Wireless Communication Basics
Which describes the RTS/CTS mechanism used to mitigate the hidden node problem in wireless LANs?
- AA terminal that wants to transmit sends a request to send (RTS), and the access point returns a clear to send (CTS); nearby terminals that receive the CTS refrain from transmitting for the duration of that exchange
- BA terminal monitors the received signal while transmitting and stops immediately if it detects a collision
- CAll terminals transmit at maximum power so that their signals reach one another
Show answer and explanation
Answer: A. A terminal that wants to transmit sends a request to send (RTS), and the access point returns a clear to send (CTS); nearby terminals that receive the CTS refrain from transmitting for the duration of that exchange
The hidden node problem occurs when signals from both terminals reach the access point, but the terminals cannot hear each other because of distance or obstacles. Unaware of each other's transmissions, they judge the channel to be free, transmit at the same time and collide at the access point. With RTS/CTS, the CTS returned by the access point states how long the exchange will take, and terminals that receive the CTS refrain from transmitting for that time. The RTS/CTS exchange costs efficiency, so it can be configured to be used only when sending large frames.
- A. Correct. The CTS also reaches hidden terminals, so they too hold off transmitting.
- B. This is the collision detection of CSMA/CD used in wired Ethernet. In radio, detecting collisions while transmitting is difficult.
- C. Raising transmit power increases interference and may still not get past obstacles. Transmit power is also capped by regulations.