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CSCI 312 – Data Communication and Computer Networks Midterm Review

CSCI 312 – Data Communication and Computer Networks Midterm Review. Rasanjalee DM. Multiple Choice Questions. Question.

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CSCI 312 – Data Communication and Computer Networks Midterm Review

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  1. CSCI 312 – Data Communication and Computer NetworksMidterm Review Rasanjalee DM

  2. Multiple Choice Questions

  3. Question • A network C advertises the CIDR network number 196.41.48/20 (and no other numbers). What network numbers (all 24 bits) could AS C own? (4-bit prefix of second-to-last octet is 0011 = 48) • 196.41.128 (10000000) • 196.41.49 (00110001) • 196.41.64 (01000000) • 196.41.1 (00000001) • 196.41.62 (00111110)

  4. Answer • A network C advertises the CIDR network number 196.41.48/20 (and no other numbers). What network numbers (all 24 bits) could AS C own? (4-bit prefix of second-to-last octet is 0011 = 48) • 196.41.128 (10000000) • 196.41.49 (00110001) • 196.41.64 (01000000) • 196.41.1 (00000001) • 196.41.62 (00111110) Minimum number : 196.41.48.1 Maximum number: (3rd octet all 1s in host part) = 0011 1111 196.41.63.255 Anything in-between is acceptable.

  5. Question • ______ refers to the physical or logical arrangement of a network.  A) Topology  B) Mode of operation  C) Data flow  D) None of the above

  6. Answer • ______ refers to the physical or logical arrangement of a network.  A) Topology  B) Mode of operation  C) Data flow  D) None of the above

  7. QUESTION • The physical layer involves movement of _______ over the physical medium. A) protocols B) dialogs C) programs D) bits

  8. Answer • The physical layer involves movement of _______ over the physical medium. A) protocols B) dialogs C) programs D) bits

  9. Question • In TCP, there can be ______ RTT measurement(s) in progress at any time. A) two • only one C) several • D) None of the choices are correct

  10. Answer • In TCP, there can be ______ RTT measurement(s) in progress at any time. A) two • only one C) several • D) None of the choices are correct

  11. Question • A serious problem can arise in the sliding window operation when either the sending application program creates data slowly or the receiving application program consumes data slowly, or both. This problem is called the ______. A) silly window syndrome B) unexpected syndrome C) window bug D) None of the choices are correct

  12. Answer • A serious problem can arise in the sliding window operation when either the sending application program creates data slowly or the receiving application program consumes data slowly, or both. This problem is called the ______. A) silly window syndrome B) unexpected syndrome C) window bug D) None of the choices are correct

  13. Question • The ports ranging from 1,024 to 49,151 are called ___________ ports. A) well-known B) registered C) dynamic D) None of the choices are correct

  14. Answer • The ports ranging from 1,024 to 49,151 are called ___________ ports. A) well-known B) registered C) dynamic D) None of the choices are correct

  15. Question • RIP uses the services of _______. A) TCP B) UDP C) IP D) None of the choices are correct

  16. Answer • RIP uses the services of _______. A) TCP B) UDP C) IP D) None of the choices are correct

  17. SHORT ANSWER Questions

  18. Question • What is a of soft state of a router. Explain in 2-3 sentences

  19. Answer • Soft state refers to some type of stored state that is not hard • Only stored temporarily by some entity • Must be constantly refreshed or will expire

  20. Question • Provide two examples of a network protocol or technology that abides by soft state

  21. Answer • ARP tables • DNS caches • DHCP addresses

  22. Question • Give two reasons that sites use Network Address Translators (NATs)

  23. Answer • Allows multiple private addresses behind single public (dedicated) address • NATs provide privacy about internal deployments.

  24. Question • You want to implement a mechanism that automates the IP configuration. Which protocol will you use to accomplish this?

  25. Answer • DHCP

  26. Question • What protocol is used to find the hardware address of a local device?

  27. Answer • ARP

  28. Question • What is the need for Network layer?

  29. Answer • The network layer is responsible for the source-to-destination delivery of a packet across multiple network links.

  30. Question • Give four causes of packet delay.

  31. Answer • Processing • Transmission • Propagation • Queueing

  32. Question • What is a tool that can be used to determine the number of hops to a destination and the round trip time (RTT) for each hop?

  33. Answer • traceroute

  34. Question • We have learned that the Internet provides “best effort service.” What is meant by a “best effort” service model?

  35. Answer • Best effort service means that the network layer doesn’t guarantee to deliver a datagram from source to destination. It will try but makes no guarantees.

  36. lONG ANSWER Questions

  37. Question • MAC and Internet architecture: • Define “hub”, “switch” and “router” . what is the difference between these devices? Under what circumstances would you prefer a hub over a switch? • Give 3 benefits of circuit switching over packet switching, and 3 benefits of packet switching over circuit switching. • Describe how bridges/switches learn. In particular, describe specifically (a) what data they store, i.e., what fields/information from packets they store (b) the algorithmic process they use to discover/learn that data

  38. Define “hub”, “switch” and “router” . what is the difference between these devices? Under what circumstances would you prefer a hub over a switch?

  39. Answer: • hub: • used as a repeater to extend networks • switch: • can create point to point connections, better for bandwidth savings • router: • this is a network-level device, forwards at IP layer (while switch/hub are MAC layer)

  40. (b) Give 3 benefits of circuit switching over packet switching, and 3 benefits of packet switching over circuit switching.

  41. Answer: • Circuits: • Guaranteed bandwidth • Not “best-effort” delivery with no real guarantees • Simple abstraction • No worries about lost or out-of-order packets • Simple forwarding • No need to inspect a packet header • Low per-packet overhead • No IP (and TCP/UDP) header on each packet • Packets: • Don’t waste bandwidth • No traffic exchanged during idle periods • Better to allow multiplexing • Different transfers share access to same links • No set-up delay

  42. (c) Describe how switches learn. In particular, describe specifically (a) what data they store, i.e., what fields/information from packets they store (b) the algorithmic process they use to discover/learn that data

  43. Answer: • when a switch observes a frame from a particular source address, it knows the frame lies out that particular port. so, it stores (source address, port) mappings.

  44. Question • Encoding, Framing and Error Detection Suppose we want to transmit the message 11100011 and protect it from errors using the CRC polynomial x3+1. • Use polynomial long division to determine the message that should be transmitted • Suppose the leftmost bit of the message is inverted due to noise on the transmission link. What is the result of the receiver’s CRC calculation? How does the receiver know that an error has occurred?

  45. Answers: • We take the message 11100011, append 000 to it, and divide by 1001 according to the method shown in Section 2.4.3. The remainder is 100; C(x)nk = 3  pad M(x) with 2 0s P(x) = 11100011000 R(x) = 100 what we transmit is the original message with this remainder appended, or 1110 0011 100. 1 1 1 0 0 0 1 1 0 0 0 1 0 0 1 0 1 1 1 0 1 0 0 1 0 0 1 1 1 0 1 0 0 1 0 0 0 1 11 1 1 0 0 1 0 0 0 0 1 10 1 1 0 0 1 0 0 0 0 0 1 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 0 0 R(x) = 100

  46. (b) Inverting the first bit of the transmission gives 0110 0011 100; dividing by1001 (x3 + 1) gives a remainder of 10; the fact that the remainder is nonzero tells us a bit error occurred.

  47. Question • Sliding Window Protocol: • Sliding window protocol can be used to implement flow control. We can imagine doing this by having the receiver delay ACKs, that is, not send the ACK until there is free buffer space to hold the next frame. In doing so, each ACK would simultaneously acknowledge the receipt of the last frame and tell the source that there is now free buffer space available to hold the next frame. Explain why implementing flow control in this way is not a good idea

  48. (a)Answer: • If the receiver delays sending an ACK until buffer space is available, it risks delaying so long that the sender times out unnecessarily and retransmits the frame.

  49. (b) Draw a timeline diagram for the sliding window algorithm with SWS = RWS = 3 frames, for the following two situations. Use a timeout interval of about 2×RTT. • Frame 4 is lost • Frames 4 to 6 are lost.

  50. (b) Answer: When the sender retransmits frames 5 and 6 (due to timeout), the receiver can determine that it is seeing a second copy of frame 5 and 6 rather than the first copies and therefore can ignore it. timeout Frame[5] timeout Frame[6]

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