Exposed Terminal Workbench

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

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

Scenarios

Controls

MAC behavior carrier sense only carrier sense only receiver-aware unsafe close receivers

Carrier sense range hears B

Interference range receiver-safe

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

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.