CSCI 4171/CSCI 6704 Networks ONLINE QUIZ NO. 2 THURSDAY, OCTOBER 29, 2020, 10 – 11.30 AM Length of the quiz: 70 minutes plus 10 minutes grace time TOPICS TOPICS Module 3: Basic LANs • Design Issue 2: Digital Encoding Schemes o Unipolar, NRZ, Manchester, Block Encoding Schemes • Design Issue 3: Framing o Length field in each frame, Starting and Ending flags in each frame • Design Issue 4: Error Control o Error detection and correction o Single bit errors and Burst errors o Parity Check – 1d and 2d o Cyclic Redundancy Check – Complete • Design Issue 5: Medium Access Control o CSMA/CD o Token Passing Module 4: Interconnected LANs • Interconnected LANs with Repeaters/Hubs • Interconnected LANs with Bridges/Switches o Bridge Operation o Bridge Learning • Collision and Broadcast Domains • Interconnected LANs with Routers o Router Functions o Operation Example 1: Single Router interconnecting multiple LANs o Operation Example 2: Multiple Routers interconnecting multiple LANs o Routing Terminology o IP Addresses o Routing Process with examples o Cisco Routing Table entries What to Study: • Lecture Notes: Modules 3 and 4 • Practice Question Bank • Assignment 2 exercises (relevant ones) CSCI 4171/CSCI 6704 Networks PRACTICE QUESTION BANK ONLINE QUIZ NO. 1 Note 1: This is meant to be a question bank covering the types of questions you will get on the quiz. The actual number of questions that will be on the quiz will be less. It will be designed to fit approximately 70 minutes of answering time. Note 2: The question bank contains representative samples of different topics. Please be sure to study all the topics for the quiz. Question 1. Multiple Choice Set - For each of the following questions, select the most appropriate answer: 1. Which encoding type always has an average non-zero DC component? a. Unipolar b. NRZ c. Manchester d. None of the above 2. A 100000 bps line experiences a voltage surge for a duration of 10 microsecs. The length of the resulting burst error that corrupts the data on that line is a. 1 bit b. 10 bits c. 100 bits d. 10000 bits 3. The generator polynomial is x8 + x7 + x3 + 1. If this is used in CRC, then a. all burst errors of 9 bits or less can be detected with 100% probability b. all burst errors of 8 bits or less can be detected with 100% probability c. all burst errors of 7 bits or less can be detected with 100% probability d. all burst errors of 6 bits or less can be detected with 100% probability 4. Which of the following interconnection devices separates LANs into different collision domains? a. Routers only b. Routers and switches c. Routers, switches and bridges d. Routers, switches, bridges and repeaters 5. Which of the following interconnection devices separates LANs into different broadcast domains? a. Routers only b. Routers and switches c. Routers, switches and bridges d. Routers, switches, bridges and repeaters 6. In CSMA/CD, a jam signal is sent by a station detecting collision for the following reason: a. To ensure that it gets to transmit first b. To ensure that the collision is absorbed c. To ensure that stations on the same segment know of the collision d. To ensure that stations behind a repeater know of the collision. 7. A minimum frame size is required in CSMA/CD for the following reason: a. To disallow one station from sending multiple frames sequentially when collisions don’t occur. b. To ensure that two stations that are at the farthest distance can hear each other’s collision. c. To ensure that collisions are reduced. d. To ensure that multiple frames from the same station are not corrupted due to one collision. 8. The following interconnection device is mainly used for boosting signal levels and distance extension. a. Repeater b. Bridge c. Router d. Gateway 9. When a collision is detected in a network using CSMA/CD, _____________________ a. The frame is immediately resent b. A jam signal is sent by the station c. The backoff value is set to 0 d. The backoff value is decremented by 1 10. A bridge updates its forwarding database based on the _____________________ address and forwards/discards the frame based on the ______________________ address in the frame, respectively: a. Destination MAC, Source MAC b. Source MAC, Destination MAC c. Source IP, Destination IP d. Destination IP, Source IP 11. Into how many classes are IPv4 addresses divided? a. Two b. Three c. Four d. Five 12. Of the above, how many classes can be used to address individual hosts? a. Two b. Three c. Four d. Five 13. The IP address 255.255.255.0 is a a. Default class A mask b. Default class B mask c. Default class C mask d. Default class D mask 14. The IP address 192.161.255.255 is a a. Host IP address b. Router IP address c. Gateway IP address d. Broadcast IP address 15. The IP address 192.161.255.255 is a a. Host IP address b. Router IP address c. Gateway IP address d. Broadcast IP address 16. The total number of networks that can be addressed in Class A is a. 125 b. 126 c. 127 d. 128 17. If there is a host specific entry in a routing table, the mask for that entry most likely is a. 255.0.0.0 b. 255.255.0.0 c. 255.255.255.0 d. 255.255.255.255 18. If the mask and the destination address are both 0.0.0.0 in a routing table, the entry is most likely a. Network specific b. Next hop c. Default d. Directly connected 19. In order to extract the network address, the router performs an AND operation on the following two entities: a. Destination IP address in the packet and Destination MAC address b. Destination IP address in the packet and corresponding mask c. Destination network address and corresponding mask d. Destination MAC address and corresponding mask 20. The administrative distance in a Cisco routing table entry specifies a. the routing protocol that generated the entry. b. the address of the next hop router. c. the mask. d. the trustworthiness of the route Question 2. The following signal is NRZ-encoded. A and -A represent +5 and -5 volts, respectively. What is the corresponding bit pattern? Question 3. The following signal is Manchester-encoded. A and -A represent +5 and -5 volts, respectively. What is the corresponding bit pattern? A 0 -A Question 4. The following bit stream is received by a host running a protocol with starting and ending flags and bit stuffing. The starting and ending flags are both 01111110. How many frames are being received? What is the actual content of each frame before the flags are added and the bits are stuffed? 0111111011111011110011111100111111000000111110101111110 Question 5. Suppose a sender-receiver pair use single bit even parity check for error detection. Suppose the sender has the message 1 1 1 0 1 1 1 to send. a) What is the actual bit pattern that is transmitted? b) Suppose during transmission, the first bit is in error. Show how the receiver is able to detect the error. c) In general, how many errors can be detected using single bit parity check? Question 6. Suppose a sender-receiver pair use 2d even parity check for error detection. Suppose the sender has the message 1010 1111 1001 0011 to be transmitted. Assume that the sender-receiver have agreed upon four-bit blocks for computing row parities. a) What is the actual bit pattern that is transmitted? b) Suppose during transmission, the first bit is in error. Show how the receiver is able to detect the error. Is the receiver also able to correct the error? c) In general, what is the error detection capability of 2d parity check? Question 7: Consider the reference polynomial x4 + x3 + 1 used in a CRC. A message M(x) 1010101010 is to be sent. Answer the following questions: a) What is the binary number represented by the CRC reference polynomial? b) How many zeros are appended to M(x) to convert it into P(x)? c) What is the remainder that is attached to M(x)? (Compute and write the answer). d) What is the value of P(x)? (Write it as a bit string). e) How many bit errors can this CRC detect with 100% probability? f) If the CRC generation was implemented with a linear feedback shift register, how many shift register blocks will the circuit have? How many XOR gates will the circuit have? Question 8: A sender and receiver communicate with each other using CRC with a reference polynomial x3 + 1. The receiver gets a bit string 11110000110. Are there any errors in this bit string? (Compute and write the answer). Question 9: Consider the following linear feedback shift register circuit used for CRC generation: a) What is the CRC reference polynomial represented by the circuit? (Write it as a binary number). b) At a particular time step, let the bits in the shift register blocks be, respectively, 1 1 1 1 0 0 1 0 0 (from left to right). Let the input bit into the circuit be 1. What will be the contents of the shift register blocks in the next time step? Question 10. Suppose nodes A and C experience collision for the third consecutive time on an Ethernet running CSMA/CD and go into the binary exponential backoff algorithm. What is the range of random numbers generated by A and C in order to calculate the backoff time on their next attempt? Question 11. How many collision and broadcast domains are there in the following network configuration? Question 12: Write the forwarding databases for Bridge B1 and B2. Assume that the bridge learning algorithm has been completed. Question 13: For the same configuration as above, assume that the forwarding databases are all empty. The table below shows some frames that are being transmitted in sequence. Write the forwarding database update in each bridge frame is transferred. If there is no update for that frame, write NO UPDATE. Also indicate how the frame is handled by the bridge: I. Forwarded on the correct port OR II. Forwarded on all out ports OR III. Discarded. (Just write the numbers I, II or III for Handling by B1 and Handling by B2). Frame FDB update in B1 Handling by B1 FDB update in B2 Handling by B2 1: A à F 2: A à E 3: C à D 4: F à D 5: B à C 6: B àA 7: F à C E F A B G C D Question 14: Study the following internetwork with routers Toronto, Halifax, Calgary and Saint John interconnecting 9 networks. Fill in the entries in the routing table for router Halifax. Assume that routing information has been exchanged and all the routers know how to reach every other network using the least number of hops. There is one host specific entry– the route for the host 192.168.4.1 from Halifax is through Saint John. Note: Default route for Halifax is Toronto. ROUTING TABLE FOR HALIFAX MASK DESTINATION NEXT HOP INTERFACE Toronto Halifax Saint John Calgary 192.168.1.0 192.168.2.0 192.168.3.0 192.168.4.0 192.168.5.0 192.168.6.0 192.168.7.0 192.168.8.0 192.168.4.1 1.1 5.1 5.2 2.1 7.1 7.2 8.2 8.1 4.2 6.2 6.1 E0 S0 S1 S1 S0 E0 S1 E0 3.1 S0 S1 E0 S0 Using the above table, show how packets with the following destination addresses are handled by Halifax: a) 192.168.2.5 b) 192.168.1.1 c) 3.4.5.6 Question 15: a) Identify the network class to which each of the following IP addresses belong. Identify the network number in each case. If it is an invalid number, indicate so. 139.108.10.2 222.100.6.37 19.3.24.119 b) The maximum number of hosts that can be supported on a class C network is: c) An IP address starts with the bit pattern 10. This must be a Class --- address. d) Classify each of the following IP addresses into one of the following special addresses: network address, direct broadcast address, limited broadcast address, specific host address, loopback address: IP ADDRESS Special address type 169.5.255.255 10.0.0.0 255.255.255.255 127.45.89.98
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