Computer Networks (BCA) 5th Sem Previous Year Solved Question Paper 2022

Stop-and-Wait ARQ (Automatic Repeat reQuest) Protocol:
Working:
    1. Sender: Sends a single frame and waits for an acknowledgment (ACK) from the receiver.
    2. Receiver: Receives the frame, checks for errors, and sends an ACK if the frame is error-free. If errors are detected, a Negative ACKnowledgment (NAK) is sent.
    3. Sender: If an ACK is received, it sends the next frame. If a NAK is received or a timeout occurs, it retransmits the same frame.
Example:
    • Sender sends Frame 1.
    • Receiver receives Frame 1, sends ACK 1.
    • Sender receives ACK 1, sends Frame 2.

Go-Back-N ARQ (Automatic Repeat reQuest) Protocol:
Working:
    1. Sender: Sends multiple frames without waiting for individual acknowledgments.
    2. Receiver: Receives frames and sends cumulative ACK for the last correctly received frame. If an error is detected, it discards the frame and sends a NAK for the last correctly received frame.
    3. Sender: If a NAK is received or a timeout occurs, it retransmits all frames from the last ACKnowledged frame.
Example:
    • Sender sends Frames 1, 2, 3, 4.
    • Receiver receives Frames 1, 2, 4, discards 3 (error detected).
    • Receiver sends NAK 2.
    • Sender retransmits Frames 2, 3, 4.

Key Differences:
    • Stop-and-Wait:
        ◦ One frame sent at a time.
        ◦ Efficient for low error rates and short distances.

    • Go-Back-N:
        ◦ Multiple frames sent before waiting for acknowledgments.
        ◦ Efficient for higher error rates and longer distances.

Advantages and Disadvantages:
    • Stop-and-Wait:
        ◦ Advantage: Simple and easy to implement.
        ◦ Disadvantage: Inefficiency for longer propagation delays.

    • Go-Back-N:
        ◦ Advantage: Higher efficiency for longer distances and higher error rates.
        ◦ Disadvantage: Complexity increases, and there is a potential for retransmitting more frames than necessary.

Considerations:
    • Timeouts:
        ◦ Both protocols use timeouts to determine when to retransmit frames.

    • Acknowledgment:
        ◦ Stop-and-Wait acknowledges each frame individually.
        ◦ Go-Back-N acknowledges multiple frames cumulatively.

These protocols play a crucial role in ensuring reliable data transmission over unreliable channels by handling errors and retransmissions effectively. The choice between Stop-and-Wait and Go-Back-N depends on the specific requirements and characteristics of the communication channel.