Section A — 1 Mark Questions
1. Define congestion in a network. [1 Mark]
Answer: Congestion occurs when the number of packets being sent to a network exceeds the capacity of the network resources to handle them.
2. What is the primary goal of congestion control? [1 Mark]
Answer: The goal is to ensure the network can handle traffic and avoid reaching the point where performance degrades significantly.
3. Name one congestion prevention policy. [1 Mark]
Answer: Retransmission policy.
4. What does QoS stand for? [1 Mark]
Answer: Quality of Service.
5. What is the purpose of traffic shaping? [1 Mark]
Answer: It is a technique to regulate the average rate and burstiness of data flow entering a network.
6. Name one type of traffic based on arrival pattern. [1 Mark]
Answer: Constant Bit Rate (CBR) traffic.
7. Does the leaky bucket algorithm smooth out bursty traffic? [1 Mark]
Answer: Yes, it converts bursty traffic into a constant rate output.
8. In a token bucket, what does a token represent? [1 Mark]
Answer: A token represents permission to send a specific amount of data into the network.
9. What is the state of the network when it reaches its maximum capacity? [1 Mark]
Answer: It is said to be saturated or congested.
10. Define jitter in the context of QoS. [1 Mark]
Answer: Jitter is the variation in the delay of received packets.
11. Which algorithm allows for some amount of burstiness by accumulating tokens? [1 Mark]
Answer: The token bucket algorithm.
12. What does an acknowledgement policy refer to in congestion control? [1 Mark]
Answer: It determines how frequently or in what manner the receiver acknowledges received packets to influence the sender's transmission rate.
13. Mention one parameter used to define QoS. [1 Mark]
Answer: Reliability.
14. Is 'flow control' the same as 'congestion control'? [1 Mark]
Answer: No, flow control is node-to-node between sender and receiver, while congestion control involves the entire subnet.
15. What happens to packets that arrive when a leaky bucket is full? [1 Mark]
Answer: They are discarded.
16. Can traffic shaping increase the total network bandwidth? [1 Mark]
Answer: No, it only regulates the flow of data.
17. What is the significance of the buffer in a leaky bucket? [1 Mark]
Answer: It holds incoming bursts of data to be released at a steady rate.
18. What is variable bit rate (VBR) traffic? [1 Mark]
Answer: Traffic where the bit rate fluctuates over time.
19. What does an out-of-order delivery impact in QoS? [1 Mark]
Answer: It impacts delay and the need for reordering, affecting latency.
20. Why is congestion control required at the transport layer? [1 Mark]
Answer: To prevent network collapse caused by packet loss and excessive retransmissions.
Section B — 2/3 Mark Questions
1. Explain the difference between congestion and flow control. [2 Marks]
Answer: Flow control is a point-to-point mechanism meant to prevent the sender from overwhelming the receiver's buffer. Congestion control is a global mechanism meant to prevent the entire network subnet from becoming overwhelmed by too much data.
2. List three key QoS parameters. [3 Marks]
Answer: The three parameters are: 1. Bandwidth (throughput), 2. Latency (delay), and 3. Jitter (variation in delay).
3. Describe the basic working principle of the leaky bucket algorithm. [2 Marks]
Answer: The leaky bucket algorithm uses a buffer that stores incoming packets. These packets are then sent out from the bucket at a constant, fixed rate regardless of how bursty the input traffic was.
4. What are the advantages of token bucket over leaky bucket? [3 Marks]
Answer: Unlike the leaky bucket, which only outputs at a fixed rate, the token bucket allows for bursts of traffic if sufficient tokens have been accumulated. This makes it more flexible for handling applications that occasionally need high throughput.
5. Explain how the timeout policy acts as a congestion prevention strategy. [2 Marks]
Answer: If a sender waits too long for an acknowledgement, it assumes the packet was lost due to congestion and triggers a retransmission, often backed by a backoff algorithm to reduce the initial load.
6. Describe two types of traffic classes based on QoS requirements. [2 Marks]
Answer: 1. Real-time traffic (e.g., VoIP), which is sensitive to delay and jitter. 2. Non-real-time traffic (e.g., file transfer), which is sensitive to packet loss but more tolerant of delay.
7. How does packet discard policy affect network congestion? [3 Marks]
Answer: When a router is congested, it may be forced to drop packets. Intelligent discard policies, like Random Early Detection (RED), drop packets before buffers are full to signal the sender to reduce the transmission rate, thereby preventing complete buffer overflow.
8. What is the role of traffic shaping in a network? [2 Marks]
Answer: Traffic shaping regulates the rate of data transmission to match the network's capacity and maintain a specific traffic profile, preventing congestion before it occurs.
9. Why is it difficult to provide guaranteed QoS in a connectionless network? [3 Marks]
Answer: In connectionless networks, packets may take different routes, encounter unpredictable loads at different routers, and do not reserve resources in advance. This makes it challenging to maintain consistent delivery times, bandwidth, or jitter levels.
10. Explain the concept of bursty traffic. [2 Marks]
Answer: Bursty traffic refers to data transmission where periods of high activity (bursts) are followed by periods of little to no activity, creating challenges for network resource allocation.
Section C — 5 Mark Questions
1. Explain the Leaky Bucket algorithm in detail including its operation and limitations. [5 Marks]
Answer:
Concept
The leaky bucket algorithm is used to control the rate at which data is sent into a network, ensuring that bursty traffic is transformed into a steady stream of data.
Operation
- Incoming packets are placed into a bucket (buffer) of finite size.
- The bucket has a small hole at the bottom.
- Packets are removed from the bucket and transmitted into the network at a constant, fixed rate.
- If the bucket is full when a new packet arrives, the packet is discarded.
Limitations
- The algorithm enforces a rigid constant output rate, which does not allow for bursts.
- If the bucket is full, excess data is simply dropped, which may be wasteful for traffic that could have been accommodated if a small burst was allowed.
2. Discuss the Token Bucket algorithm and how it addresses the shortcomings of the Leaky Bucket. [5 Marks]
Answer:
Concept
The token bucket algorithm allows for limited burstiness while still enforcing an average transmission rate over time.
Mechanism
- A bucket is filled with tokens at a constant rate until it reaches a maximum capacity.
- For a packet to be transmitted, it must consume a token from the bucket.
- If the bucket contains enough tokens, a burst of packets can be sent immediately.
- If the bucket is empty, the sender must wait for new tokens to be generated.
Comparison to Leaky Bucket
The primary advantage of the token bucket is that it provides flexibility. While the leaky bucket enforces a strict constant rate, the token bucket permits a sender to send bursts up to the capacity of the bucket, provided there is enough credit (tokens) accumulated, making it more efficient for variable-rate applications.
3. Describe the principles of congestion control and explain the difference between open-loop and closed-loop policies. [5 Marks]
Answer:
Congestion Control Principles
Congestion control is the process of managing the amount of data being injected into the network to prevent performance collapse. The goal is to keep the network load below the saturation point.
Open-Loop Policies
These policies attempt to solve the problem by good design rather than reaction. They focus on removing the cause of congestion at the source or receiver (e.g., retransmission policies, admission control). They operate based on predetermined rules without direct feedback from the network.
Closed-Loop Policies
These policies are based on the concept of a feedback loop. The network monitors the load and sends information back to the source or destination. Based on this feedback, the sender adjusts its transmission rate to alleviate congestion. Examples include backpressure signals and explicit congestion notification (ECN).
4. Elaborate on the various QoS parameters and why they are critical for modern network applications. [5 Marks]
Answer:
Introduction
QoS refers to a set of technologies that manage data traffic to reduce packet loss, delay, and jitter in a network.
Key Parameters
- Bandwidth: The capacity of the network to transfer data; critical for high-definition streaming.
- Latency (Delay): The time it takes for a packet to travel from source to destination; essential for real-time interactive apps.
- Jitter: The variance in packet delay; high jitter causes distortion in audio/video playback.
- Packet Loss: The percentage of packets that fail to reach their destination, which requires retransmissions.
Application Significance
Different applications have different priorities. For instance, VoIP requires low latency and low jitter but can tolerate minor packet loss. Conversely, file transfers require high reliability and are less sensitive to minor delays. QoS mechanisms ensure that resources are prioritized to meet these specific needs.
5. Explain traffic shaping and its role in maintaining Quality of Service (QoS). [5 Marks]
Answer:
What is Traffic Shaping?
Traffic shaping is a traffic management technique that buffers excess data packets and releases them at a controlled rate to ensure the flow conforms to a specific contract or profile.
Role in QoS
- Smooths Out Bursts: By controlling the rate, it prevents sudden spikes in traffic that could overwhelm downstream routers.
- Reduces Congestion: It prevents the buffer overflow at the next hop by regulating the flow of data.
- Improves Predictability: By creating a more consistent flow, it helps in maintaining predictable latency and jitter, which are critical for QoS-sensitive applications like video conferencing.
- Enforces Traffic Contracts: It ensures that users comply with their agreed-upon bandwidth limits, preventing any single user from monopolizing network resources.