CWS CWS MAC Layer Functions 4 — Questions and Answers
Question 1: In legacy 802.11 Power Save Mode (PSM), how does a sleeping station learn that the AP has buffered frames waiting for it?
- The AP sends a unicast wake-up frame directly to the client
- The station checks the Traffic Indication Map (TIM) in the beacon (Correct answer)
- The station listens for a PS-Poll acknowledgment
- The AP broadcasts an ATIM frame
Correct answer: The station checks the Traffic Indication Map (TIM) in the beacon
The AP includes a TIM (Traffic Indication Map) in each beacon; a sleeping station wakes up periodically to read the beacon and checks whether its AID bit is set, indicating buffered frames.
Question 2: What is the function of an IBSS ATIM window in an ad-hoc (IBSS) network power-save operation?
- A period at the start of each beacon interval during which stations announce buffered frames to sleeping peers (Correct answer)
- A centralized polling window managed by the AP
- A QoS scheduling mechanism for video streams
- A time slot reserved for RTS/CTS exchanges only
Correct answer: A period at the start of each beacon interval during which stations announce buffered frames to sleeping peers
In an IBSS, the ATIM (Announcement Traffic Indication Message) window immediately follows each beacon; stations transmit ATIM frames to notify sleeping peers of buffered data.
Question 3: Frame aggregation in 802.11n/ac/ax can combine multiple MSDUs or MPDUs into a single transmission. Which type aggregates multiple MPDUs with a shared PHY header?
- A-MSDU
- A-MPDU (Correct answer)
- RIFS bursting
- Block Acknowledgment
Correct answer: A-MPDU
A-MPDU (Aggregated MAC Protocol Data Unit) combines multiple MPDUs under a single PHY preamble, significantly increasing throughput efficiency.
Question 4: What is the maximum A-MSDU size allowed in 802.11n when the 'Large A-MSDU' capability is negotiated?
- 3,839 bytes
- 7,935 bytes (Correct answer)
- 11,454 bytes
- 65,535 bytes
Correct answer: 7,935 bytes
When Large A-MSDU is negotiated in 802.11n, the maximum A-MSDU size is 7,935 bytes, compared to the default 3,839 bytes for standard A-MSDU.
Question 5: Block Acknowledgment (Block ACK) in 802.11 allows a recipient to acknowledge multiple frames at once. What initiates a Block ACK session?
- The recipient sends a Block ACK Request to the transmitter
- The transmitter sends an ADDBA (Add Block ACK) Request action frame (Correct answer)
- The AP includes the Block ACK bitmap in the TIM
- Both stations agree during the association handshake only
Correct answer: The transmitter sends an ADDBA (Add Block ACK) Request action frame
The transmitter initiates a Block ACK session by sending an ADDBA Request action frame; upon acceptance, the recipient sends an ADDBA Response.
Question 6: Which 802.11 MAC feature allows a transmitter to fragment large MSDUs into smaller MPDUs to improve reliability in noisy environments?
- A-MPDU aggregation
- MSDU fragmentation (Correct answer)
- RTS/CTS protection
- TXOP bursting
Correct answer: MSDU fragmentation
Fragmentation splits a large MSDU into multiple smaller MPDUs, each sent with its own ACK, reducing the probability that noise causes a complete retransmission of a large frame.
Question 7: In 802.11 MAC, what field in the frame header is used to reserve the channel medium for a specified duration during an RTS/CTS exchange?
- Sequence Control field
- Duration/ID field (Correct answer)
- Frame Control field
- QoS Control field
Correct answer: Duration/ID field
The Duration/ID field in both RTS and CTS frames specifies the time (in microseconds) the medium must be reserved, causing neighboring stations to update their NAV (Network Allocation Vector).
CWS CWS MAC Layer Functions 4
In legacy 802.11 Power Save Mode (PSM), how does a sleeping station learn that the AP has buffered frames waiting for it?