ACMA Wireless Networking Fundamentals & Technologies 4 — Questions and Answers
Question 1: In 802.11, what is the purpose of the DIFS (Distributed Interframe Space)?
- It is the mandatory wait time before transmitting an ACK frame
- It is the minimum time a station must sense the channel idle before starting a transmission (Correct answer)
- It defines the beacon transmission interval
- It sets the maximum time an AP holds frames for a sleeping client
Correct answer: It is the minimum time a station must sense the channel idle before starting a transmission
DIFS is the mandatory idle period a station must observe before attempting a data transmission using the Distributed Coordination Function (DCF).
Question 2: Which term describes the technique where an AP steers a dual-band client from 2.4 GHz to 5 GHz to improve network performance?
- Channel bonding
- Band steering (Correct answer)
- Transmit beamforming
- MU-MIMO
Correct answer: Band steering
Band steering encourages capable dual-band clients to connect on the less congested 5 GHz band by manipulating probe responses or association parameters.
Question 3: What does 'spatial multiplexing' enable in MIMO wireless systems?
- Transmitting the same signal on multiple frequencies simultaneously
- Sending multiple independent data streams over the same channel at the same time (Correct answer)
- Beamforming energy toward a single client
- Combining signals from multiple APs for diversity
Correct answer: Sending multiple independent data streams over the same channel at the same time
Spatial multiplexing transmits independent data streams simultaneously using multiple antennas, multiplying throughput without requiring additional spectrum.
Question 4: In the context of 802.11 power management, what is a TIM (Traffic Indication Map)?
- A frame that schedules channel access for QoS clients
- A field in the beacon that indicates which sleeping stations have buffered frames waiting at the AP (Correct answer)
- A timer used to define the DTIM interval for multicast delivery
- A map of active channels used by the AP for DFS scanning
Correct answer: A field in the beacon that indicates which sleeping stations have buffered frames waiting at the AP
The TIM is included in AP beacons and indicates which associated sleeping stations have unicast frames buffered at the AP waiting for retrieval.
Question 5: Which 802.11 feature allows an AP to direct RF energy toward a specific client to improve signal quality?
- Spatial multiplexing
- MRC (Maximal Ratio Combining)
- Explicit transmit beamforming (Correct answer)
- STBC (Space-Time Block Coding)
Correct answer: Explicit transmit beamforming
Explicit transmit beamforming uses channel sounding feedback from the client to steer antenna energy directionally, improving SNR at the intended receiver.
Question 6: What problem does 802.11r (Fast BSS Transition) specifically address?
- Interference from non-802.11 devices on the 2.4 GHz band
- High roaming latency caused by full 802.1X re-authentication during transitions (Correct answer)
- Collision rates in high-density environments
- IP address conflicts during inter-subnet roaming
Correct answer: High roaming latency caused by full 802.1X re-authentication during transitions
802.11r reduces roaming latency by pre-establishing the security keys with the target AP before the client fully disassociates from the current AP, eliminating full 802.1X re-authentication.
Question 7: What is the key difference between infrastructure mode and ad-hoc (IBSS) mode in 802.11?
- Infrastructure mode uses WPA3 while ad-hoc uses WEP
- Infrastructure mode requires an AP to coordinate communication; ad-hoc allows direct peer-to-peer communication (Correct answer)
- Ad-hoc mode supports higher data rates than infrastructure mode
- Infrastructure mode operates only on 5 GHz while ad-hoc uses 2.4 GHz
Correct answer: Infrastructure mode requires an AP to coordinate communication; ad-hoc allows direct peer-to-peer communication
In infrastructure mode an AP acts as the central coordinator for all client communications, while in ad-hoc (IBSS) mode clients communicate directly with each other without an AP.
ACMA Wireless Networking Fundamentals & Technologies 4
In 802.11, what is the purpose of the DIFS (Distributed Interframe Space)?