CS447 – Network and Data Communication
Midterm Exam No. 2
Spring, 2004
4:30 – 5:45 P.M.
April 6, 2004
This exam is a closed-book and close note exam. There are 7 questions in this exam. You have 75 minutes to finish the 7 questions. Write your answers on separated pieces of papers. To avoid grading problem, staple your papers in the ascending order in the question numbers. In this exam, timing will be strictly enforced, and when exam is over, please do not write anything on your paper. If you need to write anything after exam is over, you need to get permission from the instructor.
Please do NOT open this exam until you are told you can.
Student ID (Last 6 Digits): ______
Notice 1: For each question in this exam, recommended time is specified. It is not necessary for you to follow each recommendation, but it is advised that you pace yourself in such a way that you do not spend too much time on a particular question. Recommended time implies the amount of details you are expected to answer. For example, if 2 minutes are recommended, you are NOT expected to write a long paragraph for your solution and what is expected is a brief (but correct) answer (you may be able to answer even by correct keywords for such questions). Although recommended time is specified, it is essentially your responsibility to pace yourself. Time may not be enough for everyone, but an exam is fair if everyone is given the same amount of time and the same questions.
Notice 2: The instructor will be willing to answer any question if there is anything wrong in the questions. However, the instructor will NOT answer any question regarding the interpretation of the questions. Understanding what a question is asking you is also a part of this exam and it is a part of evaluation this exam is making.
I understand the above suggestion: ____ (please put a check mark if the above suggestion is clear. Otherwise, ask the instructor for any question). If this is not clear, please do NOT start your exam and ask your question to the instructor.
QUESTION #1 (10 minutes)
(1) What is a non-blocking switch?
(2) Show where application-level protocols (HTTP, FTP, TELNET, POP, etc), TCP, UDP, IP and network hardware are located in OSI 7-layer model (you can show your solution by drawing a picture).
(3) Show the procedure in CSMA.
(4) Somebody says, “Packet switching requires control and address bits to be added to each packet. This introduces considerable overhead in packet switching. In circuit switching, a transparent circuit is established. No extra bits are needed. Therefore, circuit switching will be much faster to transmit data.” How do you think?
(5) What are the three different types of switches used in a circuit switching network (you do NOT have to describe them, just name the three)? Which one(s) could be non-blocking switches?
QUESTION #2 (10 minutes)
Find the shortest distance to every other nodes using Dijkstra’s algorithm.
You are currently at node D and your task is to find the shortest distance
to all the other nodes in the network (right).
· Show all your work. Credit will be given to a correct work
(show the procedure to find the shortest distances, as we
did in the classroom).
· Links are connected only to nodes (i.e., two links
can not be connected without a node).
· Show the resulting shortest-path tree graph (in a
separate piece of paper).
QUESTION #3 (10 minutes)
Suppose we are going to use a TDM switch (as shown in the picture below). In the statistical TDM, there are 110 links connected (i.e., 110 input and output lines). The peak transmission rate for each input line is 8.5Mbps (assume 1M = 106). Assume that utilization =100%. Five-byte interleave is used for frame encoding. Ignore any other overhead.
Question: In order for this TDM switch to be a non-blocking switch, what is the minimum link bandwidth (the shared link between the multiplexer and de-multiplexer) required for this TDM switch?
QUESTION #4 (up to 7.5 minutes)
Complete the following table (*: how bandwidth will be assigned to a connection that uses one of the three transmission methods):
QUESTION #5 (12.5 minutes) For these questions, a long solution is NOT expected. If know the correct idea, you answer could be very short.
(1) There are four combinations of virtual circuit and datagram packet switching for the external and internal operations. Give example for each combination (as types of network).
(2) What are the two Ethernet standards (those we discussed in the classroom, please)?
(3) What is the significance in LAN compared to WAN and Internet?
(4) Classify major MACs.
(5) What is the essential difference between bridges and routers?
(6) What are the types of routing in which routing table is not same for every node in a network?
(7) Which layer(s) in OSI 7-layer model belong to the external operations (name all that apply)?
(8) What are the two types of “hub”? Describe the difference.
(9) What is an internet? Answer this question using up to five words (yes, you have read this correctly, only up to 5 words). Any solution that contains more than five words is considered wrong, even though the idea is correct.
QUESTION #6 (up to 10 minutes)
Consider a simple telephone network consisting of two end offices and one intermediate switch with a 3.2MHz full duplex trunk between each end office and the intermediate switch. Assume a 4KHz (1M = 1,000K) channel for each voice call. The average telephone is used to make twelve calls per 8-hour workday, with a mean call duration of ten minutes. Five percent of the calls are long distance (i.e., inter-office calls). What is the maximum number of telephones an end office can support? All telephones in an end office are directly connected to a telephone switching hub in an office (as shown in the figure below). Switching hubs and inter-office hub are never bottleneck.
QUESTION #7 (up to 10 minutes)
A datagram network allows routers to drop packets whenever they need to do. The probability of a router to drop a packet is P (0.0 < P < 1.0, theoretically both 0.0 and 1.0 can be included, but either one is not realistic, so they are excluded in this question). A sender and a receiver are connected by a path that goes through three intermediate routers (thus there are four link = four hops). Any router on the path could drop a packet at probability of P and there is no coordination between the three routers for their packet drops.
Question: What is the average number of hops a packet makes per transmission?
Assumptions:
(1) If a packet is dropped at the first router after the sender, we count it “1” (one hop has been made and then dropped).
(2) Packets can be dropped only at a router (but not either at the sender or the receiver).
Hint: We did something similar for one of the subjects covered in our first midterm exam.
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CS447 – Network and Data Communication, Midterm Exam #2, Spring 2004, April 6, 2004.
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