CWNP CWNA-109 Practice Test - Questions Answers, Page 6

The scenario described suggests that while the Access Point (AP) is capable of 802.11ax (Wi-Fi 6) with four spatial streams, the clients are only achieving data rates typical of 802.11n (Wi-Fi 4) devices, which indicates that the clients are likely 802.11n devices. Here's why this is the most plausible explanation:

802.11n Limitations: Devices that adhere to the 802.11n standard have lower maximum data rates compared to 802.11ax devices due to differences in technology such as modulation, spatial streams, and channel bandwidth. An 802.11n device with a single spatial stream operating on a 20 MHz channel can achieve a maximum data rate of 72.2 Mbps. Even with two spatial streams under ideal conditions, this would only double to approximately 144.4 Mbps, which is close to the 150 Mbps mentioned.

Spatial Stream Capability: The fact that the AP supports four spatial streams suggests it can achieve much higher data rates with 802.11ax clients that also support multiple spatial streams. However, if the clients are 802.11n devices, they may not be capable of using more than two spatial streams, and many earlier 802.11n devices were limited to just one.

The other options are less likely to be the primary cause based on the information provided:

B . Two Stream 802.11ax Clients: If the clients were 802.11ax with only two spatial streams, they would likely achieve higher data rates than 150 Mbps due to the efficiency improvements in 802.11ax.

C . Contention and D. Non-Wi-Fi Interference: While these could affect performance, they would not inherently limit clients to 150 Mbps, especially in the context of an 802.11ax environment where mechanisms to handle interference and contention are more advanced.

IEEE 802.11n-2009: Enhancements for Higher Throughput.

CWNA Certified Wireless Network Administrator Official Study Guide: Exam PW0-105, by David D. Coleman and David A. Westcott.

An ACK (Acknowledgement) frame is a short control frame that is sent by the receiver of a data or management frame to confirm that the frame was received correctly. The ACK frame is sent after a SIFS (Short Interframe Space) interval, which is the shortest time gap between frames in 802.11. If the transmitter does not receive an ACK frame within a specified time, it assumes that the frame was not delivered and must be retransmitted. This is part of the 802.11 reliability mechanism that ensures reliable data delivery over an unreliable wireless medium .Reference:[CWNA-109 Study Guide], Chapter 5: IEEE 802.11 Medium Access, page 209; [CWNA-109 Study Guide], Chapter 5: IEEE 802.11 Medium Access, page 203.

In the 802.11 wireless networking protocols, when a client station sends a broadcast probe request frame with a wildcard SSID (Service Set Identifier), it is essentially asking for any nearby access points (APs) to identify themselves. The way APs respond to such a probe request is governed by standard 802.11 behavior, which includes:

Probe Request Handling: Upon receiving a broadcast probe request, each AP that can serve the client prepares a probe response. The response includes information about the AP, such as its SSID, supported data rates, and other capabilities.

Contention-Based Mechanism: Wireless networks use a contention-based mechanism (CSMA/CA - Carrier Sense Multiple Access with Collision Avoidance) for medium access. Each AP must wait for a clear channel and win the contention process before it can send its probe response.

Independent Responses: Each AP operates independently in responding to the probe request. There is no coordination between APs to decide which one responds first or at all, leading to multiple APs sending probe responses, each after winning the contention for the medium.

Option A accurately reflects this process, indicating that each AP prepares and sends a probe response in turn, contingent upon winning the medium contention. The other options suggest mechanisms (such as coordination with a DHCP server or simultaneous responses after a Short Interframe Space (SIFS)) that do not align with standard 802.11 procedures for handling broadcast probe requests.

IEEE 802.11 Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.

CWNA Certified Wireless Network Administrator Official Study Guide: Exam PW0-105, by David D. Coleman and David A. Westcott.

Shared Key Authentication is a security solution that was defined in the original 802.11 standard as an alternative to Open System Authentication, which does not provide any security at all. Shared Key Authentication uses WEP (Wired Equivalent Privacy) to encrypt and authenticate data frames between the client station and the AP. However, WEP has been proven to be extremely vulnerable to various attacks that can easily crack the encryption key and compromise the network security. Therefore, Shared Key Authentication is deprecated in the 802.11 standard and should never be used in any modern WLAN deployment .Reference:[CWNA-109 Study Guide], Chapter 10: Wireless LAN Security, page 401; [CWNA-109 Study Guide], Chapter 10: Wireless LAN Security, page 391; [Wikipedia], Wired Equivalent Privacy.

The short guard interval is an optional feature of 802.11n and 802.11ac that reduces the time between OFDM symbols from 800 ns to 400 ns. This can increase the data rate by about 11%, but also requires more precise timing and synchronization between the transmitter and the receiver. The short guard interval is only used when both the AP and the client support it and agree to use it .Reference:[CWNA-109 Study Guide], Chapter 4: Radio Frequency Signal and Antenna Concepts, page 163; [CWNA-109 Study Guide], Chapter 4: Radio Frequency Signal and Antenna Concepts, page 157.

802.11 QoS is achieved by giving high priority queues a statistical advantage at winning contention. 802.11 QoS is based on the Enhanced Distributed Channel Access (EDCA) mechanism, which defines four access categories (ACs) for different types of traffic: Voice, Video, Best Effort, and Background. Each AC has its own transmit queue and contention parameters, such as Arbitration Interframe Space (AIFS), Contention Window (CW), and Transmission Opportunity (TXOP). These parameters determine how long a station has to wait before transmitting a frame and how long it can occupy the channel. Higher priority ACs have shorter AIFS, smaller CW, and longer TXOP, which means they have more chances to access the channel and send more data than lower priority ACs. However, this does not guarantee that higher priority ACs will always win the contention, as there is still a random backoff process involved. Therefore, 802.11 QoS is a statistical service that provides different levels of service quality based on traffic categories.Reference:, Chapter 10, page 403; , Section 6.1

https://7signal.com/802-11ac-migration-part-2-whats-nobodys-telling-you-about-80mhz-and-160mhz-channel-bonding

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

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.

An IBSS is used instead of a BSS is a network configuration that would result in multiple stations broadcasting Beacon frames with the same BSSID but with different source addresses. An IBSS (Independent Basic Service Set) is a type of WLAN that does not use an AP but rather allows stations to communicate directly with each other in a peer-to-peer manner. An IBSS is also known as an ad-hoc network or a peer-to-peer network. In an IBSS, each station generates its own Beacon frames to announce its presence and capabilities to other stations within range. The Beacon frames have the same BSSID, which is randomly generated by one of the stations when creating the IBSS, but they have different source addresses, which are the MAC addresses of each station's radio interface. The BSSID is used to identify the IBSS and prevent stations from joining other IBSSs with different BSSIDs.Reference:, Chapter 1, page 25; , Section 1.1