CWNP CWNA-109 Certified Wireless Network Administrator Exam Practice Test

Answer : A

USB 3 devices in the user's work area are the most likely cause of this interference when using a spectrum analyzer to look for non-Wi-Fi interference sources. A spectrum analyzer is a tool that measures and visualizes the radio frequency activity and interference in the wireless environment. A spectrum analyzer can show the spectrum usage and energy levels on each frequency band or channel and help identify and locate the sources of interference. Interference is any unwanted signal that disrupts or degrades the intended signal on a wireless channel. Interference can be caused by various sources, such as other Wi-Fi devices, non-Wi-Fi devices, or natural phenomena. Interference can affect WLAN performance and quality by causing signal loss, noise, distortion, or errors. USB 3 devices are non-Wi-Fi devices that use USB 3.0 technology to transfer data at high speeds between computers and peripherals, such as hard drives, flash drives, cameras, or printers. USB 3 devices can generate electromagnetic radiation that interferes with Wi-Fi signals in the 2.4 GHz band, especially when they are close to Wi-Fi devices or antennas. USB 3 devices can cause significant interference across the entire 2.4 GHz band (not on a few select frequencies) within the desktop area of a user's workspace, but the interference disappears quickly after just 2 meters. This is because USB 3 devices emit broadband interference that affects all channels in the 2.4 GHz band with a high intensity near the source but a low intensity at a distance due to attenuation. The other options are not likely to cause this interference pattern when using a spectrum analyzer to look for non-Wi-Fi interference sources. Bluetooth devices in the user's work area are non-Wi-Fi devices that use Bluetooth technology to communicate wirelessly between computers and peripherals, such as keyboards, mice, headphones, or speakers. Bluetooth devices can cause interference with Wi-Fi signals in the 2.4 GHz band, but they use frequency hopping spread spectrum (FHSS) technique that changes frequencies rapidly and randomly within a range of 79 channels. Therefore, Bluetooth devices do not cause significant interference across the entire 2.4 GHz band (not on a few select frequencies), but rather intermittent interference on some channels at different times. Excess RF energy from a nearby AP is not a non-Wi-Fi interference source but rather a Wi-Fi interference source that occurs when an AP transmits more power than necessary for its coverage area. Excess RF energy from a nearby AP can cause co-channel interference (CCI) with other APs or client devices that use the same channel within range of each other. CCI reduces performance and capacity because it causes contention and collisions on the wireless medium,

Answer : C

The guard interval is a short period of time inserted between the symbols of an OFDM signal to prevent inter-symbol interference and improve the robustness of the transmission1. The guard interval can have different values depending on the 802.11 standard and the configuration of the device.For example, 802.11n supports two guard intervals: 800 ns (normal) and 400 ns (short)2.802.11ac supports the same guard intervals as 802.11n, plus an optional 200 ns guard interval for 80 MHz and 160 MHz channels3.802.11ax supports three guard intervals: 800 ns, 1600 ns, and 3200 ns4.

The guard interval affects the data rate because it determines the duration of each symbol. A shorter guard interval means more symbols can be transmitted in a given time, resulting in a higher data rate. However, a shorter guard interval also means less protection against inter-symbol interference, which may degrade the signal quality and increase the error rate. Therefore, there is a trade-off between data rate and reliability when choosing the guard interval.

The MCS tables for HT and VHT PHY devices show the data rates for different combinations of modulation, coding, channel width, spatial streams, and guard intervals.For example, for a VHT device using MCS 9 with QAM-256 modulation, 5/6 coding rate, 80 MHz channel width, and one spatial stream, the data rate is 433.3 Mbps with a normal guard interval (800 ns) and 486.7 Mbps with a short guard interval (400 ns)2. Therefore, the guard interval impacts the data rate according to the MCS tables.

Answer : C

The string of characters and digits that is entered into an AP and a client STA for WPA2 security and is 8 characters long is called apassphrase. A passphrase is a human-readable text that is used to generate a Pre-Shared Key (PSK) for WPA2-Personal security. A passphrase can be between 8 and 63 characters long and can include any ASCII character. The PSK is a 256-bit key that is derived from the passphrase using a hashing algorithm called PBKDF2. The PSK is used to encrypt and decrypt the data frames between the AP and the client STA. A MSK is a Master Session Key that is generated by an authentication server for WPA2-Enterprise security. A WEP key is a 40-bit or 104-bit key that is used for Wired Equivalent Privacy (WEP) security, which is deprecated and insecure. A PSK is not a string of characters and digits, but a binary key.Reference:[CWNP Certified Wireless Network Administrator Official Study Guide: Exam CWNA-109], page 303; [CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-109], page 293.

Answer : C

Authenticated and Unassociated.According to one of the web search results1, a STA can be authenticated to multiple APs, but it can only be associated to one AP at a time.Association is the process of establishing a logical link between the STA and the AP, which allows the STA to send and receive data frames through the AP2. Therefore, when a STA has authenticated to an AP-1, but still maintains a connection with another AP-2, it means that the STA is authenticated to both APs, but only associated to AP-2. The state of the STA on AP-1 is authenticated and unassociated, which means that the STA can switch to AP-1 without repeating the authentication process, but it cannot send or receive data frames through AP-1 until it becomes associated.

Answer : D

What prevents each station from using all the airtime when other client stations are actively communicating in the same BSS isCSMA/C

A . CSMA/CA stands for Carrier Sense Multiple Access with Collision Avoidance and is a media access control method used by WLAN devices to share the wireless medium. CSMA/CA works by having each station sense the medium before transmitting a frame. If the medium is busy (i.e., another station is transmitting), the station defers its transmission until the medium is idle. If the medium is idle, the station waits for a random backoff period before transmitting. This way, CSMA/CA reduces the chances of collisions and ensures fair access to the medium for all stations. CSMA/CA also uses positive acknowledgements to confirm successful transmissions and retransmissions to recover from errors. CSMA/CD, DOS prevention, and OFDMA are not used by WLAN devices in a BSS.Reference:[CWNP Certified Wireless Network Administrator Official Study Guide: Exam CWNA-109], page 108; [CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-109], page 98.

Answer : B

The least expensive solution available for increasing throughput for these users without implementing configuration options that are not recommended is touse 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.

Answer : A

The factors that will have the most significant impact on the amount of wireless bandwidth available to each station within a BSS are:

The number of client stations associated to the BSS

The presence of co-located (10m away) access points on non-overlapping channels

The number of client stations associated to the BSS affects the wireless bandwidth because each station shares the same channel and medium with other stations in the same BSS. The more stations there are, the more contention and collision there will be for the channel access, which reduces the throughput and efficiency of the wireless communication. The wireless bandwidth available to each station depends on how the access point allocates the channel resources and how the stations use the channel time. For example, if the access point uses a round-robin scheduling algorithm, each station will get an equal share of the channel time regardless of its data rate or traffic demand. However, if the access point uses a proportional fair scheduling algorithm, each station will get a share of the channel time that is proportional to its data rate and traffic demand, which may result in higher or lower bandwidth for different stations.

The presence of co-located (10m away) access points on non-overlapping channels affects the wireless bandwidth because even though they use different channels, they may still cause interference and noise to each other due to channel leakage or imperfect filtering. The interference and noise can degrade the signal quality and SNR of the wireless communication, which reduces the data rate and throughput of the wireless communication. The wireless bandwidth available to each station depends on how well the access point and the station can cope with the interference and noise from other channels. For example, if the access point and the station support dynamic frequency selection (DFS) or adaptive radio management (ARM), they can switch to a less congested channel or adjust their output power or antenna gain to avoid or minimize interference from other channels.