CWT Wireless Standards & Radio Frequency Fundamentals 5 — Questions and Answers
Question 1: What is the key difference between 2.4 GHz and 5 GHz Wi-Fi in terms of propagation?
- 2.4 GHz has shorter range but less interference
- 5 GHz penetrates walls better and has longer range
- 2.4 GHz has longer range but is more prone to interference from overlapping channels (Correct answer)
- 5 GHz and 2.4 GHz have identical propagation characteristics
Correct answer: 2.4 GHz has longer range but is more prone to interference from overlapping channels
The 2.4 GHz band provides longer range and better wall penetration but suffers from heavy interference due to only 3 non-overlapping channels and many competing devices.
Question 2: In OFDM, what is the role of subcarriers?
- They provide frequency hopping to avoid interference
- They carry independent data streams in parallel across narrow sub-channels within a wide channel (Correct answer)
- They provide redundancy by transmitting the same data multiple times
- They separate spatial streams in MU-MIMO
Correct answer: They carry independent data streams in parallel across narrow sub-channels within a wide channel
OFDM divides a wide channel into many narrow orthogonal subcarriers that carry data in parallel, improving spectral efficiency and robustness against multipath interference.
Question 3: What is the Hidden Node Problem in Wi-Fi, and which mechanism was designed to address it?
- Clients hidden behind NAT; solved by DHCP
- Two clients can hear the AP but not each other, causing collisions; addressed by RTS/CTS (Correct answer)
- APs hidden by building structure; solved by mesh networking
- Clients with weak signals; solved by band steering
Correct answer: Two clients can hear the AP but not each other, causing collisions; addressed by RTS/CTS
The hidden node problem occurs when two clients cannot hear each other but both transmit to the AP simultaneously; RTS/CTS (Request to Send / Clear to Send) exchanges reserve the medium to prevent collisions.
Question 4: Which 802.11 amendment introduced support for operation in the TV White Space (TVWS) spectrum below 1 GHz?
- 802.11ah
- 802.11af (Correct answer)
- 802.11ad
- 802.11p
Correct answer: 802.11af
IEEE 802.11af (also called White-Fi or Super Wi-Fi) operates in unused TV broadcast channels (54–790 MHz), offering extended range due to favorable propagation at lower frequencies.
Question 5: What is the significance of the Fresnel zone in wireless link design?
- It defines the maximum transmit power allowed on a given channel
- It is an elliptical region around the line-of-sight path that must be kept mostly clear of obstructions to avoid signal loss (Correct answer)
- It describes the frequency reuse plan for cellular networks
- It specifies the minimum antenna height for regulatory compliance
Correct answer: It is an elliptical region around the line-of-sight path that must be kept mostly clear of obstructions to avoid signal loss
The Fresnel zone is a series of ellipsoidal regions around the LOS path; obstructions within the first Fresnel zone cause significant diffraction loss, so at least 60% clearance is recommended.
Question 6: BSS Coloring, introduced in 802.11ax, primarily helps with which issue?
- Reducing co-channel interference in dense deployments by allowing concurrent transmissions from different BSSs (Correct answer)
- Assigning IP addresses to clients more efficiently
- Coloring channel maps to identify DFS radar events
- Encoding spatial streams with unique identifiers
Correct answer: Reducing co-channel interference in dense deployments by allowing concurrent transmissions from different BSSs
BSS Coloring adds a color identifier to frames so devices can distinguish between their own BSS and neighboring BSSs, allowing them to ignore foreign BSS transmissions and reduce unnecessary deferrals.
Question 7: What does a link budget calculation determine in wireless network planning?
- The cost of deploying wireless infrastructure
- Whether the received signal at the far end will exceed the receiver's sensitivity after accounting for all gains and losses (Correct answer)
- The number of clients an AP can serve simultaneously
- The optimal channel plan for a multi-AP deployment
Correct answer: Whether the received signal at the far end will exceed the receiver's sensitivity after accounting for all gains and losses
A link budget sums all gains (transmit power, antenna gain) and losses (path loss, cable loss, fade margin) to verify that received signal power exceeds the minimum receiver sensitivity threshold.
CWT Wireless Standards & Radio Frequency Fundamentals 5
What is the key difference between 2.4 GHz and 5 GHz Wi-Fi in terms of propagation?