CWS MAC Layer Functions Flashcards

WMM (Wi-Fi Multimedia) defines four Access Categories (ACs). Which AC has the HIGHEST priority?

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.

In 802.11e/WMM, what replaces the standard DIFS as the mandatory idle time before a station transmits in a QoS-enabled network?

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.

What is a TXOP (Transmission Opportunity) in 802.11 QoS?

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.

EDCA (Enhanced Distributed Channel Access) is part of which 802.11 amendment?

Answer: 802.11e

802.11e introduced EDCA and the full QoS framework (HCF), which WMM is based upon.

Which WMM Access Category maps to the 802.1p priority values 0 and 3 (best effort and video)?

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.

In EDCA, what parameter defines the minimum and maximum size of the random backoff contention window for each Access Category?

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.

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?

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.