Computer Networks · TEE · papers
Q2(a) · QoS and scheduling
2024-02-06 · 2022-23
Actual paper; 100 marks attempted. Current-syllabus alignment is stated per question.
Q2(a). Explain briefly the characteristics used to analyse Quality of Service (QoS) of flow/traffic. (Any 4). Illustrate with diagrams the different types of scheduling used to improve QoS. Briefly explain each. [10 marks]
Answer
Quality of Service (QoS) describes delivery characteristics important to an application: throughput/bandwidth, delay, delay variation (jitter) and loss/reliability. Different applications value them differently; a voice call is sensitive to delay/jitter, while a file transfer strongly needs complete data.
| Technique | Mechanism | Tradeoff |
|---|---|---|
| Scheduling | FIFO, priority or fair/weighted queues choose which packet transmits next | Strict priority may starve lower classes; fair scheduling balances shares |
| Traffic shaping | Delay departures to match a profile | Smooths load but adds queueing delay |
| Policing | Measure compliance; mark/drop nonconforming packets according to policy | Controls excess traffic but can lose packets |
| Admission control | Accept new flows only when resources can support them | Protects existing service by refusing excess demand |
| Resource allocation/reservation | Assign bandwidth/buffer/service resources | Requires capacity planning and coordination |
| Classification/marking | Identify traffic classes and carry service markings | Marking alone does not create capacity or guarantees |
Yaad rakho: Bandwidth, delay, jitter, loss; requirement ke hisaab se policy.
Scheduling diagrams
FIFO: arrivals → [one queue] → link
Priority: high → [queue H] ─┐
low → [queue L] ─┴→ choose H first → link
Weighted fair: Q1, Q2, Q3 → scheduler shares 1:2:1 → linkFIFO sends in arrival order, so a long burst can delay urgent traffic. Strict priority chooses the highest nonempty class and can starve lower classes. Fair/weighted fair scheduling shares service among queues according to equal or configured weights, preventing one flow from taking every opportunity. The four QoS dimensions are bandwidth/throughput, delay, jitter and loss/reliability.