What causes the hidden node problem in a wireless LAN?

• A. A client can receive signals from an AP but an obstacle prevents the client from receiving signals from other clients connected to the network.
• B. A client cannot hear the AP.
• C. The AP operates on a different channel than the client.
• D. Too many clients causing collisions on a single channel.

Answer: (A)

Hidden node happens when two stations can both talk to the same access point but can’t hear each other. Since each one can hear the AP, they both think the channel is idle and start transmitting to the AP. Their transmissions collide at the AP because they can’t sense each other’s activity, often due to obstacles or distance blocking the signal between the two clients. The AP ends up receiving garbled data, and the usual wireless retry mechanisms kick in.

This is what makes it different from a client just not hearing the AP (that would prevent any communication at all) or from simply having too many users on one channel (which is a general contention issue, not the specific hidden-node scenario). Using RTS/CTS can help prevent hidden-node collisions by coordinating who gets to transmit.