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BIRLA INSTITUTE OF TECHNOLOGY & SCIENCE, PILANI, DUBAI CAMPUS
DUBAI INTERNATIONAL ACADEMIC CITY, DUBAI
SECOND SEMESTER 20 23 – 2024
DEPARTMENT OF COMPUTER SCIENCE
Marking Scheme COURSE : Computer Networks (CS F303) COMPONENT : Comprehensive Exam (Closed Book) WEIGHTAGE : 35 % ( 70 Marks) DAY & DATE : Friday, 31 - May- 2024 DURATION : 3 hrs ( 8 : 30 am- 11 : 30 am) Note :
- Please write down any assumptions that you make.
- Show the intermittant steps clearly for all answers. Part A Q.1 Consider there are five nodes and IPv4 header format. Node1 computes header checksum with the following initialized values mentioned in boldface. Node1 forwards the packet in the following sequence: N1-N2-N3-N4-N5. Write the answer for the following questions in Hexadecimal representation. a. Compute the header checksum for Node1. b. The packet reached error free till Node3 and assume that least significant bit of fragment offset field is changed when it received by Node4. Illustrate how Node identifies/detects the error, if any. c. Assume that there is no error during the transmission and consider the new sequence N1-N2-N3-N4-N5-N6. What would be the next step when the packet reaches at Node6?
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Questions: 9 Pages: 4
Q.2 Consider two hosts A and B are connected via three router or four hops of links between them. The transmission rates of the links are 10Mbps, 20Mbps, 40Mbps and 80Mbps. The physical length of each hop of the link is 200,000 meters. Assume that the propagation speed over the link is 2 x 10^8 meters/sec. Currently packets of size 40Mbits are sent continuously from Host A to Host B. a. What is the A-to-B transmission delay of a single packet? b. What is the A-to-B propagation delay of a single packet? c. What is the average queuing delay of a packet at the routers if it follows circuit switching?
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Q.1 The organization created a network of 10 routers. They decided to use Open Source Shortest Path (OSPF) inside the network. The organization of considered network is as follows, The network administrator wants to prepare routing table for router A. The structure of routing table they will prefer is as follows. (Show all steps of creating routing table) Source Destination Node Path Cost
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Correct Steps of Calculation: 5M Final correct answer in table form: 5M Source Destination Node Path Cost A B A-B 3 C A-B-F-G-C 9 D A-D 4 E A-E 4 F A-B-F 6 G A-B-F-G 8 H A-E-H 6 I A-E-I 5 J A-E-I-J 12 Q.2 Consider the following network with ID 192.1 6 8.2.0/24 divided into 6 subnetworks LAN1, LAN2, LAN3, WAN link1, WAN link2, WAN link3.
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Find each subnet ID in decimal and binary, subnet mask, and subnet mask in classless notation. Assume that the subnets are organized as largest size to smallest. (Show all steps of calculations)
b. Draw the Timing diagram to illustrate the above scenario in case selective repeat is used. Also calculate the efficiency (ratio of the effective number of frames sent to the total number of frames sent in each of the above cases) Note: No corruption in either data or ACKs and no loss of ACKs
Q.
You are proposing modification in the existing Stop-and Wait protocol and see why each component of the protocol is necessary by creating example of “what could go wrong” when you modify the protocol. Answer the following questions. a. What could go wrong if we modify the original stop-and-wait in such a way that the ACK does not include the sequence number, the receiver sends ACK if and only if a message is received uncorrupted, and the sender always considers the ACK to be the most recent message sent? Complete the partially given timing diagram by turning it into a simple
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Q.
Web clients and servers may send their HTTP requests and replies using the (1) non- persistent, (2) persistent without pipelining, or (3) persistent with pipelining mode. Suppose we have a web page which contains 10 objects, a .html main file, 8 .jpg image, and a .mp background music. The 10 objects are all so small that (1) the transmission time is negligible and (2) each object can be completely transferred in one TCP packet. Assume there are no packet losses, the round-trip time between the server and client is RTT, the time to set up a TCP connection are Ts, and the time to close down a TCP connection is Tc. i. What will be the time it takes to complete transferring the web page running in the Non-persistent mode? ii. What will be the time it takes to complete transferring the web page running in the persistent without pipelining mode? iii. If the web client learns the URL of the 2nd^ to the 10th objects in the page after fully downloading the main html object, what will the entire page download time be using the persistent with pipelining mode? iv. Continue from (iii). Now suppose the html main page, 8 jpg image and the mp3 music objects are located on 10 different Web servers. The Web client can only have one download connection at a time. Assume the delay from the client to all these different Web servers are the same – RTT. What will be the entire page download time? v. Continue from (iv). If the main page and 7 of the jpg image objects are from the same server, the other two objects are from another server, what will be the entire page download time?
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(i) 10Ts+ 10RTT + 10Tc (ii) Ts+ 10RTT + Tc (iii) Ts+ 2RTT + Tc (iv) 10Ts+ 10RTT + 10Tc (v) 2Ts+ 3RTT + 2Tc ********** ALL THE BEST ********
Q. 6 Alice and Bob are located at opposite ends of a 2 0 km cable, each with 5 frames to send to the other. Each frame is 1200 Bytes in size. At t=5 microseconds, Alice tries to transmit her 2nd^ frame for the 2nd^ attempt by sensing the channel, while Bob simultaneously attempts to transmit his 1st^ frame for the 3rd^ attempt. Assume the transmission rate is 15 Mbps, it used CSMA/CD access method as shown in the flowchart. Assume the signal propagation speed is 2*10^8m/sec. a. What is the conditional probability for the following i. Alice achieving a successful transmission for her 2nd^ frame in the 2nd attempt. ii. Bob achieving a successful transmission for his 1st^ frame in the 3rd^ attempt. iii. Collision occurring between Alice's 2nd^ frame in the 2nd^ attempt and Bob's 1 st^ frame in the 3rd^ attempt. b. Following the collision between Alice's 2nd frame in the 2nd^ attempt and Bob's 1st frame in the 3rd^ attempt, Alice draws R=2 and Bob draws R= 7 in the exponential back off protocol. i. What time is Alice's 2nd^ frame completely delivered to Bob? ii. What time is Bob's attempt to sense the channel again after the collision and what is the result of channel sensing?
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Q. 7 Consider five wireless stations, A, B, C, D, and E. Station A can communicate with B, C and D. B can communicate with A, C and E. C can communicate with all the node except E. D can communicate with A, C and E. E can communicate B and D. draw the neat diagram to show the steps of the MACA protocol until the frame is successfully transmitted. Indicate which nodes must be silent (refrained from transmission) during each step. i. When B sends a frame to E. ii. When D sends a frame to A.
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********** ALL THE BEST ********
BIRLA INSTITUTE OF TECHNOLOGY & SCIENCE, PILANI, DUBAI CAMPUS
DUBAI INTERNATIONAL ACADEMIC CITY, DUBAI
SECOND SEMESTER 20 23 – 2024
DEPARTMENT OF COMPUTER SCIENCE
Marking Schme COURSE : Computer Networks (CS F303) COMPONENT : Midsem Exam OPEN BOOK WEIGHTAGE : 3 0% (60 Marks) DAY & DATE : Monday, 1 - April- 2024 DURATION : 90 Minutes ( 11 : 30 am- 1 : 00 pm) Note :
- Please write down any assumptions that you make.
- Show the intermittant steps clearly for all answers. Q. 1 Assume that a message of size 10,000 bits is sent from a Station A to a destination Station D passing through two routers B and C. All three links on the path have a delay of 20ms. Station A has a transmission rate of 100Mbps, B and C have a transmission rate of 1000 bits/sec. Assume that this is a store and forward system, there is no queuing delay. a. Find the end-to-end latency of the message when it is sent as a whole packet. b. B and C takes 5ms each to read the packet and forwards to the destination. Find the end-to-end latency of the message when it is broken into 1000bits/packet and then transmitted to the destination.
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a. 30ms + 2x10020ms = 20.07s b. 30ms + 2x1025ms = 2.08s Q. 2 a. Find the number of links required for a mesh topology of 10 devices with simplex mode b. Find the number of links required for a mesh topology of 20 devices with full duplex mode c. Find the number of links required for a star topology of 10 devices with simplex mode d. Find the number of links required for a star topology of 20 devices with full duplex mode e. Draw the topology for all the above by considering 5 devices.
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a. 10x9 = b. (20x19)/2 = 190 c. 10 d. 40 Questions: 7 Pages: 2
Transmitted value is: 101100110101100101 Q. 5 Consider a network using the pure ALOHA medium access control protocol, where each frame is of length 2000 bits. The channel transmission rate is 2Mbps. The aggregate number of transmissions across all the nodes is modelled as a Poisson process with a rate of 250 frames per second. Throughput is defined as the average number of frames successfully transmitted per second. What is the total throughput of the network?
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Transmission time of frame = Length of frame / Transmission rate = 2000 / 2Mbps = 1 ms System generates 250 frames per second. Hence, G = ¼. Average number of successful transmission are, S = G * e-2G^ = ¼ * e-2(1/4)^ = 0.151. Hence, Total throughput = 0.151 * 250 = 37.75 ≈ 38 frames Q. 6 Alice and Bob are located at opposite ends of a 20 km cable, each with 5 frames to send to the other. Each frame is 1200 Bytes in size. At t=5 microseconds, Alice tries to transmit her 2nd^ frame for the 2nd^ attempt by sensing the channel, while Bob simultaneously attempts to transmit his 1st^ frame for the 3rd^ attempt. Assume the transmission rate is 15 Mbps, it used CSMA/CD access method as shown in the flowchart. Assume the signal propagation speed is 2*10^8m/sec. a. What is the conditional probability for the following i. Alice achieving a successful transmission for her 2nd^ frame in the 2nd attempt. ii. Bob achieving a successful transmission for his 1st^ frame in the 3rd^ attempt. iii. Collision occurring between Alice's 2nd^ frame in the 2nd^ attempt and Bob's 1 st^ frame in the 3rd^ attempt. b. Following the collision between Alice's 2nd frame in the 2nd^ attempt and Bob's 1st frame in the 3rd^ attempt, Alice draws R=2 and Bob draws R= 7 in the exponential back off protocol. i. What time is Alice's 2nd^ frame completely delivered to Bob? ii. What time is Bob's attempt to sense the channel again after the collision and what is the result of channel sensing?
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Q. 7 Consider five wireless stations, A, B, C, D, and E. Station A can communicate with B, C and D. B can communicate with A, C and E. C can communicate with all the node except E. D can communicate with A, C and E. E can communicate B and D. draw the neat diagram to show the steps of the MACA protocol until the frame is successfully transmitted. Indicate which nodes must be silent (refrained from transmission) during each step. i. When B sends a frame to E. ii. When D sends a frame to A.
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