Reliable data transfer mechanisms in Computer Networks - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

How does the Stop-and-Wait protocol ensure reliable data transfer?

Which mechanism in the Stop-and-Wait protocol guarantees that data is reliably received by the receiver?

A The sender uses a checksum to detect errors but does not wait for acknowledgments.

B The sender sends all packets continuously without waiting for any response.

C The sender waits for an acknowledgment before sending the next packet.

D The receiver sends data packets back to the sender to confirm receipt.

💡 Hint

Think about how the sender knows the receiver got the data correctly.

In Stop-and-Wait, the sender sends one packet and waits for an acknowledgment (ACK) from the receiver before sending the next. This ensures that each packet is received correctly before moving on.

📋 Factual

intermediate

What is the main disadvantage of the Go-Back-N protocol?

Which of the following best describes the main drawback of the Go-Back-N reliable data transfer protocol?

A It retransmits all packets after a lost or corrupted packet, even if some were received correctly.

B It requires the receiver to buffer out-of-order packets.

C It sends only one packet at a time, causing low throughput.

D It does not use sequence numbers, causing confusion in packet order.

💡 Hint

Consider what happens when a packet is lost in the middle of a sequence.

Go-Back-N retransmits the lost packet and all subsequent packets, even if some were received correctly, which can waste bandwidth.

🔍 Analysis

advanced

Comparing Selective Repeat and Go-Back-N protocols

Which statement correctly compares Selective Repeat and Go-Back-N protocols in terms of efficiency and complexity?

A Go-Back-N requires the receiver to buffer out-of-order packets, making it more complex than Selective Repeat.

B Go-Back-N is more efficient and simpler because it retransmits only lost packets without buffering.

C Selective Repeat is simpler because it sends one packet at a time and waits for acknowledgment.

D Selective Repeat is more efficient but more complex because it retransmits only erroneous packets and requires buffering out-of-order packets.

💡 Hint

Think about how each protocol handles lost or corrupted packets and buffering.

Selective Repeat retransmits only the packets that are lost or corrupted and buffers out-of-order packets, making it more efficient but also more complex than Go-Back-N, which retransmits all packets after a loss and does not require buffering.

❓ Reasoning

advanced

Why is sequence numbering important in reliable data transfer?

What is the primary reason reliable data transfer protocols use sequence numbers for packets?

A To detect duplicate packets and ensure correct ordering of received data.

B To encrypt the data for security during transmission.

C To increase the speed of data transmission by skipping acknowledgments.

D To compress the data and reduce packet size.

💡 Hint

Consider what happens if packets arrive out of order or are repeated.

Sequence numbers help the receiver identify duplicates and place packets in the correct order, which is essential for reliable data transfer.

🚀 Application

expert

Calculating throughput in a Stop-and-Wait protocol

Assume a Stop-and-Wait protocol is used over a link with a bandwidth of 1 Mbps and a round-trip time (RTT) of 100 ms. Each data packet is 1000 bits. What is the approximate throughput of this protocol?

A 5,000 bits per second

B 10,000 bits per second

C 1,000,000 bits per second

D 100,000 bits per second

💡 Hint

Throughput = Packet size / (RTT + Transmission time). Calculate transmission time as packet size divided by bandwidth.

Transmission time = 1000 bits / 1,000,000 bps = 0.001 seconds (1 ms). RTT = 0.1 seconds (100 ms). Total time per packet = 0.101 seconds. Throughput = 1000 bits / 0.101 s ≈ 9,900 bits per second, approximately 10,000 bps.