The complementary problem to the hidden terminal: sometimes a node correctly detects that a neighbor is transmitting, but the detection causes it to unnecessarily refrain from transmitting. The node misinterprets nearby transmission as a reason to wait, even though its own transmission would not interfere with the ongoing one (because the intended receivers are far apart). This wastes available bandwidth and reduces network efficiency.
The exposed terminal problem occurs when a node (C) that is within range of an active transmitter (B) unnecessarily defers its own transmission, even though its signal would not interfere with B’s reception (because C’s intended receiver D is far away). C is “exposed” to B’s signal but doesn’t realize its own signal won’t cause problems.
- Node B is transmitting to node A (B is within range of A, not within range of C)
- Node C wants to transmit to node D (D is far from both A and B)
- C senses the channel and detects B’s ongoing transmission
- C incorrectly concludes it cannot transmit without causing interference
- C waits unnecessarily, even though its signal to D would not interfere with B’s signal to A
- Bandwidth is wasted — C could have successfully transmitted to D simultaneously
The key insight: if the intended receivers are sufficiently separated, simultaneous transmissions are possible without collision.
- Waste of bandwidth: node C unnecessarily waits when transmission would succeed
- Caused by over-conservative carrier sensing (no distinction between harmful and harmless interference)
- More subtle than hidden terminal — requires understanding of receiver locations
- Can be partially solved by RTS/CTS exchanges (MACA) which announce intended receivers
- Not as severe as hidden terminal but still reduces network efficiency significantly
- Related: Hidden Terminal Problem — the complementary problem; both arise from range limitations
- Related: MACA — MACA’s RTS/CTS handshake also helps exposed terminals by announcing clear receiver zones
- Related: CSMA — carrier sensing that causes the exposed terminal problem
- Related: Far Terminal Effect — another wireless MAC efficiency problem
- The exposed terminal problem is less severe than the hidden terminal problem in terms of call quality
- It mainly affects throughput (capacity), not call success rates
- Directional antennas can help — C can transmit in a direction different from B’s reception zone
- RTS/CTS helps resolve both hidden and exposed terminal problems by explicitly announcing receiver locations