FACULTY OF COMPUTING AND INFORMATICS

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1 namibia UniVERSITY OF SCIEnCE AnD TECHnOLOGY FACULTY OF COMPUTING AND INFORMATICS DEPARTMENT OF COMPUTER SCIENCE QUALIFICATION: Bachelor of Computer Science {Honours) QUALIFICATION CODE: 08BCSH LEVEL: 8 COURSE: Broadband Networks COURSE CODE : BBN810S DATE: JUNE 2016 SESSION: 1 DURATION: 2 HOURS 30 MARKS: 80 FIRST OPPORTUNITY EXAMINATION QUESTION PAPER EXAMINER{S) Dr GUY_ALAIN LUSILAO ZODI MODERATOR: Dr MTHULISI VELEMPINI THIS QUESTION PAPER CONSISTS OF 6 PAGES (Excluding this front page) INSTRUCTIONS 1. Answer ALL the questions. 2. Write clearly and neatly. 3. Number the answers clearly.

2 QUESTION I a) What desirable property does "statistical multiplexing" have compared to frequency division multiplexing? b) What is the difference between the flow control and congestion control mechanisms provided by TCP c) Why are ATM-based networks preferred to STM? QUESTIOND The diagram shows two network switches S 1 and S2 on the path between hosts A and B. The diagram also shows the link capacities. The propagation time on each link is p sec. "S 1 and S2 are store-and-forward packet switches". 1) Explain what store-and-forward means. 2) Derive a formula for the delay when sending a packet of s bits from A to B. [Ignore all other possible sources of delay and assume transmission starts at time 0]. [1 mark] [5 marks] QUESTION Ill Compare Go-Back-N (GBN), Selective Repeat, and TCP (no delayed ACK). Assume that the timeout values for all three protocols are sufficiently long such that 5 consecutive data segments and their corresponding ACKs can be received (if not lost in the channel) by the receiving host (Host B) and the sending host (Host A) respectively. Suppose Host A sends 5 data segments to Host B, and the 2nd segment (sent from A) is lost. In the end, all 5 data segments have been correctly received by Host B. a) How many segments has Host A sent in total and how many ACKs has Host B sent in total? What are their sequence numbers? Answer this question for all three protocols [6 marks] b) If the timeout values for all three protocol are much longer than 5 RTT, then which protocol successfully delivers all five data segments in shortest time [6 marks] interval? 1

3 QUESTION IV a) Why is round-trip time an appropriate time scale for frames retransmission? b) Let a = 0.2. Suppose for a given TCP connection three acknowledgments have been returned with RTTs: RTT for first ACK = 80 msec; RTT for second ACK = 60 msec; and R TT for third ACK = 100 msec. Determine the value of Estimated RTT after each of the three acknowledgments. [3 marks] c) Name two factors that can cause variance in the network round-trip time? QUESTIONV Suppose that a router has three input flows and one output port. It receives packets continuously as per table below, with all flows beginning at the same time and queues being empty before the arrival of the first packet. Packets order in each separate flow is listed in the table (packets 1 and 2 are the first to arrive). Length represents the number of clock ticks it takes to transmit a packet Packet ID Flow Length Arrival time PI P P P PS P P Determine the order in which packets are transmitted by the router if Fair queuing is used. [5 marks] 2

4 QUESTION VI a) What percentage of an ATM link's total bandwidth is consumed by the ATM [1 mark] cell headers? b) The IP-datagram for a TCP ACK message is 40 bytes long: it contains 20 bytes of TCP header and 20 bytes of IP header. Assume that this ACK is traversing an ATM network that uses AAL5 to encapsulate IP packets. How many ATM packets will it take to carry the ACK? c) Cell switching methods essentially always use virtual circuit routing rather than datagram routing. Give two arguments why this is so. [5 marks] QUESTION VII a) What is the goal of Fair Queueing? [1 marks] b) Assume a link of capacity 10 Mbps that is traversed by four flows with arrival rates of6, 4, 2, and 1 Mbps, respectively. What bandwidth will each [ 4 marks] flow get? (Show all your work.) QUESTION VIII a) Consider an ATM network with a leaky-bucket policer (also known as a token-bucket filter) that ensures a virtual circuit does not send more cells than expected. If the policer has a bucket depth of 10 cells and a token rate of 100 cells/second, how many cells can the virtual circuit transmit during a [3 marks] ten-second period? b) Suppose a link has a propagation delay of P seconds and transmits R cells per second. Suppose a cell arrives when the link has Q other cells awaiting [ 3 marks 1 transmission, and has just started transmitting the first cell. What delay does the new cell experience, as a function ofp, R, and Q? c) Suppose a link has a first-in-first-out queue and carries traffic for two virtual circuits. The traffic on the first virtual circuit obeys a token-bucket filter 3

5 with a bucket depth of 9 cells and a token rate of 100 cells/second, and can settle for best-effort service. The traffic on the second virtual circuit obeys a token-bucket filter with a bucket depth of one cell and a token rate of 100 cells/second, and cannot tolerate more than 100 msec of delay in delivering [ 4 marks] each cell. Suppose the propagation delay on the link is 60 msec. What is the smallest transmission rater, in cells per second that can satisfy the delay constraint, given that the link serves cells in first-in-first-out order? QUESTION IX Suppose a server transmits one frame of a video every second, and the client starts playing the video at one frame per second as soon as the first frame arrives. Suppose the first ten frames arrive at times 0, 0.9, 1.99, 3.01, 4.10, 4.98, 6.05, 8.50, 8.90, 8.99, all in seconds. Note that the counting of frame start from frame 0, that is, frame 0 arrives at time 0, frame 1 at time 0.9, frame 2 at time 1.99, and so forth. a) Which frames reach the client too late for playout? [4 marks] b) If the client imposed a one-second playout delay, which frames would reach the client too late for playout? [4 marks] 4

6 '11 O~NW~~~~~~~P~~~~~~~~NNNNNNNNNNI QUESTION X The Transmission Control Protocol uses a method called congestion control to regulate the traffic entering the network. The behavior of TCP congestion control can be represented as a graph in which the x-axis indicates the time, and they-axis indicates congestion window size. Please use the graph shown below to answer the following questions. Note that the graph does not explicitly show timeouts, but you should be able to figure out when timeouts happened based on the events shown. ~ 30 c <II E ~ 25 ~ <II N Vi "C c 15 c 0 ~ 10 <II Qtl c j 0 I U 5 I i I 0 l : j I ' I I, OPNW~~~~OO~O~NW~~~~~~~ Transmission Round L ~ a) Give two reasons why slow start is used, and explain why it does a better job than congestion avoidance for that function. b) Identify the intervals of time when TCP slow start is operating. For each interval, identify which of the above reasons apply and do not apply and explain why. c) Identify the intervals of time when TCP congestion avoidance is operating. Why congestion avoidance should be used instead of slow Start during these intervals. Please clearly identify one specific reason 5

7 d) Identify the intervals of time when TCP fast retransmission is used. Please explain what fast retransmission does and how it is triggered. e) Identify the intervals of time when TCP fast recovery is operating. What does fast recovery do and explain why is it beneficial? f) Identify the interval(s) of time when fast recovery could have happened, but did not. Identify one specific example of a circumstance that may prevent fast recovery from happening. g) Which version oftcp is represented in this Figure? ===============================End of Exam Paper============================== 6