Two laptops at opposite ends of a large office can each communicate with the central access point but cannot hear each other's transmissions. Both transmit simultaneously. What problem does this illustrate, and how does WiFi address it?
AMultipath fading — resolved by MIMO antennas combining multiple signal paths
BThe hidden node problem — addressed by CSMA/CA with optional RTS/CTS handshaking to reserve the channel
CChannel saturation — resolved by switching to the 5 GHz band's wider channels
DMAC address collision — resolved by ARP to ensure unique addressing before transmission
This is the classic hidden node problem: two devices are hidden from each other but both visible to the access point, so a collision at the AP goes undetected by either sender. CSMA/CA's RTS/CTS mechanism addresses it — a device sends a short RTS frame; the AP replies with a CTS that all nearby devices hear, causing them to defer transmission. The two hidden laptops both hear the CTS and back off.
Question 2 Multiple Choice
Why does 802.11 WiFi use CSMA/CA (Collision Avoidance) rather than CSMA/CD (Collision Detection), which is used in wired Ethernet?
ACollision avoidance is faster because it eliminates the need for backoff timers
BWireless devices transmit and receive on different frequency bands, making CD unnecessary
CA wireless transmitter cannot detect collisions because its own outgoing signal overwhelms any incoming signal during transmission
DThe 802.11 standard was designed before collision detection was invented, so avoidance was used for historical reasons
In wired Ethernet, a device can compare what it is sending to what it hears on the wire, detecting a mismatch that signals a collision. Wireless devices cannot do this because their own transmitted signal is orders of magnitude stronger than any received signal — the transmitter effectively deafens its own receiver. Since detection is impossible, avoidance through random backoff and optional RTS/CTS is the only viable strategy, even though it wastes time on precaution.
Question 3 True / False
The 5 GHz WiFi band provides longer wireless range than the 2.4 GHz band because higher frequency signals propagate further.
TTrue
FFalse
Answer: False
False. Higher frequency signals attenuate more rapidly as they pass through walls and obstacles. The 5 GHz band actually has shorter range than 2.4 GHz for this reason. Its advantages are more non-overlapping channels, less interference from Bluetooth and microwaves, and higher throughput — not range. 2.4 GHz penetrates walls better and reaches further, which is why it remains preferred for large or obstacle-heavy environments despite its congestion.
Question 4 True / False
A WiFi frame in infrastructure mode can carry up to four MAC addresses, whereas an Ethernet frame carries only two.
TTrue
FFalse
Answer: True
True. Ethernet frames identify source and destination. WiFi adds transmitter and receiver fields because the access point acts as a relay in infrastructure mode: a frame from your laptop to an internet server has your laptop's MAC as the source, the server's MAC as the destination, the AP's MAC as the receiver (for the first hop), and your laptop's MAC as the transmitter. The source/transmitter and destination/receiver distinctions only matter because packets must hop through the AP. This extended addressing supports the management frames that handle association, authentication, and beaconing.
Question 5 Short Answer
Why is CSMA/CA less efficient than CSMA/CD, and what fundamental property of wireless transmission makes avoidance the only viable option?
Think about your answer, then reveal below.
Model answer: CSMA/CA wastes channel time on random backoff periods and optional RTS/CTS handshakes even when no collision actually occurs — precautionary delays are built into every transmission. CSMA/CD wastes time only on actual collisions, reacting immediately when they happen. Wireless avoidance is necessary because a transmitting device's own signal is far stronger than any incoming signal, so it cannot hear a collision while transmitting. Without the ability to detect collisions in progress, the device must avoid them before they happen.
The efficiency gap matters especially in high-density WiFi environments where many devices share a channel. Every random backoff represents wasted airtime. 802.11ax (WiFi 6) addresses this with OFDMA, which divides the channel into sub-carriers assigned to different clients simultaneously, reducing the contention problem rather than relying purely on CSMA/CA's collision avoidance for medium access.