CSMA/CA protocol in Computer Networks - Step-by-Step Execution

Concept Flow - CSMA/CA protocol

Start: Node wants to send data

↓

Sense channel: Is it free?

No→Wait random backoff time

|Yes↓

Send RTS (Request to Send)

↓

Receive CTS (Clear to Send)?

No→Backoff and retry

|Yes↓

Send Data

↓

Receive ACK?

No→Backoff and retry

|Yes↓

Transmission done

The CSMA/CA protocol makes a node check if the channel is free, then uses RTS/CTS signals to avoid collisions before sending data, retrying with backoff if needed.

Execution Sample

Computer Networks

1. Node senses channel
2. If free, send RTS
3. Wait for CTS
4. If CTS received, send data
5. Wait for ACK
6. If ACK received, done

This sequence shows how a node uses CSMA/CA to avoid collisions by checking the channel and exchanging control messages before data transmission.

Analysis Table

Step	Action	Condition/Check	Result	Next Step
1	Node wants to send data	N/A	Start process	Sense channel
2	Sense channel	Channel free?	No	Wait random backoff
3	Wait random backoff	Backoff timer expires	Yes	Sense channel again
4	Sense channel	Channel free?	Yes	Send RTS
5	Send RTS	N/A	RTS sent	Wait for CTS
6	Wait for CTS	CTS received?	No	Backoff and retry
7	Backoff and retry	Backoff timer expires	Yes	Sense channel again
8	Sense channel	Channel free?	Yes	Send RTS
9	Send RTS	N/A	RTS sent	Wait for CTS
10	Wait for CTS	CTS received?	Yes	Send Data
11	Send Data	N/A	Data sent	Wait for ACK
12	Wait for ACK	ACK received?	No	Backoff and retry
13	Backoff and retry	Backoff timer expires	Yes	Sense channel again
14	Wait for ACK	ACK received?	Yes	Transmission done
15	Transmission done	N/A	Success	End

💡 Transmission ends successfully after ACK received confirming data delivery.

State Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	After Step 10	After Step 12	Final
Channel Status	Unknown	No (busy)	Yes (free)	N/A	N/A	N/A	N/A
Backoff Timer	0	Set and running	Reset	Set and running	Reset	Set and running	0
RTS Sent	No	No	Yes	Yes	Yes	Yes	Yes
CTS Received	No	No	No	No	Yes	No	Yes
Data Sent	No	No	No	No	Yes	No	Yes
ACK Received	No	No	No	No	No	No	Yes

Key Insights - 3 Insights

Why does the node wait a random backoff time when the channel is busy?

Waiting a random backoff time reduces the chance that multiple nodes retry sending at the same time, which helps avoid collisions. This is shown in steps 2 and 3 where the channel is busy and the node waits before sensing again.

What happens if the node does not receive a CTS after sending RTS?

If CTS is not received (step 6), the node assumes the channel is busy or collision happened, so it backs off and retries later (step 7), as shown in the execution table.

Why is ACK important after sending data?

ACK confirms the data was received successfully. Without ACK (step 12), the node retries sending data to ensure delivery, preventing data loss.

Visual Quiz - 3 Questions

Look at the execution table at step 4. What is the channel status?

A Channel is free

B Channel is busy

C Channel status unknown

D Channel is being sensed

💡 Hint

Refer to the 'Condition/Check' and 'Result' columns at step 4 in the execution table.

At which step does the node receive CTS and proceed to send data?

A Step 6

B Step 12

C Step 10

D Step 8

💡 Hint

Check the 'CTS received?' condition and the 'Next Step' column in the execution table.

If the node never receives ACK, what will happen according to the execution flow?

A Transmission ends successfully

B Node retries sending data after backoff

C Node stops trying and gives up

D Node sends RTS again immediately

💡 Hint

Look at the 'Wait for ACK' step and what happens if ACK is not received.

Concept Snapshot

CSMA/CA protocol:
- Node senses channel before sending
- If busy, waits random backoff
- Uses RTS/CTS handshake to avoid collisions
- Sends data only after CTS
- Waits for ACK to confirm delivery
- Retries with backoff if no CTS or ACK

Full Transcript

CSMA/CA is a method used by devices to avoid collisions when sending data over a shared channel. First, a device checks if the channel is free. If busy, it waits a random time before checking again. When free, it sends a Request to Send (RTS) message. If it receives a Clear to Send (CTS) reply, it sends the data. After sending, it waits for an Acknowledgment (ACK) to confirm the data was received. If CTS or ACK is not received, the device waits and retries. This process helps devices share the channel fairly and avoid data collisions.