Hidden Terminal Workbench

See why two wireless senders can both hear a receiver but not each other

Interactive hidden terminal animation showing carrier sensing, receiver-side collision, RTS/CTS reservation, NAV deferral, frame-size threshold trade-offs, and limits of control-frame protection.

Hidden terminal CSMA/CA RTS/CTS and NAV

Hidden terminal workbench

Two senders can both reach receiver B while being too far apart to hear each other. Step through physical carrier sense, receiver collision, and RTS/CTS reservation to see why the receiver's CTS matters.

A hidden from Ccarrier-sense geometry

no NAVvirtual carrier sense

collisionreceiver result

retry neededairtime impact

Scenario

MAC Controls

Channel access carrier only Carrier sense only RTS/CTS above threshold Force RTS/CTS

Carrier-sense range 260 m

CTS/NAV reach 270 m

Frame size 1200 bytes

Ready: inspect carrier sense

Watch For

Hidden terminals are diagnosed by the receiver's outcome, not by whether each sender thinks the channel is clear.

Sender viewA and C may each sense idle because they are outside each other's range.

Receiver viewB receives overlapping energy and cannot decode either frame.

CTS reachRTS/CTS helps only if C hears B's CTS and sets NAV.

OverheadControl frames can cost more than retrying tiny payloads.

Hidden Terminal Topology

A and C cannot sense each other

1. Sensecheck channel

2. Reserveno reservation

3. Sendframes pending

4. Resultreceiver outcome

Transmission Timeline

Readouts

hidden pairA and C carrier-sense relationship

RTS/CTS offreservation policy for this frame

C has no NAVvirtual carrier-sense state

collision at Bwhat the receiver experiences

retry neededairtime cost

threshold 512 BRTS/CTS decision boundary

Diagnosis

Step Transcript

Quick Reference

Hidden terminalTwo transmitters cannot hear each other, but their frames overlap at a shared receiver.

Physical carrier senseA node listens for local energy before sending. It cannot detect a sender outside its sensing range.

Virtual carrier senseRTS/CTS carries a duration. Nodes that hear CTS set NAV and defer even if they cannot hear the data frame.

How To Read The Animation

Dashed blue circles show physical carrier-sense range around A and C. The dashed green circle shows how far B's CTS can reserve the medium. If C cannot hear A and does not hear B's CTS, C may send while A is already sending.

The collision marker appears at B because receivers decode signals. A and C do not collide with themselves; they create overlapping energy at the common receiver.

Technical Accuracy Notes

RTS/CTS mitigates the hidden terminal problem; it does not guarantee collision-free operation. RTS, CTS, ACK, and data frames can still be lost, and protection depends on which nodes hear the control frames.

The 512-byte threshold used here is a teaching threshold so the trade-off is visible. Real systems use implementation-specific RTS thresholds, data rates, contention windows, retry limits, and protection modes.