Why do WLANs use CSMA/CA instead of CSMA/CD, and how does the process work?

WLANs are half-duplex — a client can't "hear" while sending, making collision detection impossible. CSMA/CA (Collision Avoidance) uses RTS/CTS handshakes to reserve the channel before transmitting, avoiding collisions proactively.

* CSMA/CA: sense, back off, transmit, ACK. — jjgarcia.tsc, Attribution, via Wikimedia Commons. *

Why CSMA/CD (Ethernet) doesn't work for wireless:

• On Ethernet, a device can send and listen simultaneously → detect collisions in real-time
• On wireless, the transmitter's own signal drowns out all other signals → collision detection is physically impossible
• Also: the "hidden node" problem — two clients may not hear each other but both reach the AP

CSMA/CA process (step by step):

1. Listen: Client checks if the channel is idle (no current traffic)
2. RTS: Client sends a "Ready To Send" frame to the AP, requesting dedicated channel access
3. CTS: AP responds with "Clear To Send," granting exclusive access (all other clients hear this and wait)
4. Wait: If no CTS received, client waits a random backoff time and retries
5. Transmit: Client sends its data frame
6. ACK: AP acknowledges receipt. If no ACK → client assumes a collision occurred and restarts

Key difference from CSMA/CD:

• CSMA/CD: Transmit first, detect collisions during transmission, retransmit if collision detected
• CSMA/CA: Request permission first (RTS/CTS), avoid collisions before they happen, then transmit

Tip: CSMA/CA is slower than CSMA/CD because of the RTS/CTS overhead — this is one reason why Wi-Fi is always slower than wired Ethernet at the same data rate.

Go deeper:

• CSMA/CA (Wikipedia) — why wireless transmitters can't detect collisions, the hidden-node problem, and RTS/CTS.