📝 DiffServ Differentiated Services in QoS (12 MCQs)
📖 From Data Communication and Networks • 30. Quality of Service • 12 questions available
What is DiffServ Differentiated Services in QoS?
Differentiated Services is a coarse-grained QoS architecture that classifies traffic into a small number of aggregate behavior classes at network edges, allowing core routers to forward packets based on simple markings without maintaining per-flow state.
📝 All DiffServ Differentiated Services in QoS MCQs
Q1. What is the primary purpose of the leaky bucket mechanism in Differentiated Services?
📖 Explanation: The leaky bucket is used as a traffic‑shaping tool that limits the average transmission rate of a flow. By allowing bursts up to the bucket size but then leaking at a constant rate, it ensures that the flow does not exceed the configured bandwidth, protecting the network from overload.
Q2. If a token bucket generates tokens at 2 Mbps and its bucket size is 1 MB, what is the maximum burst duration that can be transmitted without loss?
📖 Explanation: A token bucket can accumulate up to its bucket size before transmission. Converting 1 MB to bits gives 8 Mbit. Dividing this by the token generation rate (2 Mbps) yields 4 seconds. Therefore, a burst lasting four seconds can be sent at line rate before the bucket empties and packets would be dropped.
Q3. Which statement correctly distinguishes priority queuing from weighted fair queuing in DFFSERV?
📖 Explanation: Priority queuing processes all packets of the highest‑priority class first, ignoring lower classes until the high‑priority queue empties. Weighted Fair Queuing, on the other hand, assigns each flow a weight and distributes the link capacity proportionally, ensuring that every flow receives a share of bandwidth regardless of its class.
Q4. Given high‑priority arrivals at t=1,3,5 and low‑priority arrivals at t=2,4,6 with a service time of one unit, which packet departs first under priority queuing?
📖 Explanation: In priority queuing, any high‑priority packet present in the queue is served before low‑priority ones. At t=1 a high‑priority packet arrives and is immediately processed, so it departs before the low‑priority packet that arrives at t=2. Consequently, the first departure is the high‑priority packet from t=1.
Q5. If the low‑priority queue is full at t=4 and another low‑priority packet arrives at t=5, what happens to the arriving packet?
📖 Explanation: When a queue reaches its maximum capacity, any additional packets destined for that queue cannot be stored. The standard behavior under congestion is to discard the new arrival to avoid overflow. Therefore, the low‑priority packet arriving at t=5 is dropped rather than queued or preempting higher‑priority traffic.
Q6. Increasing the buffer size of the high‑priority queue while keeping the low‑priority queue unchanged will most likely:
📖 Explanation: A larger high‑priority buffer allows more high‑priority packets to be stored without dropping, which directly reduces loss for that class. Although low‑priority packets may experience slightly longer waiting times because the high‑priority queue can occupy the processor longer, the overall system sees fewer dropped packets, improving loss performance.
Q7. Which combination of mechanisms best ensures both bandwidth guarantee and low latency for high‑priority traffic?
📖 Explanation: A token bucket provides a guaranteed rate by limiting the average traffic, while priority queuing ensures that high‑priority packets are transmitted before lower‑priority ones, minimizing queuing delay. The combination therefore offers both a bandwidth reservation and the low‑latency treatment required for premium traffic.
Q8. According to Figure 30.2, if the high‑priority queue becomes empty at t=3 while low‑priority packets remain, what does the switch do?
📖 Explanation: Priority queuing logic dictates that the processor serves the high‑priority queue until it is empty. Once empty, the scheduler immediately begins servicing the low‑priority queue if packets are waiting. Thus, at t=3 the switch transitions to the low‑priority queue to continue processing pending packets.
Q9. A flow sends five 200 kb packets every 0.5 s. The token bucket has a rate of 1 Mbps and a bucket size of 400 kb. Will any packet be dropped?
📖 Explanation: The bucket starts full at 400 kb. After the first packet (200 kb) 200 kb remain. In the next 0.5 s, 500 kb of tokens are generated, but the bucket caps at 400 kb, refilling it. This cycle repeats, always leaving enough tokens for each 200 kb packet, so no packet experiences a shortage and none are dropped.
Q10. To guarantee a minimum rate of 1 Mbps for premium traffic while shaping excess traffic, which DFFSERV element should be configured?
📖 Explanation: A traffic conditioner combines a token‑bucket shaper with classification, allowing the network to enforce a minimum guaranteed rate for premium flows. The conditioner lets traffic up to the committed rate pass unchanged and shapes any excess, ensuring that premium traffic receives its reserved bandwidth while preventing bursts from overwhelming the network.
Q11. When the classifier marks a packet with a higher drop precedence, what is the expected behavior under congestion?
📖 Explanation: Drop precedence is a mechanism used to indicate how aggressively a packet should be treated when the network is congested. A higher precedence means the packet is placed lower in the drop hierarchy, making it more vulnerable to being discarded before lower‑precedence packets when buffers fill.
Q12. Consider Flow X (token bucket: 3 Mbps, 1.5 Mb bucket) and Flow Y (leaky bucket: 2 Mbps leak, 2 Mb bucket). Both send 3 Mb bursts simultaneously. Which flow will first experience packet loss?
📖 Explanation: Flow X can transmit up to its bucket size instantly and then sustain 3 Mbps, so it can accommodate the 3 Mb burst without immediate overflow. Flow Y, limited to a leak rate of 2 Mbps, cannot transmit the burst fast enough; its buffer fills quicker, causing overflow and packet loss before Flow X does.