CWNA-109 Practice Questions

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The clients are all 802.11n STAs or lower is the likely cause of this behavior. If a WLAN with 100

802.11ac access points is configured to use 80 MHz channels, but only 40 MHz communications are

seen in a particular BSS, it means that the clients in that BSS do not support 80 MHz channels. This

could be because they are using older standards, such as 802.11n or lower, that do not support 80

MHz channels. Alternatively, they could be using newer standards, such as 802.11ac or ax, but have

their channel width settings limited to 40 MHz or lower due to device capabilities or configuration

options. In either case, the AP will adapt to the client’s channel width and use only 40 MHz of the 80

MHz allocated bandwidth to communicate with them. This will reduce the potential throughput and

efficiency of the WLAN. Reference: , Chapter 3, page 111; , Section 3.2

OFDMA is a new modulation scheme introduced in 802.11ax that allows multiple users to share the

same channel by dividing it into smaller subchannels called resource units (RUs). This improves the

efficiency and capacity of the WLAN by reducing contention and overhead. However, to use OFDMA,

both the AP and the client must support 802.11ax and negotiate the parameters of the subchannel

allocation. Therefore, the customer needs to upgrade the clients that require OFDMA to 802.11ax

devices12.

The other options are not correct because they do not reflect the reality of OFDMA. Option B is

incorrect because OFDMA is a mandatory feature of 802.11ax for both downlink and uplink

transmissions, and all 802.11ax APs must support it1. Option C is incorrect because OFDM and

OFDMA are different modulation schemes, and OFDM does not allow multiple users to share the

same channel. Option D is incorrect because 802.11ac devices cannot support OFDMA through driver

upgrades, as they lack the hardware and firmware capabilities to do so2.

Reference: 1: CWNA-109 Official Study Guide, page 144 2: OFDMA

The response that you should give to your manager’s suggestion of implementing the security

practices of disabling the broadcasting of the SSID in Beacons and configuring the access points not

to respond to Probe Request frames that have a null SSID field is that any 802.11 protocol analyzer

can see the SSID in clear text in frames other than Beacons frames. This negates any security benefit

of trying to hide the SSID in Beacons and Probe Response frames. The SSID (Service Set Identifier) is a

human-readable name that identifies a WLAN and allows users to connect to it. The SSID is

transmitted in clear text in several types of 802.11 frames, such as Beacon frames, Probe Request

frames, Probe Response frames, Association Request frames, Association Response frames,

Reassociation Request frames, and Reassociation Response frames. Some people may think that

hiding the SSID can improve the security of the WLAN by making it invisible to potential intruders.

However, this is not true, as hiding the SSID only removes it from Beacon frames and Probe Response

frames that have a null SSID field. The SSID is still present in other types of frames that can be easily

captured and analyzed by any 802.11 protocol analyzer or wireless scanner tool. Therefore, hiding

the SSID does not provide any real security benefit and may even cause some compatibility and

performance issues for legitimate users. Reference: 1, Chapter 4, page 133; 2, Section 4.1

The least expensive solution available for increasing throughput for these users without

implementing configuration options that are not recommended is to use a 40 MHz channel on the 5

GHz radio. This solution can double the channel bandwidth and increase the data rates for the 5 GHz

capable 802.11ac clients. Using a 40 MHz channel on the 5 GHz radio is also less likely to cause co-

channel interference or overlap with other channels than using a 40 MHz channel on the 2.4 GHz

radio, which has only three non-overlapping channels. Using a 160 MHz channel on the 5 GHz radio

may provide even higher throughput, but it may also consume too much of the available spectrum

and cause more interference with other devices or networks. Installing a second AP in the coverage

area may also improve the throughput, but it may require additional costs and

configuration. Reference: [CWNP Certified Wireless Network Administrator Official Study Guide:

Exam CWNA-109], page 216; [CWNA: Certified Wireless Network Administrator Official Study Guide:

Exam CWNA-109], page 206.

What is always required to establish a high quality 2.4 GHz RF link at a distance of 3 miles (5

kilometers) is a Fresnel Zone that is at least 60% clear of obstructions. The Fresnel Zone is an

elliptical-shaped area around the line-of-sight path between two antennas that reflects and refracts

the RF waves. The Fresnel Zone radius depends on the frequency of the RF signal and the distance

between the antennas. For optimal performance, the Fresnel Zone should be at least 60% clear of

any obstructions that may cause interference, attenuation, or multipath fading. The minimum output

power level, antenna gain, and antenna type may vary depending on the environmental conditions

and regulatory constraints, but they are not always required for a high quality RF

link. Reference: [CWNP Certified Wireless Network Administrator Official Study Guide: Exam CWNA-

109], page 75; [CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-

109], page 65.

The common spectrum analyzer view that you should use for this analysis is

the Waterfall/Spectrogram view. The Waterfall/Spectrogram view shows the signal from the

interfering device over time on a three-dimensional graph. The x-axis represents frequency, the y-

axis represents time, and the z-axis represents amplitude or power. The color of each pixel indicates

the signal strength at a given frequency and time. The Waterfall/Spectrogram view can help you

identify the characteristics of the interference source, such as its frequency range, duty cycle,

modulation type, and pattern. Reference: [CWNP Certified Wireless Network Administrator Official

Study Guide: Exam CWNA-109], page 524; [CWNA: Certified Wireless Network Administrator Official

Study Guide: Exam CWNA-109], page 494.

Channel 144 is a new channel that was added to the 5 GHz band by the 802.11ac amendment, which

defines the VHT (Very High Throughput) PHY for WLANs. Channel 144 has a center frequency of 5720

MHz and a bandwidth of 20 MHz. It can also be combined with adjacent channels to form wider

channels of 40 MHz, 80 MHz, or 160 MHz. Channel 144 is available in some regions, such as North

America and Europe, but not in others, such as Japan and China . Reference: [CWNA-109 Study

Guide], Chapter 3: Antennas and Accessories, page 121; [CWNA-109 Study Guide], Chapter 3:

Antennas and Accessories, page 115; [Wikipedia], List of WLAN channels.

Fade Margin is an additional pad of signal strength designed into the RF system to compensate for

unpredictable signal fading. It is the difference between the receiver’s sensitivity and the actual

received signal level. A higher Fade Margin indicates a more robust link that can withstand

interference, attenuation, or other factors that may reduce the signal strength. A lower Fade Margin

means that the link is more susceptible to failure or performance degradation. Fade Margin is usually

expressed in decibels (dB) and can be calculated by subtracting the receiver sensitivity from the

received signal level. Reference: 1, Chapter 2, page 51; 2, Section 2.1

VHT TXOP (Very High Throughput Transmit Opportunity) power save is a feature introduced with the

802.11ac amendment, which is designed to improve the power efficiency of devices connected to a

WLAN. This feature enhances battery life in several ways, compared to the legacy Power Save mode:

Enhanced Power Saving: VHT TXOP power save allows devices to disable more components of the

WLAN transceiver when they are in a low power state. This reduces the power consumption during

periods when the device is not actively transmitting or receiving data.

Intelligent Wake-Up Mechanisms: It employs more sophisticated mechanisms for devices to

determine when they need to wake up and listen to the channel, further reducing unnecessary

power usage.

Optimized Operation: This power save mode is optimized for the high-throughput environment of

802.11ac networks, allowing devices to efficiently manage power while maintaining high

performance.

Legacy Power Save mode, introduced in earlier versions of the 802.11 standards, does not provide

the same level of component disablement or the intelligent wake-up mechanisms found in VHT TXOP

power save, making option B the correct answer.

Reference:

IEEE 802.11ac-2013 Amendment: Enhancements for Very High Throughput for Operation in Bands

below 6 GHz.

CWNA Certified Wireless Network Administrator Official Study Guide: Exam CWNA-109, by David D.

Coleman and David A. Westcott.

The device in the bridge implementation that acts as the 802.1X Authenticator is the root bridge. The

root bridge is the bridge that connects to the wired network and acts as the central point for all other

bridges in the point-to-multipoint topology. The root bridge authenticates the non-root bridges using

802.1X/EAP and forwards their authentication requests to the RADIUS server. The non-root bridges

act as the 802.1X Supplicants and use EAP methods such as EAP-TLS or EAP-PEAP to authenticate

with the root bridge. Reference: [CWNP Certified Wireless Network Administrator Official Study

Guide: Exam CWNA-109], page 459; [Cisco Aironet Wireless Bridges FAQ],