CWNP CWNP Certification CWAP-404 Questions & Answers

• Question 1:

  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?

  A. Ask a local member of staff to change the frequency of the microwave and see if the disconnections stop

  B. Ask a local member of staff to take some pictures of the microwave, including some close-ups of the door seal so that you can assess it

  C. Access the microwave remotely and run a diagnostic check

  D. Place one of the London APs into spectrum analyzer mode and monitor the situation over lunch time

  Correct Answer: D

  Explanation: 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

• Question 2:

  How many frames are exchanged for 802.11 authentication in the 6 GHz band when WPA3-Enterprise is not used, and a passphrase is used instead?

  A. 1

  B. 2

  C. 3

  D. 4

  Correct Answer: B

  Explanation: Two frames are exchanged for 802.11 authentication in the 6 GHz band when WPA3-Enterprise is not used, and a passphrase is used instead. Authentication is a process that establishes an identity relationship between a STA (station) and an AP (access point) before joining a BSS (Basic Service Set). There are two types of authentication methods defined by 802.11: Open System Authentication and Shared Key Authentication. Open System Authentication does not require any credentials or security information from a STA to join a BSS, and it consists of two frames: an Authentication Request frame sent by the STA to the AP, and an Authentication Response frame sent by the AP to the STA. Shared Key Authentication requires a shared secret key from a STA to join a BSS, and it consists of four frames: two challenge-response frames in addition to the request-response frames. However, Shared Key Authentication uses WEP (Wired Equivalent Privacy) as its encryption algorithm, which is insecure and deprecated. In the 6 GHz band, which is a newly available frequency band for WLANs, Shared Key Authentication is prohibited by the 802.11 standard, as it poses security and interference risks for other users and services in the band. The 6 GHz band requires all WLANs to use WPA3-Personal or WPA3-Enterprise encryption methods, which are more secure and robust than previous encryption methods such as WPA2 or WEP. WPA3-Personal uses a passphrase to derive a PMK (Pairwise Master Key), while WPA3-Enterprise uses an authentication server to obtain a PMK. Both methods use SAE (Simultaneous Authentication of Equals) as their authentication protocol, which replaces PSK (Pre-Shared Key) or EAP (Extensible Authentication Protocol). SAE consists of two frames: an SAE Commit frame sent by both parties to exchange elliptic curve parameters and nonces, and an SAE Confirm frame sent by both parties to verify each other's identities and generate a PMK. Therefore, when WPA3-Enterprise is not used, and a passphrase is used instead in the 6 GHz band, only two frames are exchanged for 802.11 authentication: an SAECommit frame and an SAE Confirm frame. References: [Wireless Analysis Professional Study Guide CWAP-404], Chapter 8: Security Analysis, page 220-221

• Question 3:

  Prior to a retransmission what happens to the CWmax value?

  A. Increases by 1

  B. Reset to 0

  C. Set to the value of the AIFSN

  D. Doubles and increases by 1

  Correct Answer: D

  Explanation: Before a retransmission, the CWmax (Contention Window maximum) value doubles and increases by 1. The CWmax is a parameter that determines the upper limit of the random backoff time that a STA (station) has to wait before attempting to access the medium. The random backoff time is chosen from a range of values between CWmin (Contention Window minimum) and CWmax. The CWmin and CWmax values depend on the AC (Access Category) of the traffic and the PHY type of the STA. If a transmission fails due to a collision or an error, the STA has to retransmit the frame after waiting for another random backoff time. However, to reduce the probability of another collision, the STA increases its CWmax value by doubling it and adding 1. This increases the range of possible backoff values and spreads out the STAs more evenly. The STA resets its CWmax value to its original value after a successful transmission or after reaching a predefined limit. References: [Wireless Analysis Professional Study Guide CWAP-404], Chapter 7: QoS Analysis, page 196-197

• Question 4:

  Using a portable analyzer you perform a packet capture next to a client STA and you can see that the STA is associated to a BSS. You observe the STA sending packets to the AP and the AP sending packets to the STA. Less than 2% of all packets are retransmissions. You move to capture packets by the AP and, while the retry rate is still less than 2%, you now only see unidirectional traffic from the AP to the client. How do you explain this behavior?

  A. The portable analyzer is too close to the AP causing CCI, blinding the AP to the clients packets

  B. The STA is transmitting data using more spatial streams than the potable analyzer can support

  C. There is a transmit power mismatch between the client and the AP and while the client can hear the APs traffic, the AP cannot hear the client

  D. The portable analyzer has a lower receive sensitivity than the AP and while it can't capture the packets from the client STA, the AP can receive them OK

  Correct Answer: D

  Explanation: Receive sensitivity is the minimum signal level that a receiver can detect and decode. Different devices may have different receive sensitivity levels depending on their hardware specifications and antenna configurations. In this scenario, the portable analyzer has a lower receive sensitivity than the AP, meaning that it requires a stronger signal to capture the packets from the client STA. The AP, on the other hand, has a higher receive sensitivity and can receive the packets from the client STA even if they have a weaker signal. This explains why the portable analyzer can only see unidirectional traffic from the AP to the client when capturing near the AP5 References: CWAP-403 Study Guide, Chapter 4: PHY Layer Analysis, page 121 CWAP-403 Objectives, Section 4.3: Analyze PHY layer metrics

• Question 5:

  Where would you look in a packet trace file to identify the configured Minimum Basic Rate (MBR) of a BSS?

  A. Supported Rates and Extended Supported Rates elements in a Beacon frame

  B. In the MBR Action frame

  C. In the MBR Information Element in an Association Response frame

  D. In the Minimum Basic Rate Element in a Beacon frame

  Correct Answer: A

  Explanation: The configured Minimum Basic Rate (MBR) of a BSS can be identified by looking at the Supported Rates and Extended Supported Rates elements in a Beacon frame. A Beacon frame is a type of management frame that is transmitted by an AP to advertise its presence and capabilities to potential clients. A Beacon frame contains various information elements (IEs) that provide details about the BSS configuration and operation. The Supported Rates andExtended Supported Rates IEs list the data rates that are supported by the AP for data transmission. The MBR is the lowest data rate among these supported rates that is required for all clients to join and communicate with the BSS. The MBR is usually marked with a flag bit in these IEs to indicate its mandatory status. The other options are not correct, as they do not exist or do not indicate the MBR of a BSS. References: [Wireless Analysis Professional Study Guide CWAP404], Chapter 5:

  802.11 MAC Sublayer, page 123-124

• Question 6:

  Which one of the following portions of information is communicated by bits in the PHY Header?

  A. SNR

  B. Noise

  C. Data rate

  D. Signal strength

  Correct Answer: C

  Explanation: One of the information that is communicated by bits in the PHY header is data rate. Data rate is the speed at which data is transmitted or received over the wireless medium. Data rate depends on factors such as modulation, coding, channel width, spatial streams, and guard interval. Data rate is indicated by bits in different fields of the PHY header, depending on the type of PPDU (e.g., OFDM, HT, VHT, HE). The receiver uses these bits to determine how to decode and demodulate the rest of the PPDU. The other options are not correct, as they are not communicated by bits in the PHY header. SNR (Signal-to-Noise Ratio), noise, and signal strengthare measured by the receiver based on its own capabilities and environment. References: [Wireless Analysis Professional Study Guide CWAP-404], Chapter 4: 802.11 Physical Layer, page 101-105

• Question 7:

  What does the value of the Listen Interval field in an Association Request frame indicate?

  A. How long a STA performing active scanning will listen for Probe Responses before changing channels

  B. How often a STA will go off channel to look for other BSSs

  C. How often a STA in power save mode wakes up to listen to Beacon frames

  D. How long a STA waits for an Ack before retransmitting the frame

  Correct Answer: C

  Explanation: 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.

• Question 8:

  A PHY Header is added to the PSDU at which layer?

  A. LLC

  B. Network

  C. PHY

  D. MAC

  Correct Answer: C

  Explanation: A PHY header is added to the PSDU at the PHY layer. A PHY header is a part of the PPDU that contains information such as modulation, coding, and data rate. The PHY header is added by the PHY layer when it converts a PSDU to a PPDU for transmission, or removed by the PHY layer when it converts a PPDU to a PSDU for reception. The other layers do not add or remove a PHY header. References: [Wireless Analysis Professional Study Guide CWAP-404], Chapter 4: 802.11 Physical Layer, page 97-98

• Question 9:

  Which piece of information is not transmitted in an HT PPDU header?

  A. Number of Spatial Streams

  B. PPDU length

  C. MCS index

  D. Channel number

  Correct Answer: D

  Explanation: The channel number is not transmitted in an HT PPDU header. An HT PPDU header is a part of the PPDU that contains information such as modulation, coding, data rate, and number of spatial streams for an 802.11n transmission. The channel number is not included in the HT PPDU header, as it is determined by the frequency band and channel width that are used by the transmitter and receiver. The channel number can be inferred from the frequency band and channel width, which are indicated by bits in different fields of the HT PPDU header, such as HT-SIG and HT-LTF. The other options are not correct, as they are transmitted in an HT PPDU header. The number of spatial streams, PPDU length, and MCS index are indicated by bits in the HT-SIG field of the HT PPDU header. References: [Wireless Analysis Professional Study Guide CWAP-404], Chapter 4:

  802.11 Physical Layer, page 108-109

• Question 10:

  ABC International has installed a new smart ZigBee controlled lighting system. However, the network team is concerned that this new system will interfere with the existing WLAN and has asked you to investigate the impact of the two systems operating simultaneously in the 2.4 GHz band. When performing Spectrum Analysis, which question could you answer by looking at the FFT plot?

  A. Do the ZigBee channels used by the lighting system overlap with the WLAN channels?

  B. Is the ZigBee system using more than 50% of the available airtime?

  C. Is the WLAN corrupting ZigBee system messages?

  D. Is the ZigBee system causing an increase in WLAN retries?

  Correct Answer: A

  Explanation: The FFT plot is a spectrum analysis plot that shows the RF power present at a particular frequency over a short period of time. It can help identify the sources and characteristics of RF signals in the spectrum. By looking at the FFT plot, you can determine which ZigBee channels are used by the lighting system and whether they overlap with the WLAN channels in the 2.4 GHz band. ZigBee channels are 5 MHz wide and WLAN channels are 20 MHz or 40 MHz wide, so there is a possibility of overlap and interference between them. The other questions cannot be answered by looking at the FFT plot alone, as they require other types of plots or analysis tools, such as duty cycle plot, airtime utilization plot, or protocol analyzer. References: [Wireless Analysis Professional Study Guide], Chapter 3: Spectrum Analysis, page 69-70