CWS CWS MAC Layer Functions 5 — Questions and Answers
Question 1: The Network Allocation Vector (NAV) is a virtual carrier-sense mechanism. Which event causes a station to SET its NAV timer?
- Successfully receiving an ACK frame
- Receiving a frame whose Duration field specifies a non-zero reservation time (Correct answer)
- Completing its own backoff countdown
- Detecting a physical signal above the Clear Channel Assessment threshold
Correct answer: Receiving a frame whose Duration field specifies a non-zero reservation time
When a station receives any 802.11 frame (even if not addressed to it) with a non-zero Duration field, it sets its NAV to that value, treating the medium as busy for virtual carrier sense.
Question 2: RTS/CTS protection is mandatory when the data frame size exceeds the RTS Threshold. What is the primary benefit of using RTS/CTS?
- It encrypts the data frame before transmission
- It mitigates the hidden node problem by reserving the medium before sending a large data frame (Correct answer)
- It eliminates the need for ACK frames
- It increases the data rate by pre-negotiating modulation
Correct answer: It mitigates the hidden node problem by reserving the medium before sending a large data frame
RTS/CTS allows the transmitter and receiver to exchange short reservation frames so that hidden nodes that may not hear each other will hear at least one of the exchange frames and defer.
Question 3: In 802.11ax (Wi-Fi 6), what MAC mechanism allows the AP to simultaneously trigger multiple stations to transmit on different resource units (RUs)?
- EDCA TXOP sharing
- Multi-User OFDMA with Trigger Frames (Correct answer)
- Block ACK compression
- MU-MIMO beamforming acknowledgment
Correct answer: Multi-User OFDMA with Trigger Frames
802.11ax introduces Trigger Frames that allow the AP to schedule uplink MU-OFDMA transmissions, letting multiple stations send simultaneously on assigned RUs.
Question 4: What is the EIFS (Extended Inter-Frame Space) and when is it used?
- A shorter IFS used after a management frame exchange
- A longer IFS a station must wait after receiving a corrupted frame before transmitting (Correct answer)
- The IFS used exclusively by the AP for polling
- A dynamic IFS value calculated from the current channel load
Correct answer: A longer IFS a station must wait after receiving a corrupted frame before transmitting
EIFS is a longer inter-frame space used by a station after it receives a frame with a CRC error, preventing it from interfering with an ongoing exchange it may have partially missed.
Question 5: Which MAC feature was introduced in 802.11n to allow back-to-back frame transmissions with a shorter gap than SIFS, improving aggregate throughput within a TXOP?
- Reduced Inter-Frame Space (RIFS) (Correct answer)
- Block ACK with immediate policy
- Spatial multiplexing
- Greenfield preamble
Correct answer: Reduced Inter-Frame Space (RIFS)
RIFS (Reduced Inter-Frame Space) at 2 µs is shorter than SIFS and was introduced in 802.11n to allow tighter back-to-back frame bursting within a single TXOP.
Question 6: A client's 802.11 MAC header contains a ToDS=1 and FromDS=0. What does this indicate about the frame's direction?
- The frame is moving from the DS toward a client (downlink)
- The frame is being sent from a client to the Distribution System through an AP (uplink) (Correct answer)
- The frame is a direct peer-to-peer IBSS frame
- The frame is a WDS (4-address) bridge frame
Correct answer: The frame is being sent from a client to the Distribution System through an AP (uplink)
ToDS=1 and FromDS=0 means the frame is traveling from the STA to the AP and onward into the Distribution System — a standard uplink infrastructure mode frame.
Question 7: In 802.11 MAC, what is the purpose of the MPDU Delimiter in A-MPDU aggregation?
- It carries the encryption IV for each sub-frame
- It provides length and CRC fields so a receiver can identify and parse individual MPDUs within the aggregate (Correct answer)
- It signals the end of the TXOP to neighboring stations
- It replaces the RTS/CTS exchange for aggregated frames
Correct answer: It provides length and CRC fields so a receiver can identify and parse individual MPDUs within the aggregate
Each MPDU in an A-MPDU is preceded by an MPDU Delimiter containing a length field and delimiter signature (0x4E), enabling the receiver to locate and independently decode each sub-frame.
CWS CWS MAC Layer Functions 5
The Network Allocation Vector (NAV) is a virtual carrier-sense mechanism.
Which event causes a station to SET its NAV timer?