Hidden and Exposed Station Problems in Wireless Networks

1. Hidden Station Problem

Definition: Occurs when two stations (e.g., A and C) are out of range of each other but both can communicate with a common station (e.g., B).

Problem:

• Station A senses the channel as idle and sends data to B.
• At the same time, station C (which cannot sense A) also sends data to B.
• Collision occurs at B, even though A and C could not detect each other.

As shown in the figure, stations A and C cannot detect each other’s transmissions because they are outside each other’s range (indicated by the dashed and dash-dot lines). However, both are within range of station B (solid line).

Result:

• Increased collisions and reduced network performance.
• Higher packet loss rate.
• Reduced overall throughput.
• Increased latency due to retransmissions.

Solution:

• Use RTS/CTS (Request to Send / Clear to Send) mechanism from IEEE 802.11 MAC protocol.
• Station A sends an RTS to B; B replies with CTS.
• CTS is heard by all nearby nodes (including C), so C stays silent, preventing collision.
• The duration of the intended communication is included in both RTS and CTS frames.

2. Exposed Station Problem

Definition: Occurs when a station (e.g., C) refrains from transmitting due to sensing a nearby transmission (e.g., B → A), even though its transmission (C → D) would not interfere.

Problem:

• B senses the channel and finds it free. B starts transmitting data to A.
• C also wants to transmit data to D.
• C detects B’s transmission, because B is within C’s transmission range.
• C assumes the channel to D is busy and delays its transmission, even though:
  • D is out of range of B’s signal.
  • There would be no interference between B→A and C→D communication.

Result:

• Underutilization of the channel — reduced throughput.
• Unnecessary delays in data transmission.
• Inefficient use of network capacity.
• Reduced overall network performance.

Solution: Use RTS/CTS Mechanism:

• C sends RTS to D.
• If D replies with CTS, it means the channel is clear for communication.
• Since D is not in range of B, it will respond, allowing C to send.
• This prevents C from unnecessarily delaying its transmission.