CWNP Certified Wireless Network Administrator CWNA-109 Exam Questions

Answer : B

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.

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.

Answer : C

Return loss is a measure of how well the components of an RF system are matched in terms of their impedance. Impedance is the opposition to the flow of alternating current in a circuit, and it depends on the frequency, resistance, capacitance, and inductance of the components. When the impedance of the source, the transmission line, and the load are not equal, some of the power is reflected back to the source, causing a loss of forward power. This loss is expressed in decibels (dB) as return loss. The higher the return loss, the lower the reflection and the better the impedance matching. Conversely, the lower the return loss, the higher the reflection and the worse the impedance matching.

VSWR (Voltage Standing Wave Ratio) is another way of expressing the same concept. It is the ratio of the maximum voltage to the minimum voltage along a transmission line due to the interference of the incident and reflected waves. A VSWR of 1:1 means that there is no reflection and perfect impedance matching. A VSWR higher than 1:1 means that there is some reflection and impedance mismatch. The higher the VSWR, the higher the reflection and the lower the return loss.

Therefore, a significant impedance mismatch between components in an RF system will cause high reflection, high VSWR, and low return loss.

Answer : C

It is a channel that may require DFS when used is an important fact that should be considered about channel 128. Channel 128 is a 5 GHz frequency band 20 MHz channel that has a center frequency of 5.64 GHz. Channel 128 is one of the channels that are subject to DFS (Dynamic Frequency Selection) rules, which require Wi-Fi devices to monitor and avoid using channels that are occupied by radar systems or other primary users. DFS is a feature that is defined in the IEEE 802.11h amendment and is mandated by some regulatory bodies, such as the FCC and the ETSI, to protect the licensed users of the 5 GHz band from interference by unlicensed Wi-Fi devices. DFS works by using a mechanism called channel availability check (CAC), which requires Wi-Fi devices to scan a channel for a certain period of time before using it. If a radar signal is detected during the CAC or while using the channel, the Wi-Fi devices must switch to another channel that is free from radar interference.

When configuring an access point to use channel 128, it is important to consider the implications of DFS rules, such as:

The access point must support DFS and comply with the local regulations and standards that apply to DFS channels.

The access point may experience delays or interruptions in its operation due to CAC or channel switching.

The access point may have limited channel selection or availability due to radar interference or other Wi-Fi devices using DFS channels.

The access point may have compatibility or interoperability issues with some client devices that do not support DFS or use different DFS parameters.

The access point may have performance or quality issues due to co-channel or adjacent-channel interference from other Wi-Fi devices using non-DFS channels.

Therefore, it is advisable to use channel 128 only when necessary and after performing a thorough site survey and spectrum analysis to determine the best channel for the access point.Reference:1, Chapter 3, page 117;2, Section 3.2

Answer : B

WPA2 (Wi-Fi Protected Access 2) is a security certification program developed by the Wi-Fi Alliance to secure wireless computer networks. It is important to understand the following:

WPA2 and the 802.11 Standard: While WPA2 is based on elements of the 802.11i amendment to the 802.11 standard, it is not itself a standard but rather a certification to ensure devices comply with certain security criteria, including the correct implementation of CCMP (Counter Mode Cipher Block Chaining Message Authentication Code Protocol) and AES (Advanced Encryption Standard).

CCMP/AES Implementation: WPA2 enhances the security of wireless networks by using CCMP for encryption, which is based on AES, a robust encryption algorithm. This represents a significant security improvement over WEP (Wired Equivalent Privacy) and WPA (Wi-Fi Protected Access) that used TKIP (Temporal Key Integrity Protocol).

WPA vs. WPA2: WPA was the interim security enhancement over WEP, utilizing TKIP for encryption. WPA2, however, moved to the more secure AES-based encryption method. Contrary to option C, WPA2 does not enhance security by using TKIP; it uses CCMP/AES.

Therefore, option B correctly describes WPA2 as a certification program ensuring devices properly implement the more secure CCMP/AES encryption methods.

Wi-Fi Alliance website for WPA2 certification details.

IEEE 802.11i-2004: Amendment for Enhanced Security.

Answer : B

VHT stands for Very High Throughput, which is a physical layer (PHY) specification that supports the 5 GHz frequency band and supports 80 MHz channels. VHT is used by the IEEE 802.11ac standard, which is also known as Wi-Fi 5. VHT allows for higher data rates and more spatial streams than the previous HT (High Throughput) PHY, which is used by the IEEE 802.11n standard, also known as Wi-Fi 4.HT supports the 2.4 GHz and 5 GHz bands, but only supports up to 40 MHz channels12

The other options are not correct because:

ERP (option C) stands for Extended Rate PHY, which is a physical layer specification that supports the 2.4 GHz frequency band and supports up to 20 MHz channels. ERP is used by the IEEE 802.11g standard, which is also known as Wi-Fi 3.ERP allows for higher data rates than the previous DSSS (Direct Sequence Spread Spectrum) PHY, which is used by the IEEE 802.11b standard, also known as Wi-Fi 234

OFDM (option D) stands for Orthogonal Frequency Division Multiplexing, which is a modulation technique that divides a signal into multiple subcarriers that are spaced orthogonally to each other. OFDM is not a physical layer specification, but a common feature of many PHY specifications, including ERP, HT, and VHT.OFDM allows for higher spectral efficiency and robustness against multipath interference than the previous CCK (Complementary Code Keying) modulation technique used by DSSS34

Answer : C

The HT (802.11n) physical layer (PHY) specification includes support for and compatibility with both ERP and HR/DSSS. ERP stands for Extended Rate PHY, which is an extension of the original DSSS (Direct Sequence Spread Spectrum) PHY that supports data rates up to 54 Mbps in the 2.4 GHz band. HR/DSSS stands for High Rate/Direct Sequence Spread Spectrum, which is another extension of DSSS that supports data rates up to 11 Mbps in the 2.4 GHz band. HT stands for High Throughput, which is a new PHY that supports data rates up to 600 Mbps in both the 2.4 GHz and 5 GHz bands. HT uses OFDM (Orthogonal Frequency Division Multiplexing) as its modulation scheme, but it also supports legacy DSSS and ERP devices by using a dual preamble and header structure that allows backward compatibility.Reference:, Chapter 3, page 103; , Section 3.1

Answer : D

VSWR stands for Voltage Standing Wave Ratio, which is a measure of how well the impedance of the RF cable and connectors matches the impedance of the transmitter and the antenna. Impedance is the opposition to the flow of alternating current in an RF circuit, and it depends on the frequency, resistance, capacitance, and inductance of the components. A perfect impedance match would have a VSWR of 1:1, meaning that all the power is transferred from the transmitter to the antenna, and none is reflected back. However, in reality, there is always some degree of mismatch, which causes some power to be reflected back to the transmitter, creating standing waves along the cable. This reduces the efficiency and performance of the wireless system, and can also damage the transmitter.Excessive VSWR can be caused by using poor quality or damaged cables and connectors, or by using components that have different impedance ratings123.Reference:CWNA-109 Study Guide, Chapter 2: Radio Frequency Fundamentals, page 90;CWNA-109 Study Guide, Chapter 2: Radio Frequency Fundamentals, page 86;CWNP website, CWNA Certification.