Exposed Terminal Workbench

Exposed Terminal Workbench

See why a wireless node may wait even when its transmission would not harm the active receiver

animation
wireless
mac
networking
csma-ca
Interactive exposed terminal animation showing carrier sense, receiver interference, safe parallel transmissions, and the difference between conservative CSMA waiting and receiver-aware MAC decisions.
Animation Wireless MAC CSMA/CA Throughput

Exposed Terminal Workbench

An exposed terminal waits because carrier sense says the channel is busy, even though its own receiver would still decode correctly. The problem wastes airtime: it is a throughput loss, not a collision at the active receiver.

C defersNode C decision
parallel safeReceiver outcome
1 frame/slotDelivered work
carrier senseMain cause
TrySelect carrier sense only, keep B to A active, and Step when C hears B but wants to send to D.
ObserveNode C decision becomes defer and Delivered work stays at 1 frame, even though Receiver outcome says parallel C-to-D would be safe.
ExplainFor 2 concurrent links, carrier sensing measures energy near the transmitter, not interference at A and D; conservative deferral wastes spatial reuse when both receivers remain protected.
Technical boundariesThe 4-node geometry uses binary hearing/interference ranges; it excludes fading, capture, cumulative interference, hidden nodes, rate adaptation, RTS/CTS overhead, mobility, and probabilistic packet error.
Colour keynetworking core identitycurrent / primaryreference / datasuccesscautionerror / failure

Scenarios

Controls

Ready to inspect

What To Watch

Keep the active receiver in view. C hears B's carrier, but the important question is whether C's transmission would corrupt reception at A.

Carrier SenseC hears the sender B and marks the channel busy.
Receiver CheckA is the vulnerable receiver for B to A, not B itself.
Safe ParallelC to D is safe when C does not interfere at A and B does not interfere at D.
Throughput LossThe exposed terminal waits, so one safe frame is delayed.

A-B-C-D Line Topology

B to A is active; C wants D
Exposed terminal wireless topology Four wireless nodes A, B, C, and D in a line with carrier sense and interference ranges highlighted.
C hears BCarrier sense result
A stays cleanActive receiver result
parallel would workThroughput opportunity

Current State

B sends to AActive flow
C wants DBlocked flow
no collision at AReceiver safety
50% airtimeSlot efficiency

Learning Diagnosis

Event Transcript

Quick Reference
Exposed TerminalA node defers because it hears a sender, even though its own transmission would be safe at the receivers.
Classic TopologyB sends to A. C hears B and wants to send to D. C to D can be safe if A is not affected.
EffectUnlike a hidden terminal, the exposed terminal causes unnecessary waiting rather than collision.
How To Read The Animation
  • Blue shows the active B to A transmission.
  • Orange dashed C to D means C is waiting because carrier sense says busy.
  • Green C to D means receiver-aware logic allows safe parallel delivery.
  • Red means the alternate transmission would now interfere at the active receiver, so deferral is correct.
Technical Accuracy Notes
  • The classic exposed-terminal explanation uses B to A as the active flow and C to D as the waiting flow. If A to B is active, C is often too close to B and the same conclusion may not hold.
  • Carrier-sense range and interference-at-receiver range are not identical. A sender can be heard by a neighbor without that neighbor's own receiver being unsafe.
  • RTS/CTS mainly helps hidden terminals. Receiver-aware MAC variants can reduce exposed waiting when a node can infer that the receiver of the active flow is not at risk.
  • The simplified ranges here are conceptual; real deployments depend on SINR, capture effect, rate, antenna pattern, power control, and fading.