Procedural Explanation of CSMA/CA Concepts

Below is a structured explanation of key CSMA/CA mechanisms: Collision, Backoff, Hidden Node Problem, and RTS/CTS, along with their role in the CSMA/CA procedure.

Step-by-Step CSMA/CA Procedure

Step 1: Carrier Sensing (Check if Channel is Free) Before transmitting, a device listens to the channel.

If the channel is free, it proceeds to transmit.

If the channel is busy, it triggers the backoff mechanism to avoid collision.

Step 2: Collision & Backoff Mechanism Collision happens when two or more devices transmit at the same time, causing interference and data loss.

2.1 What Happens During a Collision?

Since CSMA/CA tries to avoid collisions instead of detecting them, it does not allow transmission on a busy channel.

If a device finds the channel occupied, it does not transmit immediately. Instead, it waits and uses the backoff mechanism.

2.2 How the Backoff Mechanism Works

The device generates a random backoff time within a set range (Contention Window).

It waits for this time before trying again.

If the channel is still busy after the backoff, it picks another random time and waits again.

If the channel is free, it moves to transmission.

🔹 Why Backoff?

Prevents multiple devices from retransmitting at the same time.

Reduces collision probability.

Step 3: The Hidden Node Problem & RTS/CTS Solution The Hidden Node Problem occurs when two devices cannot detect each other but can both communicate with the same access point.

3.1 Hidden Node Problem Example

Device A and Device C both want to send data to Device B.

Device A cannot "hear" Device C's transmission and vice versa.

If both transmit at the same time, Device B will receive garbled data due to interference.

3.2 RTS/CTS Handshake (Solution to Hidden Node Problem)

🔹 Request to Send (RTS) / Clear to Send (CTS) is a mechanism that ensures only one device transmits at a time.

Device A sends an RTS (Request to Send) to Device B asking for permission to transmit.

Device B replies with CTS (Clear to Send) if the channel is free.

Device A then transmits its data safely, knowing no one else will interfere.

Other devices (like Device C) hearing the CTS will remain silent until the transmission is done.

🔹 Why Use RTS/CTS?

Prevents collisions due to hidden nodes.

Helps in high-traffic wireless networks where multiple devices compete for transmission.

Step 4: Data Transmission & Acknowledgment (ACK) After the RTS/CTS or direct sensing, the device transmits data.

The receiver sends an ACK (Acknowledgment) if the data is successfully received.

If no ACK is received, the sender assumes a collision happened and starts the backoff process again.