- "Do I Know This Already?" Quiz
- Foundation Topics
- Exam Preparation Tasks
This chapter is from the book
This chapter is from the book
This chapter is from the book
Foundation Topics
Configuring 802.11 Support
Cisco controllers and most APs can support wireless LANs in both the 2.4- and 5-GHz bands. By default, both bands are enabled; however, you can view or change a number of parameters by browsing to the Wireless tab in the controller, shown in Figure 13-1.
)
Figure 13-1 Wireless Tab on a Cisco Controller GUI
The wireless parameters are organized under a list of links that are found on the left side of the web page. At the CCNA level, you should be familiar with the following links:
- Access Points—Used to verify and configure RF things like transmit power level and channel number on individual APs
- 802.11a/n/ac—Used to configure global parameters for the 5-GHz band
- 802.11b/g/n—Used to configure global parameters for the 2.4-GHz band
The initial web page displays a list of all APs that are currently joined to the controller, as if you had selected Wireless > Access Points > All APs. The remaining configuration is covered in the sections that follow.
Configuring Data Rates
You can enable or disable the 2.4- or 5-GHz bands by selecting 802.11b/g/n or 802.11a/n/ac, respectively, and then clicking the Network link. Figures 13-2 and 13-3 show the two network configuration pages. Make sure that the 802.11b/g or 802.11a Network Status check box is checked to enable the 2.4- or 5-GHz radios on all APs.
)
Figure 13-2 Configuring 2.4-GHz Radios
On the right side of the network web pages, as shown in Figures 13-2 and 13-3, you can configure the individual data rates (and the corresponding modulation and coding schemes) that are supported on each band. Each data rate can have one of the following states:
- Mandatory—A client must be able to use the data rate and Modulation Coding Scheme (MCS) to associate with an AP.
- Supported—A client can associate with an AP even if it cannot use the data rate.
- Disabled—An AP will not use the data rate with any clients.
By default, all data rates are enabled and supported. In the 2.4-GHz band, the 1-, 2-, 5.5-, and 11-Mbps rates are all marked as mandatory, based on the initial IEEE requirement that all clients be able to support each possible modulation type defined in 802.11b. In the 5-GHz band, the 6-, 12-, and 24-Mbps rates are marked as mandatory.
)
Figure 13-3 Configuring 5-GHz Radios
You can change the state of any data rate by selecting a new state from the drop-down menu. Remember that you can disable lower data rates to decrease the AP cell size and make channel use more efficient. Just make sure that your actions do not shrink the cells too much, leaving holes or gaps in the coverage between APs. Also be sure that all of your wireless clients can use the same set of mandatory and supported data rates.
Be sure to click the Apply button to make any configuration changes active. Any wireless networks that are already in production on the controller might be disrupted while the new configuration takes effect.
Configuring 802.11n and 802.11ac Support
You might have noticed that you can configure plenty of data rates, but 802.11n and 802.11ac are never mentioned on the wireless network configuration pages. That is because 802.11n and 802.11ac are considered to be rich sets of high-throughput enhancements and must be configured separately.
By default, 802.11n and 802.11ac are enabled. To check or change their state, go to Wireless > 802.11a/n/ac or 802.11b/g/n > High Throughput (802.11n/ac). Figure 13-4 shows the 5-GHz 802.11n/ac configuration page. Check the 11n Mode and 11ac Mode check boxes to enable 802.11n and 802.11ac, respectively. By default, every possible MCS is enabled and supported.
)
Figure 13-4 Configuring 802.11n and 802.11ac Support
Recall that 802.11n can bond one 20-MHz channel to an adjacent 20-MHz channel to effectively double the channel width; 802.11ac can scale even further By default, the controller will use only a single 20-MHz channel on each AP. You can configure channel bonding as a part of the dynamic channel allocation (DCA) configuration for the 5-GHz band only, as covered in the following section.
Understanding RRM
Suppose that you need to provide wireless coverage in a rectangular-shaped building. Using the information you have learned from this book, you decide to use six APs and locate them such that they form a staggered, regular pattern. The pattern shown in Figure 13-5 should create optimum conditions for roaming and channel use. (The building dimensions have not been mentioned, just to keep things simple.)
)
Figure 13-5 Hypothetical AP Layout
So far, you have considered the layout pattern and an average cell size, but you still have to tackle the puzzle of selecting the transmit power level and channel number for each AP. The transmit power level will affect the final cell size, and the channel assignment will affect co-channel interference and roaming handoff. At this point, if all the APs are powered up, they might all end up transmitting at maximum power on the same channel. Figure 13-6 shows one possible scenario; each of the AP cells overlaps its neighbors by about 50 percent, and all the APs are fighting to use channel 1!
)
Figure 13-6 Poorly Configured RF Coverage
Where do you begin to prevent such mayhem? Because the AP locations are already nailed down, you can figure out the transmit power level that will give the proper cell overlap. Then you can work your way through the AP layout and choose an alternating pattern of channel numbers. With six APs, that might not be a daunting task.
Do not forget to repeat the task for both 2.4- and 5-GHz bands.
Also, if you plan on using 802.11n or 802.11ac with channel widths greater than 20 MHz, do not forget to reserve the extra channels needed for that. Be aware that only the 5-GHz band is capable of supporting wide channels.
If you happen to notice that an AP fails one day, you could always reconfigure its neighboring APs to increase their transmit power level to expand their cells and cover the hole.
If you introduce another AP or two in the future, do not forget to revisit the entire configuration again to make room for cells and channels.
Did your life as the wireless LAN administrator just become depressing and tedious? Cisco Radio Resource Management (RRM) can handle all these tasks regularly and automatically. RRM consists of several algorithms that can look at a large portion of a wireless network and work out an optimum transmit power level and channel number for each AP. If conditions that affect the RF coverage change over time, RRM can detect that and make the appropriate adjustments.
RF Groups
RRM works by monitoring a number of APs and working out optimal RF settings for each one. The APs that are included in the RRM algorithms are contained in a single RF group. An RF group is formed for each band that is supported—one group for 2.4-GHz AP radios and another for 5-GHz AP radios. By default, an RF group contains all the APs that are joined to a single controller.
You can also configure a controller to automatically populate its RF group. In that case, the RF group can expand to include APs from multiple controllers, provided the following two conditions are met:
- The controllers share a common RF group name.
- At least one AP from one controller can be overheard by an AP on another controller.
When an RF group touches more than one controller, the controllers form a type of cluster so that they all participate in any RF adjustments that are needed. Every AP sends a Neighbor Discovery Packet (NDP) at maximum transmit power and at 60-second intervals, by default. If two controllers are close enough in proximity for an AP on one to hear an AP on the other at a received signal strength indicator (RSSI) of –80 dBm or greater, they are close enough to belong to the same RF group. Up to 20 controllers and 1000 APs can join to form a single RF group.
Figure 13-7 shows a simple scenario with four controllers and four APs, resulting in two separate RF groups. AP-1 and AP-2 are both joined to controller WLC-1, so they are members of one RF group by default. AP-3, joined to WLC-2, is located near enough to AP-1 and AP-2 that neighbor advertisements are overheard. As a result, controller WLC-2 joins the RF group with WLC-1. However, AP-4, joined to controller WLC-3, is not close enough to pass the neighbor test. Even though AP-4’s cell intersects the cells of AP-2 and AP-3, the APs themselves are not within range. Therefore, controller WLC-3 resides in a different RF group by itself.
)
Figure 13-7 Automatic RF Group Discovery and Formation
One controller in each group is elected as an RF group leader, although you can override that by configuring one controller as a static leader. The leader collects and analyzes information from all APs in the group about their RF conditions in real time. You can access the RF group leader configuration information by selecting Wireless > 802.11a/n/ac or 802.11b/g/n > RRM > RF Grouping. In Figure 13-8, the controller is in automatic RF group mode and is a member of an RF group along with two other controllers. The RF group leader is controller WLC-1.
)
Figure 13-8 Displaying RF Group Information
Radio resource monitoring is used to gather and report information from the APs. Each AP is assigned to transmit and receive on a single channel, so it can easily detect noise and interference on that channel, as well as the channel utilization. The AP can also keep a list of clients and other APs that it hears transmitting on that channel.
Each AP can also spend a short bit of time (less than 60 ms) tuning its receiver to all of the other channels that are available. By scanning channels other than the one normally used, an AP can measure noise and interference all across the band from its own vantage point. The AP can also detect unexpected transmissions coming from rogue clients and APs, or devices that are not formally joined to the Cisco wireless network.
Based on the radio resource monitoring data, RRM can make the following decisions about APs in an RF group:
- Transmit power control (TPC)—RRM can set the transmit power level of each AP.
- Dynamic channel allocation (DCA)—RRM can select the channel number for each AP.
- Coverage hole detection mitigation (CHDM)—Based on information gathered from client associations, RRM can detect an area with weak RF coverage and increase an AP’s transmit power level to compensate.
The RRM algorithms are designed to keep the entire wireless network as stable and efficient as possible. The TPC and DCA algorithms run independently because they perform very different functions. By default, the algorithms are run every 600 seconds (10 minutes). If conditions in the RF environment change, such as interference or the addition or failure of an AP, RRM can discover and react to the changes at the next interval. The RRM algorithms are discussed in more detail in the following sections.
TPC
The TPC algorithm focuses on one goal: setting each AP’s transmit power level to an appropriate value so that it offers good coverage for clients while avoiding interference with neighboring APs that are using the same channel. Figure 13-9 illustrates this process. APs that were once transmitting too strongly and overlapping each other’s cells are adjusted for proper coverage, reducing the cell size more appropriately to support clients.
)
Figure 13-9 Basic Concept of the TPC Algorithm
Controllers have no knowledge of the physical location of each AP. By looking at Figure 13-9, you can see that the APs are arranged in a nice, evenly spaced pattern, but the controller cannot see that. When an AP joins a controller, only the AP’s MAC address, IP address, and some basic information are advertised to the controller. If the locations of neighboring APs cannot be known, each AP must resort to using the RSSI of its neighbors as a measure of how closely their cells touch or overlap its own.
During the time each AP scans the channels to listen for RF conditions and other APs, it forms a list of its neighbors and their RSSI values. Each of those lists is sent to the local controller and on to the RF group leader where they are used by the TPC algorithm.
TPC works on one band at a time, making adjustments to APs as needed. If an AP has been heard with an RSSI above a threshold (–70 dBm by default) by at least three of its neighbors, TPC considers the AP’s cell to be overlapping the cells of its three neighbors too much. The AP’s transmit power level will be decreased by 3 dB, and then its RSSI will be evaluated again. This process is repeated for all APs at regular intervals until the neighbor that is measuring the third-strongest RSSI value for the AP no longer measures the RSSI greater than the threshold.
Although you probably will not have to make any configuration changes for the TPC algorithm, it is still useful to understand its settings. TPC runs on the 2.4- and 5-GHz bands independently. You can see the settings by selecting Wireless > 802.11a/n/ac or 802.11b/g/n > RRM > TPC. Figure 13-10 shows the TPC configuration for the 5-GHz band.
)
Figure 13-10 Adjusting the RRM TPC Algorithm Parameters
Actually, there are two different TPC algorithms as you can see in the figure. TPCv1 (the default), also known as Coverage Optimal Mode, works toward making adjustments that give the best RF coverage, while keeping signals sufficient and stable. TPCv2, also known as Interference Optimal Mode, focuses on avoiding negative impacts that TPCv1 might have had, where the power among AP cells ends up being imbalanced, causing some cells to interfere with others. TPCv2 requires proper tuning of RF parameters in order to work properly. While TPCv2 might sound superior, it should only be enabled in specific cases that are outside the scope of the CCNA Wireless exam or when directed by Cisco TAC.
By default, TPC runs automatically every 10 minutes. This is the recommended mode because any changes in the RF environment can be detected and compensated for without any intervention. As an alternative, you can select On Demand to run the algorithm immediately; then the resulting transmit power levels will be frozen until TPC is manually triggered again. If you would rather have the controller set the transmit power level on all APs to one fixed value, you can select Fixed and choose the power level from the drop-down menu.
Cisco controllers determine the transmit power level according to an index from 1 to 8, rather than discrete dBm or mW values. A value of 1 corresponds to the maximum power level that is allowed in the AP’s regulatory domain. Each increment in the power level number reduces the transmit power by 3 dBm. You might remember from Chapter 1 that reducing by 3 dBm also means that the power in mW is cut in half. As an example, Table 13-2 lists the power levels used in the 2.4-GHz and 5-GHz bands on a Cisco 3700 AP in the Americas or European domains.
Table 13-2 AP Transmit Power Level Numbers, dBm, and mW Values in the 2.4-GHz Band
Power Level |
dBm (2.4 GHz) |
dBm (5 GHz) |
mW |
1 |
23 |
23 |
200 |
2 |
20 |
20 |
100 |
3 |
17 |
17 |
50 |
4 |
14 |
14 |
25 |
5 |
11 |
11 |
12.5 |
6 |
8 |
8 |
6.25 |
7 |
5 |
Unused |
3.125 |
8 |
2 |
Unused |
1.56 |
With every iteration, the TPC algorithm can continue adjusting the transmit power levels until no further changes are needed. As a result, some APs might end up higher or lower than you might want. For example, it is usually best to match the AP transmit power level with that of the clients. Suppose that some of the clients have a fixed power level of 25 mW; if TPC ends up reducing some APs to 10 mW, the AP and client power levels will be mismatched.
To prevent such a condition, you can set minimum and maximum power level boundaries for the TPC algorithm. By default, the minimum level is set to –10 dBm and the maximum to 30 dBm, as shown in Figure 13-10.
Whenever you change the TPC parameters in a controller configuration, remember to make the same changes to all controllers that might be members of the same RF group. No matter which controller might become the RF group leader, the parameters should be identical.
DCA
Recall from Chapter 7, “Planning Coverage with Wireless APs,” and Chapter 12, “Understanding Roaming,” that a proper channel assignment is vital for efficient use of air time and for client mobility. When neighboring APs use the same channel, they can interfere with each other. Ideally, adjacent APs should use different, non-overlapping channels. Working out a channel layout for many APs can be a difficult puzzle, but the DCA algorithm can work out optimum solutions automatically for all APs in an RF group.
When a new AP first powers up, it uses the first non-overlapping channel in each band—channel 1 for 2.4 GHz and channel 36 for 5 GHz. Consider a simplistic scenario where all APs are new and powered up for the first time. You would end up with a building full of overlapping cells competing for the use of 2.4-GHz channel 1, as shown in simplified form in Figure 13-11. The DCA algorithm works to correct this situation by finding a channel that each AP in the RF group can use without overlapping or interfering with other APs. Like TPC, DCA works out one channel layout in the 2.4-GHz band and another layout in the 5-GHz band.
)
Figure 13-11 Basic Concept of the DCA Algorithm
DCA does not just solve the channel layout puzzle once for all APs. The algorithm runs every 10 minutes by default, so that it can detect any conditions that might require an AP’s channel to change. APs in the RF group are monitored for the metrics listed in Table 13-3 that can influence the channel reassignment decision.
Table 13-3 Metrics Affecting DCA Decisions
Metric |
Default State |
Description |
RSSI of neighboring APs |
Always enabled |
If DCA detects co-channel interference, it may move an AP to a different channel. |
802.11 interference |
Enabled |
If transmissions from APs and devices that are not part of the wireless network are detected, DCA may choose to move an AP to a different channel. |
Non-802.11 noise |
Enabled |
If excessive noise is present on a channel, DCA may choose to avoid using it. |
AP traffic load |
Disabled |
If an AP is heavily used, DCA may not change its channel to keep client disruption to a minimum. |
Persistent interference |
Disabled |
If an interference source with a high duty cycle is detected on a channel, DCA may choose to avoid using it. |
The DCA algorithm tends to look at each AP individually to find the ones with the worst RF conditions. Changing the channel of even one AP can affect many other APs if there are not other alternative channels available. Channel layout is a puzzle that may require several iterations to solve. For this reason, the controller that is the RF group leader will undergo an RRM startup mode after it is elected. The startup mode consists of ten DCA iterations at 10-minute intervals, or a total of 100 minutes before the channel layout reaches a steady state.
The end result of DCA is a channel layout that takes a variety of conditions into account. The channel layout is not just limited to the two dimensions of a single floor space in a building; it also extends to three-dimensional space because the RF signals from one floor can bleed through to another. As long as the APs on different floors belong to the same RF group, co-channel interference between them should be minimized.
You can display and configure the DCA parameters of either the 2.4- or 5-GHz band by selecting Wireless > 802.11a/n/ac or 802.11b/g/n > RRM > DCA. Figure 13-12 shows the 802.11a/n/ac configuration.
By default, the DCA algorithm runs automatically at 10-minute intervals. You can change the interval time, select Freeze to run DCA manually on demand, or turn it Off completely. You can also select the conditions to avoid, which will trigger a channel change on an AP.
The DCA parameters also include the 802.11n channel width. By default, 20-MHz channels will be used. If you have enabled 802.11n in the 5-GHz band and want to enable 40-MHz channels, be sure to select 40 MHz as the channel width. If you have 802.11ac enabled, you can choose between 20-, 40-, and 80-MHz channel width. The DCA algorithm will solve the channel assignment puzzle automatically, even with wide channels.
TIP
You might be wondering why 802.11ac can support 80- and 160-MHz channel widths, but 160 MHz is not an option on the controller depicted in Figure 13-12. The reason is twofold: (1) Full 160-MHz channel width is not supported until 802.11ac Wave 2; and (2) the CCNA Wireless 200-355 exam uses AireOS 8.0, which supports only Wave 1. In addition, the available spectrum does not currently support more than two 160-MHz channels. Both reasons will be solved over time, as new hardware is developed and as new spectrum is reclaimed and set aside in the 5-GHz band.
)
Figure 13-12 Adjusting the RRM DCA Algorithm Parameters
The bottom portion of the web page contains a list of channels that DCA can use as it assigns channels to APs in the respective band. This list is populated with channel numbers by default, but you can edit the list as needed. You can also enable or disable individual channel use by using the list of Select check boxes.
The DCA algorithm normally runs on an automatic schedule or manually on demand. Event-Driven RRM (ED-RRM) takes this a step further; DCA can be triggered based on RF events that occur in real time. The CleanAir feature, covered in more detail in Chapter 19, “Dealing with Wireless Interference,” provides the triggers for ED-RRM. By default, ED-RRM is disabled. You can enable it with the EDRRM check box at the very bottom of the web page.
Coverage Hole Detection Mitigation
The TPC algorithm normally reduces AP transmit power levels to make cell sizes appropriate. Sometimes you might find that your best intentions at providing RF coverage with a good AP layout still come up short. For example, you might discover that signals are weak in some small area of a building due to the building construction or surrounding obstacles. You might also have an AP radio that happens to fail, causing a larger coverage hole. How would you discover such a condition? You could make a habit of surveying the RF coverage often. More likely, your wireless users will discover a weakness or hole in the coverage and complain to you about it.
A Cisco controller-based wireless network offers an additional RRM algorithm that can detect coverage holes and take action to address them. Coverage hole detection mitigation (CHDM) can alert you to a hole that it has discovered and it can increase an AP’s transmit power level to compensate for the hole.
CHDM is useful in two cases:
- Extending coverage in a weak area
- Rapidly healing a coverage hole caused by an AP or radio failure, sooner than the TPC algorithm can detect and correct
The algorithm does not run at regular intervals like TPC and DCA do. Instead, it monitors the RF conditions of wireless clients and decides when to take action. In effect, the algorithm leverages your wireless users who are out in the field and tries to notice a problem before they do.
Every controller maintains a database of associated clients and their RSSI and signal-to-noise ratio (SNR) values. It might seem logical to think that a low RSSI or SNR would mean a client is experiencing a hole in coverage. Assuming the client and its AP are using the same transmit power levels, if the AP is receiving the client at a low level, the client must also be receiving the AP at a low level. This might not be true at all; the client might just be exiting the building and getting too far away from the AP. The client might also have a “sticky” roaming behavior, where it maintains an association with one AP until the RSSI falls to a very low level before reassociating elsewhere.
CHDM tries to rule out conditions that are experienced by small numbers of clients and signal conditions due to client roaming behavior. A valid coverage hole is detected when some number of clients, all associated to the same AP, have RSSI values that fall below a threshold. In addition, the coverage hole condition must exist longer than a threshold of time without the client roaming to a different AP.
By default, the following conditions must all be met for a coverage hole to be detected:
- Client RSSI at the AP is at or below –80 dBm.
- The low RSSI condition must last at least 60 seconds over the past 180 seconds.
- The condition must affect at least three clients or more than 25 percent of the clients on a single AP.
Be aware that CHDM runs on a per-band basis. Unlike TPC and DCA, which operate on the entire RF group of controllers, CHDM runs on each controller independently, on a per-AP radio basis.
You can display and configure the CHDM thresholds by selecting Wireless > 802.11a/n/ac or 802.11b/g/n > RRM > Coverage. Figure 13-13 shows the threshold parameters for the 5-GHz 802.11a band.
)
Figure 13-13 Displaying Coverage Threshold Parameters for the 5-GHz 802.11a Band
Manual RF Configuration
You might sometimes want to keep RRM from changing the RF conditions in parts of your wireless network. For instance, you might have client devices that operate at a fixed transmit power level. Ideally, the AP and client power levels should be identical or matched. If RRM raises or lowers AP power levels at a later time, then asymmetric power levels would result.
You can override RRM on a per-AP basis by selecting Wireless > Access Points > Radios > 802.11a/n/ac or 802.11b/g/n. From the list of APs displayed, choose a specific AP and select the drop-down menu at the far-right side of the list. From this menu, select Configure, as shown in Figure 13-14.
)
Figure 13-14 Selecting an AP for Manual Configuration
On the AP configuration page, as shown in Figure 13-15, you can set the channel under RF Channel Assignment or the transmit power under Tx Power Level Assignment. By default, the Global radio button is selected for each, which allows the value to be determined globally within the RF group. You can set a specific channel or power level by selecting the Custom radio button and then choosing a value from the drop-down list. In the figure, the AP’s transmit power level has been manually set to 3.
)
Figure 13-15 Manually Setting the Transmit Power Level of an AP
Verifying RRM Results
The RRM algorithms can either run at regular intervals or on demand. You can display the channel number and transmit power level that are being used on every AP by selecting Wireless > Access Points > Radios > 802.11a/n/ac or 802.11b/g/n, as shown in Figure 13-15. The controller displays an asterisk next to values that have been set through RRM. Otherwise, if no asterisk appears, the value has been set manually.
To get a much better feel for the RRM results, you can use the Cisco Prime Infrastructure management system (covered in Chapter 18, “Managing Cisco Wireless Networks”) to view APs on a graphical representation of an area. The map in Figure 13-16 displays each AP’s location on a building floor plan, along with its channel number and transmit power level for the 2.4-GHz band. Figure 13-17 shows the same map for the 5-GHz band. Seeing the physical arrangement of APs and their cells can help you get a much better idea how the channels are assigned and reused.
)
Figure 13-16 Displaying 2.4-GHz RRM Results in Cisco Prime Infrastructure Maps
)
Figure 13-17 Displaying 5-GHz RRM Results in Cisco Prime Infrastructure Maps