QUESTION: 21

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

LLC
Network
PHY
MAC

Answer(s): 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.

Reference:

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

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QUESTION: 22

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

SNR
Noise
Data rate
Signal strength

Answer(s): 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 strength are measured by the receiver based on its own capabilities and environment.

Reference:

[Wireless Analysis Professional Study Guide CWAP-404], Chapter 4: 802.11 Physical Layer, page 101-105

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QUESTION: 23

When performing protocol analysis, you capture an 802.1 lac data frame on channel 52, transmitted at MCS 8. At what data rate was the PHY Preamble transmitted?

54 Mbps
86.7 Mbps
6 Mbps
78 Mbps

Answer(s): C

Explanation:

The data rate at which the PHY preamble was transmitted is 6 Mbps. 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 detect and synchronize with the signal. The PHY

preamble is always transmitted at a fixed data rate that depends on the type of PPDU (e.g., OFDM, HT, VHT, HE). For an 802.1 lac data frame on channel 52, which uses VHT PPDUs, the data rate for the PHY preamble is 6 Mbps. This data rate does not depend on MCS (Modulation and Coding Scheme), which only affects the data rate for the PSDU.

Reference:

[Wireless Analysis Professional Study Guide CWAP-404], Chapter 4: 802.11 Physical Layer, page 99-100

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QUESTION: 24

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

Number of Spatial Streams
PPDU length
MCS index
Channel number

Answer(s): 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.

Reference:

[Wireless Analysis Professional Study Guide CWAP-404], Chapter 4: 802.11 Physical Layer, page 108-109

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