CWS CWS MAC Layer Functions 2 — Questions and Answers
Question 1: What is the primary purpose of the Distributed Coordination Function (DCF) in 802.11 MAC?
- To assign fixed time slots to each station
- To provide contention-based channel access using CSMA/CA (Correct answer)
- To allow the AP to poll stations for data
- To prioritize voice traffic over data traffic
Correct answer: To provide contention-based channel access using CSMA/CA
DCF is the fundamental 802.11 MAC access method, using CSMA/CA so stations contend for the channel rather than being assigned fixed slots.
Question 2: In 802.11 CSMA/CA, a station must sense the medium idle for a DIFS period before transmitting. What does DIFS stand for?
- Distributed Inter-Frame Space (Correct answer)
- Dynamic Inter-Frame Spacing
- Data Interval Frame Sequence
- Deferred Inter-Frame Slot
Correct answer: Distributed Inter-Frame Space
DIFS (Distributed Inter-Frame Space) is the mandatory idle time a station must observe before initiating a new transmission under DCF.
Question 3: After detecting the channel is busy, a station invoking binary exponential backoff selects a random number from the contention window. What happens to the contention window size after each successive collision?
- It resets to its minimum value
- It is halved
- It doubles up to a defined maximum (Correct answer)
- It remains constant
Correct answer: It doubles up to a defined maximum
Binary exponential backoff doubles the contention window after each collision, up to a maximum CW value, to reduce the probability of repeated collisions.
Question 4: Which inter-frame space is the SHORTEST and is used to give priority to ACK and CTS frames?
- DIFS
- PIFS
- SIFS (Correct answer)
- AIFS
Correct answer: SIFS
SIFS (Short Inter-Frame Space) is the shortest IFS and is used for high-priority responses like ACKs and CTS frames, preventing other stations from interrupting an exchange.
Question 5: A station completes a successful data frame exchange. According to DCF, what must happen before the receiving station sends an ACK?
- Wait a DIFS period
- Wait a SIFS period (Correct answer)
- Run a backoff timer
- Send a CTS frame first
Correct answer: Wait a SIFS period
After receiving a unicast data frame, the recipient waits a SIFS before transmitting the ACK, ensuring the ACK gets channel priority over new transmissions.
Question 6: What mechanism does 802.11 MAC use to detect and discard duplicate frames caused by retransmissions?
- CRC check on the frame body
- The Sequence Control field and the Retry bit in the MAC header (Correct answer)
- Comparing source MAC addresses
- The Duration/ID field value
Correct answer: The Sequence Control field and the Retry bit in the MAC header
Receiving stations track the Sequence Control field and examine the Retry bit to identify and discard frames that are retransmissions of already-received frames.
Question 7: Which MAC frame type is responsible for managing 802.11 association and authentication between a client and an AP?
- Data frames
- Control frames
- Management frames (Correct answer)
- Extension frames
Correct answer: Management frames
Management frames handle high-level operations like authentication, association, deauthentication, and probe requests/responses between STAs and APs.
CWS CWS MAC Layer Functions 2
What is the primary purpose of the Distributed Coordination Function (DCF) in 802.11 MAC?