Discussion
For the IEEE 802.11 MAC protocol for wireless communication, which of the following statements is/are TRUE?
I. At least three non-overlapping channels are available for transmissions.
II. The RTS-CTS mechanism is used for collision detection.
III. Unicast frames are ACKed.
Answer: B
The true statements are I and III only.
I is true: in the 2.4 GHz band, channels 1, 6 and 11 do not overlap, so at least three non-overlapping channels are available (and many more at 5 GHz).
III is true: 802.11 uses a stop-and-wait acknowledgement at the MAC layer, so every unicast data frame received correctly is ACKed; broadcast frames are not.
II is false: a wireless station cannot reliably detect collisions while transmitting, so 802.11 uses CSMA/CA, collision avoidance. The RTS-CTS exchange reserves the medium to avoid collisions, notably from hidden terminals; it does not detect them. That rules out every option containing II.
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