🔒 Hidden Node Problem

The Hidden Node Problem occurs in wireless networks when two devices, A and C, can communicate with a central device B, but A and C cannot hear each other’s transmissions.

  • Scenario: A and C transmit data to B simultaneously, causing a collision at B. Since A and C cannot detect each other, this results in hidden collisions.

Impact:

  • Reduced throughput and increased retransmissions.
  • Inefficiency in the network due to wasted bandwidth and collisions.

Solution: Request to Send / Clear to Send (RTS/CTS)

  • RTS/CTS is used to reserve the medium before data transmission. Device A sends a Request to Send (RTS) to device B, and B responds with a Clear to Send (CTS).
  • If another device (like C) hears the CTS, it knows the channel is reserved and refrains from transmitting.

🌐 Exposed Node Problem

The Exposed Node Problem happens when a device, A, hears the transmission of device B but is not in the range of the receiver (C). Device A may unnecessarily hold off from transmitting, thinking it will cause interference, even though it won’t.

  • Scenario: A wants to transmit data to C but senses B’s transmission and backs off. However, the transmission from A does not interfere with B’s communication with C, resulting in inefficient channel usage.

Impact:

  • Reduced throughput due to unnecessary waiting, leading to underutilization of the medium.

Solution: Carrier Sensing Mechanisms

  • Using carrier sensing more effectively can help A realize that its transmission will not interfere with B’s communication with C, allowing for parallel transmission without collisions.

🧠 Deep Insight

Both the Hidden Node and Exposed Node problems are symptomatic of shared medium networks, especially in wireless communication. These issues highlight the challenge of managing access in networks where devices cannot always hear each other, leading to collisions and inefficiencies. Proper MAC layer protocols like RTS/CTS (for Hidden Node) and exposed carrier sensing (for Exposed Node) are necessary to mitigate these issues and improve performance.

“In a wireless network, the greatest challenge isn’t the nodes that can’t hear each other, but the ones that think they can.”

🧭 Key Takeaways

  • Hidden Node Problem causes collisions due to devices being out of range of each other.
  • Exposed Node Problem leads to unnecessary transmission delays when a device backs off unnecessarily.
  • Both problems can be alleviated by more advanced MAC protocols like RTS/CTS and smarter carrier sensing.

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