Not compatible with 802.11 standard
  • Up to eight devices can be paired or linked into a PAN, with one device taking a master role and the others operating as slaves

ZigBee

  • Defined in the IEEE 802.15.4 standard
  • allocates the 2.4-GHz ISM band into 16 channels of 5 MHz each
  • has a low duty cycle and does not utilize a channel much of the time
  • Low power consumption
  • Low transmit power level hence less interference
  • Low data rates ( 20 to 250 Kbps).
  • commonly used for energy management and home and building automation applications

Cordless Phones

  • Phones that are advertised to use the 2.4- and 5.8-GHz bands can cause significant interference with nearby WLANs
  • Can use one channel at a time, but can also change channels dynamically
  • Transmit power can rise up to 250mW (more than AP maximum power)
  • DECT phones do not use the 2.4-GHz ISM band hence do not cause interference with 802.11 WLANs. They operate in the upper portion of the 1.8 GHz band in Europe, Asia, Australia, and South America. For America – 1.9 GHz.

Microwave Ovens

  • Microwave ovens are free to use the 2.4-GHz ISM band and most produce a signal that spreads over a large portion of the band
  • Microwaves are commonly rated to generate around 700 W of power inside the oven. Leaked energy often interferes with nearby APs.

WiMAX (Worldwide Interoperability for Microwave Access)

  • Specified in the IEEE 802.16 standard and not compatible with 802.11 WLANs
  • Provides “last mile” broadband access to consumers within a geographic area
  • WiMAX does not require line of sight with a base station, so it can offer connectivity to many fixed and mobile users within a 3 to 10-km radius
  • Uses several bands between 2 and 11 GHz and from 10 to 66 GHz
  • Can cause interference but highly unlikely.

Cisco CleanAir

This is a spectrum analysis capability built right in to the radio hardware that enables the AP to operate normally and also monitor RF energy on that channel, analyze the data, and report specific information about any interfering devices without interrupting normal WLAN operation. 802.11 frames are processes normally using the split MAC architecture whereas the non 802.11 signals are processed by the spectrum analysis hardware in the AP then sent tp WLC which can also send the information to the MSE so that the interference is located. Using Radio Resource Management (RRM) process and Event-driven RRM, the interference can be interacted automatically – AP can be moved to a different channel.

Check Channel quality for AP

Monitor > Access Points > Radios > 802.11a/n or 802.11b/g/n .

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Enable CleanAir

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Interference

The duty cycle is the percentage of time the source is transmitting on the channel, which indicates its persistence or how much of the airtime the interferer is consuming. The AP combines the RSSI and duty cycle into a severity index value. Severity ranges from 0 (not severe) to 100 (very severe).

Interference device reports

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Cisco WLCs can do a better analysis by calculating an air-quality index (AQI) for each AP and its channels which indicates WiFi health within an AP’s cell (0 -unusable, 100 – perfect).

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Event-Driven RRM (ED-RRM)

CleanAir and RRM can work together so that controllers actually take some action on interference events at the regular RRM intervals which by default is 10 minutes (when the dynamic channel algorithm is run by the controller).

With (ED-RRM) the RRM DCA process is triggered immediately an interference is reported by an AP. You must enable it then specify AQI threshold that will be used as the baseline.

Enable ED-RRM and chose AQI threshold

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