CS 447 Network and Data Communication Midterm Exam No.2 Spring, 2003 7:30-8:45 P.M. April 8, 2003 This question is a closed book and closed note exam. There are 6 questions in this exam. You have 75 minutes to finish the questions. Please write your answers on separate pieces of papers. To avoid grading problems, please staple your papers in the ascending order in the question number. Please bring a calculator (no calculator sharing, please). Name: _______________________________ Student ID (optional): ___________________ Question #1 (10 minutes - 10 points) (1) What are the two potential problems for throughput in “buffered-statistical” TDM? Mention two possible problems (2 minutes) – 2 points (2) What are the two types of switches used in a circuit-switching network? (1 minutes) – 2 points (3) What are the internal and external operations in the Internet? (1 minutes) – 2 points (4) What are the two types of switching networks? (1 minutes) – 2 points (5) Compare, circuit switching, virtual circuit packet switching and datagram packet switching networks in terms of bandwidth sharing of links and message storing at each router (you can answer by just mentioning the keywords, you do NOT have describe your solution) (5 minutes) – 2 points 1 Question #2 (15 minutes - 20 points) (1) What is the minimum requirement (i.e., how much transmission rate is required) for the shared transmission cable for a statistical TDM shown below (make sure this TDM is not a blocking statistical TDM)? – (10 minutes) 10 points N input lines Multiplexer N output lines De-Multiplexer Shared Transmission Cable Assumptions are: Each slot now has a slot header (the slot header includes only the information that indicates the destination output line – nothing else). Assuming there are 128 input lines. All the input lines have the same transmission rate of 10Mbps. The slot includes 35 bits as user payload for each input line (excluding the slot header) (2) Assume that the buffered-statistical TDM will be used. If the link bandwidth of the shared transmission cable is 900 Mbps, and each input line always transmits data at 120 Mbps, what is the requirement for the throughput of the memory buffer (in the number of bits that have to go through the memory unit per second), if N = 10? – (5 minutes) 10 points Shared Transmission Cable N input lines Multiplexer De-Multiplexer Memory Buffer 2 N output lines Question #3 (10 minutes - 10 points) (Exercise #8.13) Ten 57.6-Kbps lines are to be multiplexed using TDM. Ignoring overhead bits, what is the total capacity required for synchronous TDM? Assuming that we wish to limit average line utilization of 75%, and assuming that each line is busy 45 percent of the time, what is the capacity required for statistical TDM? Question #4 (10 minutes - 10 points) (Exercise #9.2) Consider a simple telephone network consisting of two end offices and the intermediate switch with a 1-MHz full-duplex trunk (trunk = a shared transmission cable) between each end office and the intermediate switch. The average telephone is used to make four calls per 8-hour workday, with a mean call duration of six minutes. Ten percent of the calls are long distance (i.e., not local – they go through the intermediate switch). What is the maximum number of telephones and end office can support? Question #5 (10 minutes - 15 points) (Exercise #10.2) Define the following parameters: N: Number of hops between two given end hosts L: Message length in bits B: Data rate in bps on all links P: Fixed packet size in bits H: Overhead bits per packet D: Signal propagation delay Assume that the signaling message to set up a path and the ACK message for path set up so small that the packet transmission delay is negligible for both of them. Also assume that there is no overhead to process the signaling message. N = 4, L = 3200, B = 9600, P = 1024, H = 16, D = 0.001, compute the estimated time required to complete the data transmission. 3 Question #6 (20 minutes - 10 points) This question is about TDM bus switch. The figure below shows the internal structure of typical 33 TDM bus switch. The transmission rate of the shared bus is usually R n Mbps where R is the transmission rate (in Mbps) of each input (assume that all input lines have the same transmission rate and they are always transmitting at their full transmission rate and there are n input lines). TDM Bus switch Input Buffer Input lines Switch Control Unit A B Output switch C Input switch X Y Z Output lines Shared Bus The length of the TDM bus (i.e., the shared cable within a TDM bus switch) will be a significant problem if transmission rate of each input line increases. The problem is the signal propagation delay in the bus. Even after a bit has been transmitted at an input switch, the output switch must hold a connection until the bit signal propagates in the bus cable to the target output switch (this means that there must be some waiting time as much as the signal propagation delay for the bus cable). Only after this, the switch of the next input line can start transmit its data. Question (a): In order to demonstrate this problem, calculate the utilization of the transmission cable if R = 1,000 Mbps (= 1 Gbps) and n = 3. Assume that bit interleave is used (i.e., the bus transmits a bit for each input at a time) and the transmission rate of the bus cable is 3 Gbps. Assume that the distance of the bus cable is 10 meters and the signal propagation speed is 300,000Km/second. Assume that the bus length is same for any input-output port pair. (15 minutes) Hints: 1 bit at 1Gbps = 1ns, which is 10-9 second. Question (b): If two-bit interleaving (the bus transmits two bits for an input line at a time) is used, what will happen to the bus cable utilization (will it increase or decrease)? (5 minutes) Hint: this question is really easy, if you understand Question (a) above. You could answer this question within two minutes without calculator. CS 447-Network and Data Communication, Midterm Exam #2, April 8, 2003 4