Section A — 1 Mark Questions
1. What is the primary function of the Data Link Layer regarding flow control? [1 Mark]
Answer: It ensures that the sender does not overwhelm the receiver with data by controlling the rate of transmission.
2. Define a noiseless channel. [1 Mark]
Answer: A noiseless channel is one where the link is assumed to be perfect and no frames are lost or damaged.
3. What is the simplest protocol for a noiseless channel? [1 Mark]
Answer: The Simplest Protocol, which has no flow or error control.
4. In the Stop-and-Wait protocol, what does the sender do after sending a frame? [1 Mark]
Answer: It waits for an acknowledgment from the receiver before sending the next frame.
5. What is the main purpose of an ARQ protocol? [1 Mark]
Answer: To provide error control by combining error detection with retransmission of lost or damaged frames.
6. In Go-Back-N ARQ, what happens if a frame is damaged? [1 Mark]
Answer: The receiver discards the damaged frame and all subsequent frames, and the sender must retransmit all frames starting from the damaged one.
7. What does ARQ stand for? [1 Mark]
Answer: Automatic Repeat Request.
8. What is a piggybacking acknowledgment? [1 Mark]
Answer: It is the process of sending an acknowledgment inside a data frame going in the opposite direction.
9. In Selective Repeat ARQ, what frames are retransmitted? [1 Mark]
Answer: Only the specific frames that are lost or damaged are retransmitted.
10. Define the sender window size in Go-Back-N. [1 Mark]
Answer: It is the maximum number of unacknowledged frames that the sender can hold.
11. Does the Simplest Protocol use flow control? [1 Mark]
Answer: No, it does not use any flow control.
12. What is a time-out event in Stop-and-Wait ARQ? [1 Mark]
Answer: An event where the sender retransmits a frame if an acknowledgment is not received within a specified period.
13. Is the Go-Back-N protocol efficient for noisy channels with high error rates? [1 Mark]
Answer: No, because it discards all subsequent frames after an error occurs, leading to significant retransmissions.
14. What is a sequence number in data link protocols? [1 Mark]
Answer: A number assigned to each frame to allow the receiver to identify frames and reorder them if necessary.
15. What happens to the window size in Selective Repeat ARQ? [1 Mark]
Answer: It is usually smaller than the window size of Go-Back-N, typically half the sequence number space.
16. What does the receiver do in the Stop-and-Wait protocol when it receives a frame? [1 Mark]
Answer: It sends an acknowledgment to the sender.
17. Identify one major difference between Stop-and-Wait and Go-Back-N. [1 Mark]
Answer: Stop-and-Wait sends only one frame at a time, whereas Go-Back-N allows multiple frames in transit.
18. What is the main advantage of Selective Repeat ARQ? [1 Mark]
Answer: It is more efficient than Go-Back-N as it only resends the specific lost frames.
19. What is a noisy channel? [1 Mark]
Answer: A channel where data can be corrupted, lost, or delayed during transmission.
20. Why is a window size used in ARQ protocols? [1 Mark]
Answer: To limit the number of outstanding frames and manage flow control efficiently.
Section B — 2/3 Mark Questions
1. Explain why the Simplest Protocol is impractical for real networks. [2 Marks]
Answer: The Simplest Protocol assumes a noiseless channel and has no flow control. In real networks, channels are inherently noisy and receivers have limited buffers, meaning frames will be lost and the receiver will be easily overwhelmed.
2. Compare Go-Back-N and Selective Repeat ARQ in terms of efficiency. [3 Marks]
Answer: Go-Back-N is less efficient because it discards all subsequent frames after an error, causing redundant retransmissions. Selective Repeat is more efficient as it selectively retransmits only the corrupted or lost frames. However, Selective Repeat requires more complex logic and sorting at the receiver end.
3. What is the role of a timer in Stop-and-Wait ARQ? [2 Marks]
Answer: The timer handles frame loss. If an acknowledgment is not received within the timeout interval, the sender assumes the frame or the acknowledgment was lost and retransmits the original frame.
4. Why is Selective Repeat ARQ preferred for channels with high error rates? [3 Marks]
Answer: In high-error-rate channels, frames are lost frequently. Go-Back-N would waste bandwidth by constantly resetting and retransmitting entire windows of data. Selective Repeat avoids this by resending only the specific problematic frames, preserving channel capacity for new data.
5. Explain the concept of the window size in sliding window protocols. [2 Marks]
Answer: The window size defines the maximum number of outstanding frames the sender can transmit without receiving an acknowledgment. It acts as a flow control mechanism to prevent the sender from flooding the receiver.
6. Describe how an acknowledgment works in Stop-and-Wait ARQ. [2 Marks]
Answer: When the receiver successfully gets a frame, it sends an acknowledgment back to the sender. Upon receiving the acknowledgment, the sender clears the buffer and proceeds to send the next frame.
7. Differentiate between noiseless and noisy channels. [3 Marks]
Answer: A noiseless channel assumes no frames are lost, damaged, or delayed. Protocols like the Simplest Protocol suffice here. A noisy channel accounts for transmission errors, requiring protocols like Stop-and-Wait ARQ, Go-Back-N, or Selective Repeat to detect and correct errors via retransmission.
8. What is the drawback of having a very small window size? [2 Marks]
Answer: A very small window size reduces the channel utilization because the sender spends too much time waiting for acknowledgments, leading to low throughput despite the protocol's safety.
9. How are frames identified in ARQ protocols? [2 Marks]
Answer: Frames are identified using sequence numbers. These numbers allow both the sender and receiver to keep track of which frames have been sent, received, or need to be retransmitted.
10. Explain the concept of ARQ. [3 Marks]
Answer: ARQ is a protocol for error control that combines error detection (such as CRC) with acknowledgments and retransmissions. If an error is detected or an acknowledgment is missing, the sender automatically repeats the transmission of the frame to ensure reliability.
Section C — 5 Mark Questions
1. Explain the mechanism of the Stop-and-Wait ARQ protocol in detail. [5 Marks]
Explanation
Stop-and-Wait ARQ is an enhancement of the Stop-and-Wait protocol used in noisy channels. It provides a reliable data transfer mechanism by requiring an acknowledgment for every transmitted frame.
Working Principle
- The sender transmits a single frame and starts a timer.
- The receiver receives the frame, checks for errors, and sends an ACK if the frame is correct.
- If the sender receives the ACK before the timer expires, it sends the next frame.
- If the timer expires before an ACK is received, the sender assumes the frame or the ACK was lost and retransmits the frame.
Key Features
- Uses sequence numbers (0 and 1) to identify frames.
- Handles lost data frames and lost acknowledgment frames.
- Provides simple error detection and recovery at the cost of low link utilization.
2. Discuss the working and limitations of Go-Back-N ARQ. [5 Marks]
Working Principle
Go-Back-N allows the sender to send multiple frames up to a defined window size without waiting for an individual ACK for each. The receiver keeps track of the expected sequence number.
Error Handling
- If a frame is corrupted, the receiver rejects it and all subsequent frames received until the correct one is re-sent.
- The sender, upon timeout, retransmits the lost frame and all frames that followed it.
Limitations
- Inefficient in noisy channels as it discards correctly received frames following a bad one.
- High overhead in terms of bandwidth due to the retransmission of a large number of potentially healthy frames.
- Requires the sender to maintain a large buffer to store transmitted but unacknowledged frames.
3. Elaborate on Selective Repeat ARQ and how it overcomes the drawbacks of Go-Back-N. [5 Marks]
Explanation
Selective Repeat ARQ is designed to maximize efficiency by retransmitting only the specific frames that were lost or damaged. Unlike Go-Back-N, the receiver in Selective Repeat accepts and buffers out-of-order frames.
Mechanism
- The receiver maintains a receive window and accepts frames that arrive out of order, provided they are within the window.
- When a frame is found to be lost or corrupted, the receiver sends a negative acknowledgment (NACK) or the sender times out specifically for that frame.
- Only the specific missing frame is retransmitted.
- Once all missing frames arrive, the receiver delivers the data to the upper layer in the correct sequence.
Advantages
Selective Repeat offers higher throughput compared to Go-Back-N, especially in high-error environments, as it avoids unnecessary retransmissions of successfully received data.
4. Compare the three main ARQ protocols: Stop-and-Wait, Go-Back-N, and Selective Repeat. [5 Marks]
| Protocol | Efficiency | Complexity | Retransmission Logic |
|---|---|---|---|
| Stop-and-Wait | Low | Low | Resends the single frame |
| Go-Back-N | Medium | Medium | Resends the bad frame + all following |
| Selective Repeat | High | High | Resends only the bad frame |
Summary
Stop-and-Wait is the simplest but least efficient. Go-Back-N balances complexity and performance for moderate error rates. Selective Repeat is the most sophisticated and efficient, ideal for noisy links, but requires complex buffering and logic at both ends.
5. Explain the concept of flow control and how sliding window protocols implement it. [5 Marks]
Concept of Flow Control
Flow control is a set of procedures used to restrict the amount of data that a sender can transmit before it waits for an acknowledgment from the receiver. This prevents a fast sender from overflowing a slow receiver's buffer.
Sliding Window Mechanism
- The sender and receiver both maintain a window of a certain size.
- The sender's window represents the frames that can be sent or are currently in transit.
- As acknowledgments arrive, the sender's window slides forward, allowing new frames to be transmitted.
- This method provides a continuous stream of data while still ensuring the receiver is not overwhelmed.
Significance
By defining window sizes (such as N in Go-Back-N or smaller sizes in Selective Repeat), the protocol effectively manages the balance between throughput and link reliability, ensuring data integrity in the Data Link Layer.