You're the WLAN administrator for a large retailer based at the HQ in New York. The London-based office has been complaining about WLAN disconnections around lunch time each day. You suspect this might be interference from the staff microwave, how might you test your theory from the New York office?
Correct Answer:
D
The best way to test the theory of microwave interference from the New York office is to use a remote spectrum analyzer. By placing one of the London APs into spectrum analyzer mode, you can capture and analyze the RF spectrum in the London office over lunch time. You can then look for any signs of microwave interference, such as high duty cycle, high amplitude, or frequency hopping on the 2.4 GHz band. This method does not require any physical access tothe microwave or any changes to its
frequency. References: [Wireless Analysis Professional Study Guide], Chapter 3: Spectrum Analysis, page 64
What is the function of the PHY Preamble?
Correct Answer:
D
The function of the PHY preamble is to allow the receiver to detect and synchronize with the signal. The PHY preamble is a part of the PPDU that is transmitted before the PHY header and the PSDU. The PHY preamble consists of a series of training fields that help the receiver to adjust its parameters, such as frequency, timing, and gain, to match the incoming signal. The PHY preamble also helps the receiver to estimate the channel conditions and noise level. References: [Wireless Analysis Professional Study Guide CWAP-404], Chapter 4: 802.11 Physical Layer, page 99-100
What does the value of the Listen Interval field in an Association Request frame indicate?
Correct Answer:
C
The value of the Listen Interval field in an Association Request frame indicates how often a STA in power save mode wakes up to listen to Beacon frames. The Listen Interval is expressed in units of Beacon Intervals (typically 100 TU or 102.4 ms). For example, if the Listen Interval is set to 10, it means that the STA will wake up every 10 Beacon Intervals (or about 1 second) to check for buffered frames at the AP. The Listen Interval is used by the AP to determine how long it can hold frames for a STA in power save mode before discarding them . References: CWAP-404 Certified Wireless Analysis Professional Study and Reference Guide, Chapter 6: MAC Sublayer Frame Exchanges, page 197; CWAP-404 Certified Wireless Analysis Professional Study and Reference Guide, Chapter 6: MAC Sublayer Frame Exchanges, page 198.
Which one of the following is required for Wi-Fi integration in laptop-based Spectrum Analyzer software in addition to the spectrum analysis adapter?
Correct Answer:
A
An 802.11 wireless adaptor is required for Wi-Fi integration in laptop-based spectrum analyzer software in addition to the spectrum analysis adapter. The spectrum analysis adapter is a hardware device that captures the RF signals in the wireless environment and sends them to the spectrum analyzer software for analysis and display. The 802.11 wireless adapter is a hardware device that connects the laptop to the wireless network and allows the spectrum analyzer software to correlate the RF data with the Wi-Fi data, such as SSID, channel, and BSSID. This enables the spectrum analyzer software to provide more context and insight into the spectrum activity and its impact on the Wi-Fi network. A firmware upgrade for the spectrum analysis adapter is not required for Wi-Fi integration, but it may be needed to fix bugs or add features to the device. A directional antenna is an antenna that focuses the RF energy in a specific direction and has a high gain and a narrow beamwidth. A directional antenna can be used with a spectrum analysis adapter to pinpoint the location or source of interference or noise in the wireless environment, but it is not required for Wi-Fi integration. SNMP read credentials to the WLAN controller or APs are not required for Wi-Fi integration, but they may be useful for obtaining additional information about the wireless network configuration and performance from the network devices.References:
✑ CWAP-404 Study Guide, Chapter 4: Spectrum Analysis and Troubleshooting, page 123
✑ CWAP-404 Objectives, Section 4.2: Integrate Wi-Fi data with spectrum analysis data
✑ CWAP-404 Study Guide, Chapter 4: Spectrum Analysis and Troubleshooting, page 131
What is an AIFS?
Correct Answer:
B
An AIFS is a variable interframe space introduced by 802.11e to help prioritize medium access for different Access Categories (ACs). An interframe space is a period of time that a STA (station) has to wait before attempting to access the medium. An AIFS is a type of interframe space that varies depending on the AC of the traffic. An AC is a logical queue that corresponds to a QoS (Quality of Service) level for different types of traffic. There are four ACs defined by 802.11e: AC_VO (Voice), AC_VI (Video), AC_BE (Best Effort), and AC_BK (Background). Each AC has a different AIFSN (Arbitration Interframe Space Number) value, which determines how long it has to wait before attempting to access the medium. A lower AIFSN value means a higher priority and a shorter waiting time. The other options are not correct, as they do not describe what an AIFS is. An AIFS is not a medium access method introduced by 802.11n, but never implemented, as it is part of the 802.11e standard and widely used in QoS-enabled WLANs. An AIFS is not a form of aggregation performed at the PHY layer based on 802.11e UP values interpreted from DSCP values, as aggregation is a technique that combines multiple frames into one larger frame to improve efficiency and throughput, not prioritization or medium access. An AIFS is not the shortest period of time a STA can sleep, as sleeping is a power saving mode that allows a STA to conserve battery power by
periodically turning off its radio, not accessing the medium. References: [Wireless Analysis Professional Study Guide CWAP-404], Chapter 7: QoS Analysis, page 194-195
