CWS CWS 802.11 Standards and Protocols 3 — Questions and Answers
Question 1: In 802.11 OFDM, what is the purpose of the cyclic prefix (guard interval)?
- Encrypt each subcarrier independently
- Prevent inter-symbol interference caused by multipath (Correct answer)
- Boost signal amplitude at the transmitter
- Synchronize channel access timing between STAs
Correct answer: Prevent inter-symbol interference caused by multipath
The cyclic prefix provides a guard interval between OFDM symbols so that multipath echoes from one symbol do not corrupt the next symbol.
Question 2: What technology does 802.11ax use to allow simultaneous uplink transmissions from multiple clients?
- TDMA
- UL MU-MIMO and OFDMA (Correct answer)
- CDMA
- CSMA/CA only
Correct answer: UL MU-MIMO and OFDMA
802.11ax introduced uplink MU-MIMO and OFDMA, enabling the AP to schedule multiple clients to transmit simultaneously on different resource units.
Question 3: Which 802.11 feature allows an AP to request that associated clients adjust their transmit power to reduce co-channel interference?
- TPC – Transmit Power Control (Correct answer)
- DRS – Dynamic Rate Switching
- APSD – Automatic Power Save Delivery
- BSS Coloring
Correct answer: TPC – Transmit Power Control
Transmit Power Control (TPC), defined in 802.11h, lets the AP signal clients to lower their transmit power, reducing interference on shared channels.
Question 4: What is the primary function of BSS Coloring introduced in 802.11ax?
- Visually distinguish APs on management dashboards
- Reduce unnecessary deferrals from overlapping BSSs by tagging frames (Correct answer)
- Assign different QoS colors to traffic classes
- Color-code spatial streams for MU-MIMO scheduling
Correct answer: Reduce unnecessary deferrals from overlapping BSSs by tagging frames
BSS Coloring adds a numeric color field to PHY headers so STAs can identify frames from a foreign BSS and potentially ignore them rather than deferring unnecessarily.
Question 5: How many non-overlapping 20 MHz channels are available in the 2.4 GHz band in the United States?
- 1
- 3 (Correct answer)
- 6
- 11
Correct answer: 3
In the US 2.4 GHz band, only channels 1, 6, and 11 are non-overlapping when using 20 MHz channel widths.
Question 6: What does the 802.11r amendment specifically optimize?
- Multi-band channel bonding
- Fast BSS Transition (roaming) handoff times (Correct answer)
- Radio resource measurement reporting
- Wireless network management frames
Correct answer: Fast BSS Transition (roaming) handoff times
802.11r (Fast BSS Transition) pre-authenticates clients with target APs before the handoff, reducing roaming latency critical for voice and real-time applications.
Question 7: Which 802.11 PHY layer mode is used when legacy 802.11b devices must coexist with 802.11g devices on the same BSS?
- ERP-OFDM only mode
- ERP mixed mode with CTS-to-self or RTS/CTS protection (Correct answer)
- DSSS-CCK high-rate mode
- HR-DSSS compatibility mode
Correct answer: ERP mixed mode with CTS-to-self or RTS/CTS protection
When 802.11b clients are present, 802.11g APs activate ERP protection using CTS-to-self or RTS/CTS to prevent legacy devices from transmitting during OFDM frames.
CWS CWS 802.11 Standards and Protocols 3
In 802.11 OFDM, what is the purpose of the cyclic prefix (guard interval)?