CWNA-109 Premium Exam Questions

TWT is the feature of 802.11ax (HE) that is managed with beacon and trigger frames and is primarily a power management method, but also provides more efficient access to the channel used within a BSS. TWT stands for target wake time, which is a mechanism that allows an access point and a client device to negotiate and schedule specific times for data transmission and reception. This enables the client device to enter a low-power sleep mode when it is not expected to communicate with the access point, which saves battery life and reduces power consumption. TWT also reduces contention and interference on the channel used within a BSS, as it coordinates the transmissions of multiple client devices and avoids collisions. TWT is managed with beacon and trigger frames, which are two types of management frames that are used to announce and initiate data exchanges. A beacon frame is a frame that is periodically sent by an access point to advertise its presence, capabilities, and parameters to client devices. A trigger frame is a frame that is sent by an access point or a client device to request or initiate a data transmission with another device. BSS color, UL-MU-MIMO, and OFDMA are other features of 802.11ax (HE) that are not primarily power management methods, but rather performance enhancement methods. BSS color is a feature that assigns a color code to each BSS to differentiate it from other BSSs that use the same channel. This reduces interference and improves spatial reuse of the channel. UL-MU-MIMO is a feature that allows an access point to receive multiple simultaneous transmissions from different client devices using multiple spatial streams. This increases capacity and throughput of the uplink direction. OFDMA is a feature that divides a channel into smaller subchannels called resource units (RUs) that can be allocated to different devices for concurrent transmissions. This increases efficiency and flexibility of the channel utilization. References: CWNA-109 Study Guide, Chapter 10: Wireless LAN Operation, page 323

A 5 GHz channel should be used for this AP because channels 1 and 6 are 2.4 GHz channels and they have no impact on the decision. The 5 GHz frequency band offers more non-overlapping channels than the 2.4 GHz frequency band, which reduces interference and improves performance. The 5 GHz frequency band also supports higher data rates and wider channel bandwidths than the 2.4 GHz frequency band, which increases capacity and throughput. The 5 GHz frequency band also has less interference from other devices and sources than the 2.4 GHz frequency band, which enhances reliability and quality of service. Therefore, it is recommended to use the 5 GHz frequency band for WLANs whenever possible. Channels 1 and 6 are two of the three non-overlapping channels in the 2.4 GHz frequency band (the other one is channel 11). They are used by a neighbor network in this scenario, but they do not affect the channel selection for this AP because they operate in a different frequency band than the 5 GHz frequency band. Channel 6 is not always best to use; it depends on the interference and congestion level in the environment. Channel 1 is not best to use because it has a lower frequency than channel 6; frequency does not determine channel quality or performance. Channel 11 is not best to use because it is also a 2.4 GHz channel and it may interfere with channels 1 and 6. References: CWNA-109 Study Guide, Chapter 4: Antenna Systems and Radio Frequency (RF) Components, page 113

The center frequency of channel 4 in the 2.4 GHz band is 2.427 GHz (2427 MHz). The center frequency of a channel is the midpoint of its frequency range, where the signal strength is highest and most concentrated. The center frequency of channel 1 in the 2.4 GHz band is 2.412 GHz (2412 MHz), as given in the question. The center frequency of each subsequent channel is obtained by adding 5 MHz to the previous channel’s center frequency, since the channels are spaced 5 MHz apart from each other in this band. Therefore, to find the center frequency of channel 4, we need to add 15 MHz (5 MHz x 3) to the center frequency of channel 1:

2.412 GHz + 0.015 GHz = 2.427 GHz

Alternatively, we can use a formula to calculate the center frequency of any channel in the 2.4 GHz band:

Center frequency (GHz) = 2.407 + (0.005 x Channel number)

Using this formula for channel 4, we get:

Center frequency (GHz) = 2.407 + (0.005 x 4)

Center frequency (GHz) = 2.407 + 0.02

Center frequency (GHz) = 2.427 References: 1, Chapter 3, page 85; 2, Section 3.2

In addition to coverage analysis results, what should be included in a post-deployment site survey report to ensure WLAN users experience acceptable performance is Capacity analysis results. Capacity analysis is a method of testing the ability of the WLAN to support the expected number and type of users, devices, and applications. Capacity analysis can help to determine the optimal number and placement of access points, the appropriate channel and power settings, the required QoS policies, and the expected throughput and latency levels. Capacity analysis results can help to verify that the WLAN meets the performance requirements and service level agreements (SLAs) of the organization. References: [CWNP Certified Wireless Network Administrator Official Study Guide: ExamCWNA-109], page 548; [CWNA: Certified Wireless Network Administrator Official Study Guide: ExamCWNA-109], page 518.