Computer Networking Basics
What is the main reason wireless LAN uses CSMA/CA rather than CSMA/CD, which has been used in wired Ethernet?
- AOn a wireless channel it is hard to detect a collision by receiving other stations' weak signals while transmitting, so stations wait before sending to avoid collisions as far as possible
- BWireless is faster than wired, so collisions have little effect on performance
- CIn wireless LAN a station and the access point always communicate one-to-one, so collisions cannot happen
Show answer and explanation
Answer: A. On a wireless channel it is hard to detect a collision by receiving other stations' weak signals while transmitting, so stations wait before sending to avoid collisions as far as possible
CSMA/CD is used on shared wired media such as old hub-based Ethernet: a station that detects a collision while sending stops and resends after a random time. On a wireless channel, a station's own signal is so strong that it is hard to receive a distant station's weak signal at the same time, and collisions between stations that cannot hear each other (hidden stations) are invisible to the sender. So with CSMA/CA, a station confirms the channel is idle and then waits a fixed time plus a random backoff before sending. The receiver returns an ACK when it receives the frame correctly, and if no ACK arrives the sender assumes a collision or similar and retransmits. In today's wired Ethernet, connected full duplex to switches, collisions do not occur and CSMA/CD is effectively unused.
- A. Correct. Since collisions cannot be detected, they are avoided in advance and delivery is confirmed by ACK.
- B. Wireless is generally slower than wired, and losses from collisions are a bigger problem, not a smaller one.
- C. Many stations share one access point on the same radio channel, so collisions can occur.