GATE CS 2016 Set 2 Q54 — Data Link Layer: Framing, Error Detection, MAC, Ethernet Bridging

Let us analyze each statement for the IEEE 802.11 MAC protocol:

• Statement I: At least three non-overlapping channels are available for transmissions. (TRUE)
  In the widely used $2.4\text{ GHz}$ ISM band of IEEE 802.11, there are 11 to 13 channels. Among these, channels 1, 6, and 11 do not overlap with each other. Thus, there are indeed at least three non-overlapping channels available.
• Statement II: The RTS-CTS mechanism is used for collision detection. (FALSE)
  Wireless transceivers cannot transmit and detect collisions simultaneously because their own transmission drowns out any incoming signal (the near-far problem). Therefore, IEEE 802.11 uses CSMA/CA (Collision Avoidance) rather than Collision Detection (CD). The RTS/CTS (Request to Send / Clear to Send) mechanism is used for collision avoidance and to address the hidden terminal problem, not for collision detection.
• Statement III: Unicast frames are ACKed. (TRUE)
  Due to the high bit-error rates of wireless mediums and the inability to detect collisions during transmission, IEEE 802.11 relies on an explicit link-layer acknowledgement (ACK) frame for every successfully received unicast frame. If the sender does not receive an ACK, it retransmits the frame.

Since statements I and III are true, the correct option is B.