Why do we need to send an acknowledgement in 802.11 protocols when a frame is successfully received?
Try an answer before revealing the guidance below.
Key Concepts
- Wireless collision detection
- Hidden terminal
- Fading
- CSMA/CA
- Link-layer retransmission
Answer Approach
- Ask whether a wireless sender can detect a collision while it is transmitting.
- Consider hidden terminals and signal fading.
- Explain how the sender otherwise learns the frame arrived.
Full Answer
Answer status: Draft answer (unofficial). Revision notes, not an official marking scheme.
A wireless station cannot reliably detect collisions while transmitting: its own signal overwhelms weak incoming signals, hidden terminals may collide at the receiver without the sender hearing them, and fading can corrupt frames. So 802.11 uses CSMA/CA without collision detection, and the receiver sends an ACK after a short interval (SIFS) when a frame passes its CRC. If the sender gets no ACK it assumes the frame was lost and retransmits after backoff, giving reliable delivery over an error-prone link.
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