ELEC3506

Topics

Wireless NetworksLecture 11 PDF10 min

Hidden and exposed stations

Why two wireless senders that cannot hear each other collide at a receiver both can reach, how the CTS frame in the CSMA/CA handshake prevents it, and what the exposed station problem is.

By the end of this page you should be able to

  • Describe the hidden station problem using stations A, B and C
  • Explain how the CTS frame prevents a collision from a hidden station
  • Say what the exposed station problem is
  • Say what the lecture does and does not claim RTS and CTS fix

The idea

Carrier sensing assumes a sender can hear everyone who might disturb its receiver. On a wire that is true. In the air it is not. Two stations on opposite sides of a receiver can each be in range of it while out of range of each other. Each listens, hears silence and sends.

How it works

Hidden terminal problem

The lecture’s slide frames this as a problem created by having multiple wireless senders and receivers, beyond multiple access in general. Take three stations, A, B and C.

  • B and A hear each other.
  • B and C hear each other.
  • A and C cannot hear each other.

So A and C are unaware of their interference at B. The lecture shows two causes, in two figures. In one, A and C are simply too far apart. In the other, signal fading stops them hearing each other even though the geometry would otherwise allow it. Either way, the stations that collide cannot sense the collision, and the damage lands at B.

Two wireless scenarios drawn with carrier-sense range ellipses. Hidden node: A and B sit either side of access point AP, each within range of AP but outside range of each other, so both can transmit to AP at once without either sensing the other, causing a collision at AP. Exposed node: A transmits to B while C, within range of A but not of A's destination, wants to transmit to a separate node D; C hears A's transmission and defers even though sending to D would not have collided with anything.Hidden node — A and B cannot hear each other, both hear APAAPBcollision hereExposed node — C hears A and defers, though C→D would not collideABCDdeferred unnecessarily
Same failure, opposite direction: hidden node stays silent when it should speak — a collision AP never sees coming — and exposed node stays silent when it is free to speak, wasting bandwidth for no reason. RTS/CTS fixes the first by making AP's neighbourhood declare itself; it does nothing for the second, which is why the lecture notes RTS/CTS solves hidden but not exposed node.

The diagram above is drawn from Lecture 4 and shows the same hidden station layout with an access point in the middle. Its distances are illustrative. The Lecture 11 figures themselves did not extract.

How it works

How the CTS helps

The lecture’s statement is brief. The CTS frame in the CSMA/CA handshake can prevent a collision from a hidden station.

The reasoning follows from the handshake in the previous topic. A hidden station cannot hear the source’s RTS, but it can hear the destination. The destination broadcasts the CTS, and the CTS carries the duration. Stations that hear it start a NAV and keep quiet until the exchange is finished.

How it works

Exposed station problem

The lecture gives the exposed station problem its own slides, and those slides are figures only. Nothing from them extracted, so the lecture’s own description is not available here and this page does not fill it in. The label is the name of the problem itself, the mirror image of the hidden station. A hidden station fails to hold off when it should. An exposed station holds off when it did not need to. The Module 4 topic on CSMA/CA, RTS/CTS and the hidden node problem gives a worked layout from Lecture 4.

Aside

The unreadable slides leave this topic thinner than the lecture. If the exposed station figures matter for the exam, check the lecture PDF directly.

Where marks get lost

The lecture's claim is narrow

The lecture only says the CTS frame can prevent a collision from a hidden station. Do not extend that to say RTS and CTS solve the exposed station problem on this unit’s authority.

In the exam

  • Hidden terminal setup. B hears A and C. A and C cannot hear each other. They cannot know they interfere at B. Say that the interference is at the receiver.
  • Cause. Distance, or signal fading.
  • Fix. The CTS frame in the CSMA/CA handshake.
  • Exposed station. Name it as the opposite failure and do not claim the CTS fixes it.

Check yourself

  • Hidden stations hear the receiver but not each other, so both transmit and collide at the receiver.
  • The CTS from the destination reaches the hidden station and starts its NAV.
  • The exposed station problem is the case where a station defers although it would not have interfered.

Check yourself

  1. A and C cannot hear each other but both can hear B. They both send to B. What is this?
  2. How does the CSMA/CA handshake stop a hidden station from transmitting?
  3. Which statement matches the lecture?
  4. What does a station gain from the exposed station problem when it defers?