QUESTION: 25

Given: The Frame Check Sequence (FCS) is a 32 CRC used for error detection.
The CRC is calculated over what?

Mac Header and Frame Body only
Frame Body only
PHY Header, MAC Header and Frame Body
PHY Header and Mac Header only

Answer(s): A

Explanation:

The CRC is calculated over the MAC Header and Frame Body only. The CRC (Cyclic Redundancy Check) is a 32-bit value that is used for error detection in wireless transmissions. The CRC is calculated over the MAC Header and Frame Body of a PSDU, which are the parts of the data unit that contain information such as source and destination addresses, frame type, frame control, sequence number, payload, etc. The CRC is appended to the end of the PSDU as a FCS (Frame Check Sequence) field. The CRC is not calculated over the PHY Header or PHY Preamble, which are parts of the PPDU that contain information such as modulation, coding, data rate, etc. The PHY Header and PHY Preamble are added or removed by the PHY layer during the conversion between PSDU and PPDU.

Reference:

[Wireless Analysis Professional Study Guide CWAP-404], Chapter 4: 802.11 Physical Layer, page 97-98

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QUESTION: 26

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

Supported Rates & Extended Supported Rates elements in a Beacon frame
In the MBR Action frame
In the MBR Information Element in an Association Response frame
In the Minimum Basic Rate Element in a Beacon frame

Answer(s): 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 and Extended 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.

Reference:

[Wireless Analysis Professional Study Guide CWAP-404], Chapter 5: 802.11 MAC Sublayer, page 123-124

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QUESTION: 27

After examining a Beacon frame decode you see the SSID Element has a length of 0.
What do you conclude about this frame?

The frame is corrupted
SSID elements always have a length of 0
This is a common attack on WISP backend SQL databases
The beacon is from a BSS configured to hide the SSID

Answer(s): D

Explanation:

If the SSID element has a length of 0 in a Beacon frame decode, it means that the beacon is from a BSS configured to hide the SSID. The SSID element is a part of the Beacon frame that contains the name or identifier of the BSS. The SSID element has two fields: length and value. The length field indicates how many bytes are used for the value field, which contains the actual SSID string. If the length field is 0, it means that there is no value field or SSID string in the element. This is a common technique used by some APs to hide their SSID from passive scanning clients or potential attackers. However, this technique does not provide much security, as there are other ways to discover or reveal the hidden SSID, such as active scanning or capturing probe response or association frames.

Reference:

[Wireless Analysis Professional Study Guide CWAP-404], Chapter 5: 802.11 MAC Sublayer, page 122-123

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QUESTION: 28

A client is operating in an unstable RF environment. Out of five data frames transmitted to the client it only receives four. The client sends a Block Ack to acknowledge the receipt of these four frames but due to frame corruption the Block Ack is not received by the AP.
Which frames will be retransmitted'

All data frames
Both the corrupted data and Block Ack
Only the data frame which was corrupted
Only the Block Ack

Answer(s): A

Explanation:

All data frames will be retransmitted in this scenario. This is because the AP uses a Block Ack (BA) mechanism to acknowledge the receipt of multiple data frames from a client in a single frame. The BA contains a bitmap that indicates which data frames were received correctly and which were not. If the BA is not received by the AP due to frame corruption, the AP will assume that none of the data frames were received by the client and will retransmit all of them. The other options are not correct, as they do not account for the loss of the BA or the use of the bitmap.

Reference:

[Wireless Analysis Professional Study Guide CWAP-404], Chapter 6: 802.11 Frame Exchanges, page 167-168

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