Q12Mobile Computing
Question
Explain the hidden terminal and exposed terminal problems and their solutions (MACA).
Answer
A critical analysis of the Hidden and Exposed Terminal problems in wireless MAC. Explains how CSMA/CD mathematically fails in RF, and how MACA utilizes violent RTS/CTS handshakes to prevent catastrophic packet collisions.
In a wired Ethernet network, CSMA/CD works perfectly because electricity on a wire can be sensed everywhere. In wireless networks, radio waves have strict physical boundaries. This physical limitation creates two catastrophic mathematical failures in standard carrier sensing.
Assume Node A and Node C are both in range of Node B, but A and C are completely out of range of each other. Node A wants to send data to B. A mathematically senses the air, hears silence, and starts transmitting. A millisecond later, Node C also wants to send to B. C senses the air, hears silence (because it cannot physically hear A), and transmits. The Catastrophe: The two RF waves violently collide at Node B. The packet is completely destroyed.
Assume Node B is transmitting to Node A. Node C (in range of B) wants to transmit to Node D (out of range of B). Node C mathematically senses the air, hears B transmitting, and violently aborts its transmission to avoid a collision. The Catastrophe: This is mathematically stupid. C's transmission to D would have absolutely no impact on B's transmission to A. Bandwidth is catastrophically wasted due to a false alarm.
MACA completely abandons raw carrier sensing and replaces it with a violent, mathematical handshake using RTS/CTS (Request To Send / Clear To Send).
- Solving Hidden Terminal: Node A sends an RTS to B. B broadcasts a CTS. Node C mathematically hears the CTS from B and instantly freezes, knowing B is receiving data. Collision averted.
- Solving Exposed Terminal: Node B sends an RTS to A. Node C hears the RTS, but NOT the CTS from A. Node C mathematically deduces that it is perfectly safe to transmit to D. Wasted bandwidth recovered.