CITS3002 Computer Networks  
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Hidden Node, Crouching Dragon Exposed Node

As a first guess, it would appear that the standard (wired) Ethernet protocol should also work for wireless networks - simply wait until the medium (airwaves) becomes clear, transmit, and then retransmit if a collision occurs.

However, this simple approach will not work, primarily because not all nodes are within range of each other.

Consider these two typical situations:

  • (a) A wishes to send a frame to B, but A cannot 'hear' that B is busy receiving a message from C. If A transmits after detecting an idle medium, a collision may result near B. This is described as the hidden terminal or the hidden node problem. C is hidden from A, but their communications can interfere.

  • (b) B wishes to transmit to C, but hears that A is transmitting (possibly to someone to the left of A). B incorrectly concludes that it cannot transmit to C, for fear of causing a collision. This is described as the exposed terminal or the exposed node problem.


CITS3002 Computer Networks, Lecture 4, Local Area Networks (LANs and WLANs), p18, 20th March 2024.