CCNA Wireless 200-355 Official Cert Guide (2016)

As interference sources are detected and classified according to interference type, the AP also measures the received signal strength indicator (RSSI) and the duty cycle of the interferer. 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 with a high severity rating can render a channel unusable.

Interference detection reports sent to a controller include the AP name, interference type, affected channel, time stamp, severity, duty cycle, and RSSI. The controller then tries to determine whether the same interference source is involved in reports coming from multiple APs. If so, the source is uniquely identified by assigning it a cluster ID. A cluster ID is actually a pseudo-MAC address that represents the non-802.11 device with a familiar wireless identifier.

You can display a list of interference detection reports on a controller by selecting Monitor > CleanAir > 802.11a/n/ac or 802.11b/g/n > Interference Devices. Figure 19-7 shows an example list.

Figure 19-7 Example List of Interference Device Reports

Air-Quality Index

As APs and controllers work together to generate a list of interference reports, the list can grow quite long. For example, the controller from Figure 19-7 has a list of 25 reports from the 5-GHz 802.11a/n/ac band. In addition, the same controller has 401 reports on the 2.4-GHz 802.11b/g/n band. To get a feel for the RF conditions on any one channel and AP, you would have to read through the list of reports manually and guess the cumulative effect that various interferers were having.

Cisco WLCs can do a better analysis by calculating an air-quality index (AQI) for each AP and its channels. The AQI indicates Wi-Fi health within an AP’s cell, as indicated by a scale from 0 (unusable) to 100 (perfect). You can display the air quality metrics for every AP on a controller by selecting Monitor > CleanAir > 802.11a/n/ac or 802.11b/g/n > CleanAir > Air Quality Report, as shown in Figure 19-8. You can see a summary of the AP with the worst air-quality rating in each band by selecting Worst Air-Quality Report.

Figure 19-8 Displaying Air-Quality Indices for all APs on a Controller

Air-quality ratings are updated dynamically. Each AP measures RF conditions once a second and then calculates an air quality value every 15 seconds and a summary every 30 seconds. By default, these values are reported to the AP’s controller every 15 minutes.

You can also view an AP’s CleanAir activity more frequently through the rapid update mode. To access this mode, use Monitor > Access Points > 802.11a/n/ac or 802.11b/g/n to display a list of APs. Choose an AP, and then select CleanAir from the blue triangular drop-down menu on the right side of the list. In rapid update mode, the controller automatically refreshes the page every 30 seconds with updated CleanAir data.

Rapid update mode also shows more detailed CleanAir information from an AP. In Figure 19-9, the AP has detected interference from a microwave oven. The air quality is shown as a bar graph. Normally only the one channel used by an AP is shown; if the AP is in monitor mode, all channels are shown. The same page also shows channel utilization and interference power measured by the AP.

Figure 19-9 Displaying Detailed CleanAir Data in Rapid Update Mode

Controllers display air quality on a per-AP (and channel) basis, and this information is aggregated further by Prime Infrastructure (PI). Air quality is summarized for the entire set of controllers managed in the enterprise.

Using Event-Driven RRM

By default, APs and controllers work together to detect, classify, and report wireless interference from non-802.11 devices. CleanAir and RRM can work together so that controllers actually take some action on interference events at the regular RRM intervals. By default, RRM runs the dynamic channel allocation (DCA) algorithm every 10 minutes, but you can increase the interval up to 24 hours. If non-802.11 interference occurs near an AP, the controller must wait until the next DCA interval before it can move the AP away from the unusable channel.

When Event-Driven RRM (ED-RRM) is enabled, the normal periodic RRM DCA process is triggered immediately in response to interference reported by an AP. ED-RRM must be enabled and then triggered based on an AQI threshold. You can set the threshold to Low (AQI 35), Medium (AQI 50), or High (AQI 60).

First, enable ED-RRM globally on a controller. ED-RRM uses the DCA function to change an AP’s channel and work out any other channel allocation changes that might be needed. Therefore, navigate to the DCA configuration by selecting Wireless > 802.11a/n/ac or 802.11b/g/n > RRM > DCA. Scroll to the bottom of the page to the Event Driven RRM section, as shown in Figure 19-10. Once you check the EDRRM box, you will be able to select a Sensitivity Threshold from the drop-down list. By default, the threshold is set to Medium. You can select Low, Medium, High, or Custom.

Figure 19-10 Enabling ED-RRM

Exam Preparation Tasks

As mentioned in the section “How to Use This Book” in the Introduction, you have a couple of choices for exam preparation: the exercises here, Chapter 21, “Final Review,” and the exam simulation questions on the DVD.

Review All Key Topics

Review the most important topics in this chapter, noted with the Key Topic icon in the outer margin of the page. Table 19-3 lists a reference of these key topics and the page numbers on which each is found.

Table 19-3 Key Topics for Chapter 19

Define Key Terms

Define the following key terms from this chapter and check your answers in the glossary:

air-quality index (AQI)

Cisco CleanAir

cluster ID

duty cycle

Event-Driven RRM (ED-RRM)

interference

noise

piconet

pseudo-MAC address

rogue AP

spectrum analyzer