CWS CWS MAC Layer Functions 3 — Questions and Answers
Question 1: WMM (Wi-Fi Multimedia) defines four Access Categories (ACs). Which AC has the HIGHEST priority?
- AC_BE (Best Effort)
- AC_BK (Background)
- AC_VI (Video)
- AC_VO (Voice) (Correct answer)
Correct answer: AC_VO (Voice)
AC_VO (Voice) has the highest priority under WMM, using the smallest AIFS and contention window values to gain channel access fastest.
Question 2: In 802.11e/WMM, what replaces the standard DIFS as the mandatory idle time before a station transmits in a QoS-enabled network?
- SIFS
- PIFS
- AIFS (Arbitration Inter-Frame Space) (Correct answer)
- EIFS
Correct answer: AIFS (Arbitration Inter-Frame Space)
AIFS (Arbitration Inter-Frame Space) replaces DIFS in EDCA; each Access Category has its own AIFS value, with higher-priority ACs using shorter AIFS values.
Question 3: What is a TXOP (Transmission Opportunity) in 802.11 QoS?
- A token a station must acquire from the AP before transmitting
- A bounded time interval during which a station may send multiple frames (Correct answer)
- The process of reserving the channel using RTS/CTS
- A power-save polling mechanism
Correct answer: A bounded time interval during which a station may send multiple frames
A TXOP grants a station a bounded time window in which it can send multiple frames without re-contending for the channel after each frame.
Question 4: EDCA (Enhanced Distributed Channel Access) is part of which 802.11 amendment?
- 802.11a
- 802.11e (Correct answer)
- 802.11n
- 802.11ac
Correct answer: 802.11e
802.11e introduced EDCA and the full QoS framework (HCF), which WMM is based upon.
Question 5: Which WMM Access Category maps to the 802.1p priority values 0 and 3 (best effort and video)?
- AC_BK
- AC_BE and AC_VI respectively (Correct answer)
- AC_VO
- AC_VI and AC_VO respectively
Correct answer: AC_BE and AC_VI respectively
802.1p UP 0 maps to AC_BE (Best Effort) and UP 3 maps to AC_VI (Video) in the WMM Access Category mapping table.
Question 6: In EDCA, what parameter defines the minimum and maximum size of the random backoff contention window for each Access Category?
- TXOP Limit
- CWmin and CWmax (Correct answer)
- AIFS Number (AIFSN)
- MSDU Lifetime
Correct answer: CWmin and CWmax
Each AC has its own CWmin and CWmax parameters; higher-priority ACs use smaller values, making them statistically win channel access more often.
Question 7: U-APSD (Unscheduled Automatic Power Save Delivery) is a QoS-aware power-save mechanism. How does a client trigger delivery of buffered frames from the AP?
- By sending a PS-Poll frame
- By sending a QoS Data or QoS Null trigger frame to the AP (Correct answer)
- By setting the More Data bit in its beacon
- By sending a Probe Request frame
Correct answer: By sending a QoS Data or QoS Null trigger frame to the AP
In U-APSD, the client sends a trigger frame (a QoS Data or QoS Null frame) to the AP, which then delivers buffered downlink frames for that AC immediately.
CWS CWS MAC Layer Functions 3
WMM (Wi-Fi Multimedia) defines four Access Categories (ACs).
Which AC has the HIGHEST priority?