Feedback Homework 9 Help You submitted this homework on Sat 22 Mar 2014 5:51 AM PDT. You got a score of 5.00 out of 5.00. Question 1 Lecture 9-1 discussed the quality of service requirements for different applications. This question explores your understanding of the lecture material. Select ALL statements that are correct. When interpreting the lecture slides, note that a "high stringency" for property means that the property matters a great deal to the application. Thus a high stringency for delay means that the delay should be low (not high!) since this is what matters for delay. However, a high stringency for bandwidth means that the bandwidth should be high because this is what matters for bandwidth. We avoid using the term "stringent" below because of the potential for confusion. Telephony (as in voice calls) has demanding quality of service requirements for bandwidth. Voice-over-IP requires lower jitter than web access. Video-conferencing has more demanding quality of service requirements than file sharing. Telephony (as in Skype audio calls) needs the same kind of quality of service as file sharing (as in BitTorrent downloads) 0.25 No, voice calls need little bandwidth (say in the order of about 10 KB/s) from the network. 0.25 Yes, low jitter (or variation in delay) is important for VoIP to prevent gaps, whereas web access is not particularly sensitive to jitter. 0.25 Yes, both video-conferencing and file sharing require high bandwidth, but video-conferencing also requires low packet delays and low jitter. 0.25 No, file sharing wants high bandwidth and is insensitive to delay. Telephone wants low delay/jitter and is needs little bandwidth. https://class.coursera.org/comnetworks-003/quiz/feedback?submission_id=1071407 1/5
Total / Question 2 Consider a router that uses fair queueing, with two queues, for flows F1 and respectively. F1 uses packets of length 1200 bits, and uses packets of length 400 bits. To what flows do the first five packets sent belong? To answer the question, compute the finish times. Assume that the queues are always backlogged and that a router can transmit a packet from either one of the two flows at any point in time.,,,,,,, F1, Yes, this option sends the packets with the lowest finish times first., F1,, F1, F1 F1,,,, Total / Finish times for F1 are 1200, 2400, 3600... Finish times for are 400, 800, 1200, 1600... Packets are transmitted based on the finish time Question 3 https://class.coursera.org/comnetworks-003/quiz/feedback?submission_id=1071407 2/5
Two flows F1 and are shaped by token buckets (R1, B1) = (50 KB/sec, 2000 bytes) and (R2, B2) = (100 KB/sec, 1000 bytes). Which statements are TRUE regarding the flows after they are shaped by the token buckets? Select ALL answers that are correct. For simplicity, use 1 KB = 1000 bytes. F1 can transmit a maximum of 102KB over *two* seconds F1 can transmit a maximum of 50KB over a second can transmit a maximum of 100KB over a second can transmit a maximum of 101KB over a second 0.25 The maximum data transmitted in t secs is bounded by R*t + B = 50000*2 + 2000. 0.25 Did you take into account the fact that traffic can exceed the rate R by B bytes? 0.25 Did you take into account the fact that traffic can exceed the rate R by B bytes? 0.25 The maximum data transmitted in t secs is bounded by R*t + B = 100000*1 + 1000 Total / Question 4 This question covers the differentiated services concept discussed in lecture 9-6. Which statements are TRUE concerning how differentiated services are implemented and used? Select ALL statements that are correct. Quality of service markings on a packet are modified at each router Best effort service traffic typically has the lowest priority 0.25 No, the markings are carried through the network and used to ensure that the packet receives the same service at each router in the network. 0.25 Quality of service markings are sent along with 0.25 https://class.coursera.org/comnetworks-003/quiz/feedback?submission_id=1071407 3/5
each packet Each router maintains state regarding the quality of service level for each flow 0.25 No, routers need only consider the different services because each packet is marked with its service (regardless of its flow). Total / Review the differentiated services architecture; the correct statements are descriptive of it. Question 5 This question explores the strong quality of service guarantee provided by the scheme of lecture 9-7. Consider a (2MBps, 100KB) shaped traffic flow (i.e., rate is set to 2 MBps and burst limited to 100KB). Assume that it traverses two routers R1 and R2 with the following settings: Weight attached to the flow at R1 is 10 Sum of weights of all R1 flows is 100 Capacity of link coming out of R1 is 10 MBps Weight attached to the flow at R2 is 10 Sum of weights of all R2 flows is 200 Capacity of link coming out of R2 is 40 MBps End-to-end end delay for the flow is 100 ms when there is no queueing at R1 or R2 Which statements about the bandwidth and delay guarantees provided for the flow are TRUE? Select ALL statements that are correct. Note that we have given both rates and bursts using bytes (B). For simplicity use K = 1,000 and M = 1,000,000 in your calculations. The flow is bottlenecked at R1 when there is cross-traffic in the network. 0.25 Correct. R1 guarantees only 1 MBps, while R2 guarantees 2 MBps and the token bucket allows 2 MBps. The flow is 0.25 Check what rate is guaranteed by R1. https://class.coursera.org/comnetworks-003/quiz/feedback?submission_id=1071407 4/5
bottlenecked at R2 The flow can achieve a bandwidth rate of 1MBps irrespective of the amount of cross traffic The worst case delay for the flow's packets is 200 ms 0.25 The flow is constrained at router R1 to a rate of 10/100*10MBps = 1MBps 0.25 This is tricky. Since the token bucket can let R = 2 MBps into the network but the network only guarantees 1 MBps at R1, then the queue at R1 can build endlessly and drive up the worst case delay until there is loss. (If the traffic is shaped so that R Total / Work out the rate that the flow is guaranteed at R1 and R2. This will let you find the lowest rate that is guaranteed across the network and reason about how this interacts with the flow after it has been shaped. https://class.coursera.org/comnetworks-003/quiz/feedback?submission_id=1071407 5/5