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Question 1802.11 Physical (PHY) Layer Frame Formats and Technologies

Which field of the 802.11 MAC header is present in all three frame types (management, control, and data)?

  1. Frame Control Correct answer.
  2. Duration/ID
  3. HT Control
  4. Sequence Control

Explanation

The Frame Control field is the first field of every 802.11 MAC header regardless of frame type. It carries the Protocol Version, Type, Subtype, and various flag bits that determine how the rest of the frame is interpreted. Fields such as Sequence Control and HT Control appear only in specific frame types or configurations.

Question 2802.11 Physical (PHY) Layer Frame Formats and Technologies

In the 802.11 Frame Control field, how many bits are allocated to the Type and Subtype subfields combined?

  1. 6 bits
  2. 12 bits
  3. 8 bits Correct answer.
  4. 4 bits

Explanation

The Type subfield occupies 2 bits and the Subtype subfield occupies 4 bits, giving 6 bits total for both. Together these identify whether a frame is management, control, or data and its specific variant. This 6-bit combination is what allows dozens of distinct frame subtypes to be distinguished.

Question 3802.11 Physical (PHY) Layer Frame Formats and Technologies

A station receives a frame whose Protocol Version bits are set to a value other than 0. What is the correct behavior?

  1. Process the frame normally using the existing header layout
  2. Discard the frame because the version is not supported Correct answer.
  3. Respond with a Protocol Version mismatch management frame
  4. Forward the frame to the distribution system for translation

Explanation

The Protocol Version subfield identifies the version of the 802.11 MAC protocol in use, and the standard defines version 0. A station that receives a frame with an unrecognized protocol version cannot reliably parse the remainder of the frame, so it discards it. There is no defined management exchange for negotiating protocol version at the MAC layer.

Question 4802.11 Physical (PHY) Layer Frame Formats and Technologies

Which 802.11 PHY is specified to operate in the 60 GHz band and uses channel bonding with widths up to 2.16 GHz?

  1. ERP-OFDM
  2. DMG Correct answer.
  3. VHT
  4. HR/DSSS

Explanation

DMG (Directional Multi-Gigabit), defined by 802.11ad, operates in the 60 GHz millimeter-wave band. It supports very wide channels, with a single channel being 2.16 GHz wide and bonding options beyond that. The other listed PHYs operate in 2.4 GHz or 5 GHz bands with much narrower maximum channel widths.

Question 5802.11 Physical (PHY) Layer Frame Formats and Technologies

An OFDM PHY uses a 20 MHz channel. How many data subcarriers does a legacy 802.11a/g OFDM symbol carry?

  1. 56
  2. 64
  3. 52
  4. 48 Correct answer.

Explanation

A 20 MHz legacy OFDM symbol uses a 64-point FFT, of which 52 subcarriers are used: 48 for data and 4 for pilots. The remaining subcarriers include the DC null and guard bands. The 48 data subcarriers are what carry the coded bits after modulation mapping.

Question 6802.11 Physical (PHY) Layer Frame Formats and Technologies

Which modulation and coding scheme provides the highest data rate in a single 20 MHz legacy 802.11a OFDM channel?

  1. 64-QAM with rate 3/4 coding Correct answer.
  2. BPSK with rate 1/2 coding
  3. QPSK with rate 3/4 coding
  4. 16-QAM with rate 3/4 coding

Explanation

Legacy 802.11a defines eight MCS entries, and the fastest is 64-QAM with rate 3/4 convolutional coding, yielding 54 Mbps in a 20 MHz channel. Higher-order QAM carries more bits per subcarrier, and the coding rate determines how many of those bits are information bits. 64-QAM with rate 3/4 is the top entry of the legacy table.

Question 7802.11 Physical (PHY) Layer Frame Formats and Technologies

What is the primary purpose of the Short Training Field (STF) at the start of an OFDM PPDU?

  1. Carry the SERVICE field and scrambler seed
  2. Provide channel estimation for coherent demodulation
  3. Enable packet detection, AGC, and coarse frequency offset estimation Correct answer.
  4. Carry the SIGNAL field with rate and length

Explanation

The STF consists of repeated short training symbols used by the receiver for signal detection, automatic gain control, and coarse frequency offset estimation. It does not carry data. Fine frequency offset and channel estimation are handled by the Long Training Field.

Question 8802.11 Physical (PHY) Layer Frame Formats and Technologies

In a legacy OFDM PPDU, which field tells the receiver the modulation and coding used for the DATA field?

  1. SERVICE field
  2. LENGTH field within the MAC header
  3. SIGNAL field Correct answer.
  4. Frame Check Sequence

Explanation

The SIGNAL field in the PLCP header carries the RATE (modulation and coding) and LENGTH values needed to decode the DATA field. It is transmitted at the most robust rate so all receivers can decode it. The SERVICE field instead carries scrambler initialization bits.

Question 9802.11 Physical (PHY) Layer Frame Formats and Technologies

Which 802.11 amendment introduced the 256-QAM modulation used to reach higher throughput in a 40 MHz channel?

  1. 802.11ax
  2. 802.11ac
  3. 802.11n Correct answer.
  4. 802.11g

Explanation

802.11n (HT) introduced 256-QAM as MCS 31, allowing higher throughput than the 64-QAM ceiling of legacy OFDM. It also introduced MIMO spatial streams and 40 MHz channels. Later amendments added 1024-QAM and wider channels.

Question 10802.11 Physical (PHY) Layer Frame Formats and Technologies

An HT station uses 4 spatial streams with 64-QAM rate 5/6 in a 20 MHz channel. Which factor most directly increases throughput compared with a single-stream configuration?

  1. Use of DSSS spreading
  2. Reduction of the FFT size
  3. Spatial multiplexing across multiple transmit/receive chains Correct answer.
  4. Use of a shorter guard interval only

Explanation

MIMO spatial multiplexing transmits independent data streams on separate spatial paths, multiplying throughput roughly by the number of spatial streams when channel conditions permit. The other options either do not apply to HT OFDM or provide only marginal gains. Spatial streams are the dominant throughput multiplier in HT and VHT.

Question 11802.11 Physical (PHY) Layer Frame Formats and Technologies

What is the duration of the guard interval used by a legacy 802.11a OFDM symbol?

  1. 3.2 microseconds
  2. 0.8 microseconds Correct answer.
  3. 0.4 microseconds
  4. 1.6 microseconds

Explanation

Legacy OFDM uses a 0.8 microsecond guard interval preceding each 3.2 microsecond useful symbol period. The guard interval is a cyclic extension that protects against inter-symbol interference from delay spread. HT introduced an optional 0.4 microsecond short guard interval.

Question 12802.11 Physical (PHY) Layer Frame Formats and Technologies

In an 802.11ac VHT PPDU, which field indicates the number of spatial streams and the MCS used?

  1. VHT-STF
  2. VHT-SIG-A Correct answer.
  3. L-SIG
  4. VHT-SIG-B

Explanation

VHT-SIG-A carries the channel width, number of spatial streams, MCS, and other parameters needed to decode the remainder of the PPDU. It is transmitted in a robust format so all VHT receivers can decode it. VHT-SIG-B instead carries per-user information for MU-MIMO.

Question 13802.11 Physical (PHY) Layer Frame Formats and Technologies

Which field in an 802.11 frame carries a value used by stations to set their Network Allocation Vector?

  1. Reason Code
  2. Capability Information
  3. Sequence Control
  4. Duration/ID Correct answer.

Explanation

The Duration/ID field carries a duration value that other stations use to update their NAV, implementing virtual carrier sense. In some control frames this field instead carries an association ID. The NAV prevents stations from transmitting during an expected exchange.

Question 14802.11 Physical (PHY) Layer Frame Formats and Technologies

A data frame is fragmented into three fragments. Which field allows the receiver to detect a duplicate fragment and reassemble in order?

  1. Power Management bit
  2. Protected Frame bit
  3. Order bit in Frame Control
  4. More Fragments bit and Fragment Number in Sequence Control Correct answer.

Explanation

The Sequence Control field contains a 4-bit Fragment Number and a 12-bit Sequence Number, and the More Fragments bit in Frame Control indicates whether more fragments follow. Together these let the receiver order fragments and detect duplicates. The other flags serve unrelated purposes.

Question 15802.11 Physical (PHY) Layer Frame Formats and Technologies

Which 802.11 control frame subtype is sent by a station to acknowledge receipt of a unicast data frame?

  1. ACK Correct answer.
  2. CTS
  3. Block Ack Request
  4. RTS

Explanation

The ACK control frame is a short frame sent by the recipient of a unicast frame to confirm successful reception. It contains only the RA field and FCS. RTS and CTS are used for medium reservation, and Block Ack Request initiates a block acknowledgment exchange.

Question 16802.11 Physical (PHY) Layer Frame Formats and Technologies

Which frame type is used by an access point to announce its SSID and supported rates to stations?

  1. Association Request
  2. Beacon Correct answer.
  3. Authentication
  4. Probe Request

Explanation

The Beacon is a management frame transmitted periodically by the AP that advertises the SSID, supported rates, capabilities, and other parameters. Stations use beacons to discover and evaluate networks. Probe Requests are sent by stations, not the AP.

Question 17802.11 Physical (PHY) Layer Frame Formats and Technologies

What is the primary function of the PLCP preamble in a DSSS PHY transmission?

  1. Provide synchronization and signal detection for the receiver Correct answer.
  2. Encode the SSID
  3. Carry the MAC frame body
  4. Encrypt the payload

Explanation

The PLCP preamble in DSSS allows the receiver to detect the incoming signal and synchronize to it before decoding the PLCP header. It is transmitted at a fixed 1 Mbps rate for the long preamble. Payload and encryption are handled at higher layers.

Question 18802.11 Physical (PHY) Layer Frame Formats and Technologies

Which modulation does the 802.11b HR/DSSS PHY use to achieve 11 Mbps?

  1. 64-QAM
  2. QPSK
  3. CCK Correct answer.
  4. BPSK

Explanation

HR/DSSS uses Complementary Code Keying (CCK) at 5.5 and 11 Mbps, providing higher throughput than the earlier Barker-based 1 and 2 Mbps rates. CCK encodes multiple bits per symbol using a set of complementary code words. 64-QAM is an OFDM modulation not used by 802.11b.

Question 19802.11 Physical (PHY) Layer Frame Formats and Technologies

Which frequency band and channel width is used by the 802.11a PHY?

  1. 5 GHz with 20 MHz channels Correct answer.
  2. 60 GHz with 2.16 GHz channels
  3. 5 GHz with 40 MHz channels
  4. 2.4 GHz with 20 MHz channels

Explanation

802.11a operates in the 5 GHz band using OFDM with 20 MHz channels. It does not support 40 MHz channels, which were introduced later by 802.11n. 802.11a offers up to 54 Mbps per channel.

Question 20802.11 Physical (PHY) Layer Frame Formats and Technologies

How does the 802.11n HT preamble provide backward compatibility with legacy OFDM devices?

  1. It uses a separate 2.4 GHz-only control channel
  2. It begins with L-STF, L-LTF, and L-SIG that legacy devices can decode Correct answer.
  3. It relies on DSSS spreading for legacy detection
  4. It omits the legacy preamble entirely

Explanation

The HT-mixed format begins with legacy training fields and an L-SIG field so that legacy stations can detect the transmission and set their NAV correctly from the L-SIG duration. The HT-specific fields follow, which legacy devices ignore. This coexistence mechanism allows HT and legacy devices to share a channel.

Question 21802.11 Physical (PHY) Layer Frame Formats and Technologies

Which parameter determines the maximum theoretical data rate of a single spatial stream in an 802.11ac 80 MHz channel?

  1. Beacon interval
  2. SSID length
  3. MCS, coding rate, and symbol rate including guard interval Correct answer.
  4. Number of pilot subcarriers only

Explanation

The data rate is computed from the number of data subcarriers, bits per subcarrier from the MCS, the coding rate, and the symbol rate determined by the guard interval. For VHT, 80 MHz provides 234 data subcarriers. SSID and beacon interval are MAC parameters unrelated to PHY rate.

Question 22802.11 Physical (PHY) Layer Frame Formats and Technologies

An 802.11ax HE PPDU uses 1024-QAM. Which condition is required for this MCS to be usable?

  1. A DSSS-compatible preamble
  2. A single spatial stream only
  3. A very high SNR channel with sufficient signal quality Correct answer.
  4. A 2.4 GHz-only channel

Explanation

1024-QAM encodes 10 bits per subcarrier and requires very high signal-to-noise ratio to demodulate reliably. It is therefore only usable in favorable channel conditions, typically close to the AP. HE also supports multiple spatial streams and both bands.

Question 23802.11 Physical (PHY) Layer Frame Formats and Technologies

Which field in the 802.11 MAC header indicates whether the frame body is encrypted?

  1. More Data bit
  2. To DS bit
  3. Retry bit
  4. Protected Frame bit Correct answer.

Explanation

The Protected Frame bit in the Frame Control field indicates that the frame body is protected by a security mechanism such as WPA2 or WPA3. It allows receivers to know whether decryption is needed. The other bits control retransmission, buffered data, and distribution system routing.

Question 24802.11 Physical (PHY) Layer Frame Formats and Technologies

A VHT station transmits an 80 MHz PPDU. How many data subcarriers does the 80 MHz VHT configuration provide?

  1. 108
  2. 52
  3. 468
  4. 234 Correct answer.

Explanation

An 80 MHz VHT channel uses a 256-point FFT per 80 MHz segment with 234 data subcarriers and 8 pilot subcarriers. The 234 figure is used when computing the maximum rate for 80 MHz. Wider 160 MHz configurations use two such segments.

Question 25802.11 Physical (PHY) Layer Frame Formats and Technologies

Which aspect of the PHY is described by the term 'channel bonding' as used in 802.11n and later amendments?

  1. Combining two adjacent 20 MHz channels into a 40 MHz channel Correct answer.
  2. Bonding multiple antennas to a single radio chain
  3. Combining multiple MAC frames into one A-MPDU
  4. Merging multiple SSIDs into one BSS

Explanation

Channel bonding combines two adjacent 20 MHz channels into a single 40 MHz channel, doubling the available subcarriers and thus the maximum data rate. It was introduced in 802.11n and extended to 80 and 160 MHz in 802.11ac. The other options describe unrelated concepts.

Question 26802.11 Physical (PHY) Layer Frame Formats and Technologies

An 802.11a OFDM PPDU is being decoded. Which field of the PLCP header is used by the receiver to determine the number of OFDM symbols in the DATA field?

  1. The SERVICE field, which contains the scrambler seed
  2. The LENGTH field, which is expressed in octets of the PSDU Correct answer.
  3. The RATE field, which encodes the modulation and coding scheme
  4. The LENGTH field, which is expressed in microseconds for the PSDU

Explanation

In the 802.11a OFDM PLCP header, the LENGTH field is a 12-bit value that gives the number of octets in the PSDU. The receiver divides this by the data rate indicated by the RATE field to compute the transmission duration and the number of OFDM symbols. The SERVICE field carries scrambler initialization, not length information.

Question 27802.11 Physical (PHY) Layer Frame Formats and Technologies

Which statement accurately describes the channel spacing and occupied bandwidth of an 802.11a OFDM channel?

  1. Channels are spaced 20 MHz apart and occupy approximately 20 MHz of occupied bandwidth
  2. Channels are spaced 20 MHz apart and occupy approximately 16.6 MHz of occupied bandwidth Correct answer.
  3. Channels are spaced 25 MHz apart and occupy approximately 20 MHz of occupied bandwidth
  4. Channels are spaced 10 MHz apart and occupy approximately 8.3 MHz of occupied bandwidth

Explanation

An 802.11a channel is defined with 20 MHz channel spacing, but the OFDM signal uses 52 subcarriers (48 data, 4 pilot) with 312.5 kHz spacing, giving an occupied bandwidth of about 16.6 MHz. The remaining spectrum provides guard bands between adjacent channels. This distinction matters when analyzing spectral masks and adjacent channel interference.

Question 28802.11 Physical (PHY) Layer Frame Formats and Technologies

A wireless engineer is analyzing an 802.11n HT PPDU capture. Which field in the HT-SIG identifies whether the PPDU uses 20 MHz or 40 MHz?

  1. The STBC field in HT-SIG
  2. The Channel Width bit in HT-SIG Correct answer.
  3. The MCS field in HT-SIG
  4. The Short GI bit in HT-SIG

Explanation

HT-SIG contains a Channel Width bit that indicates whether the HT PPDU occupies a 20 MHz or 40 MHz channel. The MCS field defines modulation and coding, the STBC field indicates space-time block coding streams, and the Short GI bit selects the guard interval length. These are separate subfields within HT-SIG.

Question 29802.11 Physical (PHY) Layer Frame Formats and Technologies

How does an 802.11ac VHT PPDU signal to a legacy 802.11a station that the channel is occupied for the duration of the VHT transmission?

  1. By sending a beacon frame with a TIM element that includes the VHT duration
  2. By using a VHT-SIG-A field that legacy stations can decode to set their NAV
  3. By including a legacy OFDM L-SIG field with a LENGTH value that causes the legacy station to defer for the correct duration Correct answer.
  4. By transmitting a separate RTS/CTS exchange before every VHT PPDU

Explanation

VHT PPDUs begin with a legacy-compatible preamble that includes L-STF, L-LTF, and L-SIG. The L-SIG LENGTH and RATE values are set so that a legacy OFDM station computes a duration covering the entire VHT PPDU, thereby deferring access. VHT-SIG-A is not decodable by legacy stations, so it cannot be used for this purpose.

Question 30802.11 Physical (PHY) Layer Frame Formats and Technologies

Which modulation and coding scheme is used by the 802.11b HR/DSSS PHY to achieve a 5.5 Mbps data rate?

  1. PBCC with 8PSK at 1.375 Msymbol/s
  2. CCK with 4 bits per symbol at a 1.375 Msymbol/s rate Correct answer.
  3. DQPSK at 1.375 Msymbol/s with a 1/2 rate convolutional code
  4. BPSK at 1.375 Msymbol/s with no coding

Explanation

The 5.5 Mbps HR/DSSS mode uses Complementary Code Keying (CCK) to carry 4 bits per symbol at a chip rate that yields 1.375 Msymbol/s. The 11 Mbps mode uses CCK with 8 bits per symbol. DQPSK and BPSK are used in the lower-rate 802.11b modes, and PBCC is an optional modulation.

Question 31802.11 Physical (PHY) Layer Frame Formats and Technologies

An 802.11ax HE PPDU is captured. Which field in HE-SIG-A indicates whether the PPDU is a single-user or multi-user transmission?

  1. The SU/MU bit Correct answer.
  2. The UL/DL bit
  3. The Coding bit
  4. The LDPC extra symbol segment bit

Explanation

HE-SIG-A contains a SU/MU bit that distinguishes single-user from multi-user PPDUs. The UL/DL bit indicates direction, the Coding bit selects BCC or LDPC, and the LDPC extra symbol segment bit signals an additional OFDM symbol for LDPC. These are distinct subfields.

Question 32802.11 Physical (PHY) Layer Frame Formats and Technologies

What is the primary role of the Long Training Field (LTF) in a legacy 802.11a OFDM PPDU?

  1. To provide channel estimation and fine frequency offset correction Correct answer.
  2. To signal the modulation and coding rate
  3. To indicate the length of the PSDU
  4. To scramble the data bits before modulation

Explanation

The LTF consists of two long training symbols that allow the receiver to estimate the channel response per subcarrier and to perform fine frequency offset correction. The LENGTH field is in the SIGNAL field, scrambling is handled by the SERVICE field seed, and modulation is indicated by the RATE field. The LTF is therefore essential for coherent demodulation.

Question 33802.11 Physical (PHY) Layer Frame Formats and Technologies

In an 802.11ac VHT PPDU, which field conveys the total number of space-time streams and the MCS for a single-user transmission?

  1. VHT-SIG-A Correct answer.
  2. VHT-SIG-B
  3. The SERVICE field
  4. L-SIG

Explanation

For a single-user VHT PPDU, VHT-SIG-A contains the MCS, the number of space-time streams, and other parameters needed to decode the data. VHT-SIG-B carries user-specific information mainly for multi-user transmissions. L-SIG provides legacy duration information, and the SERVICE field holds scrambler initialization.

Question 34802.11 Physical (PHY) Layer Frame Formats and Technologies

A VHT 80 MHz PPDU is analyzed. How many data subcarriers are available per OFDM symbol in this configuration?

  1. 52
  2. 242
  3. 108
  4. 234 Correct answer.

Explanation

An 80 MHz VHT channel uses a 256-point FFT with 234 data subcarriers and 8 pilot subcarriers per OFDM symbol. The 242 value corresponds to the 20 MHz HT/VHT configuration, 108 to 40 MHz, and 52 to legacy 20 MHz OFDM. These subcarrier counts directly affect throughput calculations.

Question 35802.11 Physical (PHY) Layer Frame Formats and Technologies

Which guard interval duration is mandatory for 802.11n HT OFDM transmissions?

  1. 800 ns Correct answer.
  2. 400 ns
  3. 1600 ns
  4. 3200 ns

Explanation

802.11n defines a mandatory 800 ns guard interval for HT OFDM symbols, with an optional short guard interval of 400 ns. The longer 1600 ns and 3200 ns values are associated with later amendments such as 802.11ax for outdoor or extended range operation. The guard interval mitigates inter-symbol interference.

Question 36802.11 Physical (PHY) Layer Frame Formats and Technologies

An 802.11ax HE PPDU uses 1024-QAM. Which condition must be satisfied for this MCS to be usable?

  1. The channel must be bonded to 160 MHz
  2. The station must support the optional 1024-QAM capability and the received signal quality must be sufficient Correct answer.
  3. The PPDU must be a multi-user transmission
  4. The station must use at least four spatial streams

Explanation

1024-QAM in 802.11ax is an optional capability that a station must advertise, and it requires a high signal-to-noise ratio to be decoded reliably. It can be used in single-user or multi-user PPDUs and does not require a specific channel width or number of spatial streams. The MCS index and modulation are selected based on channel conditions.

Question 37802.11 Physical (PHY) Layer Frame Formats and Technologies

Which statement best describes the purpose of the SIGNAL field in an 802.11a OFDM PPDU?

  1. It performs channel estimation for the receiver
  2. It contains the MAC header of the frame
  3. It carries the PSDU data bits
  4. It provides the receiver with the rate and length needed to decode the DATA field Correct answer.

Explanation

The SIGNAL field in the 802.11a PLCP header contains the RATE and LENGTH fields, which tell the receiver the modulation/coding and the number of octets in the PSDU. This information is required to demodulate and decode the DATA field. The PSDU data, MAC header, and channel estimation are carried or performed elsewhere.

Question 38802.11 Physical (PHY) Layer Frame Formats and Technologies

A network administrator observes an 802.11n HT PPDU with a 40 MHz channel width. Which statement about the subcarrier allocation is correct?

  1. It uses 234 data subcarriers and 8 pilot subcarriers per OFDM symbol
  2. It uses 108 data subcarriers and 6 pilot subcarriers per OFDM symbol Correct answer.
  3. It uses 114 data subcarriers and 6 pilot subcarriers per OFDM symbol
  4. It uses 52 data subcarriers and 4 pilot subcarriers per OFDM symbol

Explanation

A 40 MHz HT channel uses a 128-point FFT with 108 data subcarriers and 6 pilot subcarriers per OFDM symbol. The 52/4 combination is for 20 MHz legacy OFDM, and 234/8 is for 80 MHz VHT. The 114 value is not a standard HT allocation.

Question 39802.11 Physical (PHY) Layer Frame Formats and Technologies

Which field in an 802.11ac VHT PPDU is used to indicate the modulation and coding scheme for a multi-user transmission?

  1. VHT-SIG-A
  2. L-SIG
  3. VHT-SIG-B Correct answer.
  4. HT-SIG

Explanation

In a multi-user VHT PPDU, VHT-SIG-B carries per-user information including the MCS and the number of space-time streams for each user. VHT-SIG-A contains common parameters such as channel width and group ID. L-SIG is legacy, and HT-SIG is not present in a VHT PPDU.

Question 40802.11 Physical (PHY) Layer Frame Formats and Technologies

What is the effect of using a 400 ns short guard interval in an 802.11n HT PPDU?

  1. It changes the modulation from OFDM to DSSS
  2. It doubles the number of data subcarriers
  3. It increases the OFDM symbol duration, reducing throughput
  4. It reduces the OFDM symbol duration, increasing throughput but requiring better channel conditions Correct answer.

Explanation

The short guard interval reduces the guard time from 800 ns to 400 ns, which shortens the total OFDM symbol duration and thus increases the data rate for the same modulation and coding. However, it provides less protection against delay spread, so it is only effective in environments with low intersymbol interference. It does not alter the number of subcarriers or the modulation type.

Question 41802.11 MAC Layer Frame Formats and Technologies

Which field in the 802.11 MAC header immediately follows the Frame Control field?

  1. Sequence Control
  2. Address 1
  3. FCS
  4. Duration/ID Correct answer.

Explanation

The MAC header begins with the two-octet Frame Control field, and the next two octets are the Duration/ID field. It carries either a NAV duration value, an association identifier in PS-Poll frames, or a fixed value depending on frame type. Address fields come after it.

Question 42802.11 MAC Layer Frame Formats and Technologies

How many octets long is the Frame Control field of an 802.11 MAC header?

  1. 6
  2. 1
  3. 4
  4. 2 Correct answer.

Explanation

The Frame Control field is exactly two octets. It packs the Protocol Version, Type, Subtype, To DS, From DS, More Fragments, Retry, Power Management, More Data, Protected Frame, and Order subfields into 16 bits.

Question 43802.11 MAC Layer Frame Formats and Technologies

A frame has the To DS bit set to 1 and the From DS bit set to 0. How many address fields does its MAC header contain?

  1. Two
  2. Four
  3. Five
  4. Three Correct answer.

Explanation

A frame traveling from a wireless station toward the distribution system uses three addresses: Address 1 is the BSSID, Address 2 is the source station, and Address 3 is the destination. Four addresses appear only when both DS bits are set.

Question 44802.11 MAC Layer Frame Formats and Technologies

In a data frame where both To DS and From DS are set to 1, which address is the transmitter address (TA)?

  1. Address 3
  2. Address 4
  3. Address 2 Correct answer.
  4. Address 1

Explanation

When both DS bits are 1, the frame traverses a wireless distribution system, so four addresses are used. Address 2 always carries the transmitter address, while Address 1 is the receiver address and Address 4 is the source address.

Question 45802.11 MAC Layer Frame Formats and Technologies

Which frame type is identified by a Type value of 00 in the Frame Control field?

  1. Management Correct answer.
  2. Control
  3. Data
  4. Extension

Explanation

The two-bit Type subfield encodes 00 for management, 01 for control, 10 for data, and 11 for extension frames. Management frames include beacons, probes, and authentication frames.

Question 46802.11 MAC Layer Frame Formats and Technologies

What is the length of the Frame Check Sequence appended to every 802.11 frame?

  1. 4 octets Correct answer.
  2. 2 octets
  3. 8 octets
  4. 3 octets

Explanation

The FCS is a 32-bit (four-octet) CRC calculated over the frame header and body. Receivers recompute it and silently discard frames that fail the check.

Question 47802.11 MAC Layer Frame Formats and Technologies

Which subfield of the Sequence Control field increments for each new MSDU sent to a given receiver?

  1. Order bit
  2. Retry bit
  3. Sequence Number Correct answer.
  4. Fragment Number

Explanation

Sequence Control is 16 bits split into a 12-bit Sequence Number and a 4-bit Fragment Number. The Sequence Number increments for each new MSDU, while the Fragment Number starts at zero and increments per fragment.

Question 48802.11 MAC Layer Frame Formats and Technologies

How many bits are allocated to the Fragment Number within the Sequence Control field?

  1. 12
  2. 16
  3. 4 Correct answer.
  4. 8

Explanation

Sequence Control totals 16 bits: 12 bits for the Sequence Number and 4 bits for the Fragment Number. That limits fragmentation to 16 fragments per MSDU.

Question 49802.11 MAC Layer Frame Formats and Technologies

Which information element is always present in a Beacon frame and identifies the network to scanning stations?

  1. TIM element
  2. Country element
  3. SSID element Correct answer.
  4. Supported Rates element

Explanation

The SSID element is mandatory in beacons and carries the network name, or zero length for a hidden SSID. Supported Rates is also typically present but the SSID is the defining identifier for scanners.

Question 50802.11 MAC Layer Frame Formats and Technologies

Which frame does a station transmit to announce it is entering power-save mode?

  1. A Null Data frame with the More Data bit set
  2. A PS-Poll frame
  3. A Control frame with the Retry bit set
  4. A Data frame with the Power Management bit set to 1 Correct answer.

Explanation

A station signals a change into power-save mode by setting the Power Management bit to 1 in the Frame Control field of a frame it transmits. The AP then buffers frames and indicates them through the TIM.