Data and computer communication exam ll
- 1. Data and Computer Exam 2 Question 1 A LAN has a data rate of 4 Mbps and a propagation delay between two stations at opposite ends of 20us. For what range of PDU sizes does stop-and-wait give an efficiency of at least 70%. A simple medium access control protocol would be to use a fixed assignment Time Division Multiplexing (TDM) scheme. Each station is assigned one time slot per cycle for transmission. For the bus, the length of each slot is the time to transmit 100 bits plus the end-to-end propagation delay. Stations monitor all time slots for reception. Assume a propagation time of 200000000 m/s. For N stations, what is the throughput per station for a 1-km, 10-Mbps baseband bus? Answer: 6.67 x 10^6N Question 3 System A consists of a single ring with 60 stations, one per repeater. If the probability of a link failure is Pl, a repeater failure is Pr, and a bridge failure is Pb, Assuming Pr=Pl=Pb=0.01, What is Probability of failure of system A Answer: 0.70 Question 4 System B consists of four 15-station rings linked by a bridge. If the probability of a link failure is Pl, a repeater failure is Pr, and a bridge failure is Pb, Assuming Pr=Pl=Pb=0.01, What is Probability of complete failure of system B Answer: 0.004591 Question 5 3 / 5 points A disadvantage of the contention approach for LANs is the capacity wasted due to multiple stations attempting to access the channel at the same time. Suppose that time is divided into discrete slots, with each of N = 8 stations attempting to transmit with probability P = 0.10 during each slot. What fraction of slots are wasted due to multiple simultaneous transmission attempts? Answer: 0.52, 0.18 Question 6 5 / 5 points If it takes 25 micro seconds to send a frame using the pure ALOHA method, then the vulnerable time is ……………. Answer: 373 bits Question 2
- 2. 12.5 micro seconds 50 ms 25 micro seconds 50 micro seconds Question 7 A network using the CSMA random access method with P equal to 0.25 will refrain from sending …………percent of the time after accessing an idle line. 25 50 75 100 Question 8 In the p-persistent approach, when a station finds an idle time, it……………………. Waits a time equal to the propagation time before sending Waits a time equal to the frame transmission time before sending Sends with probability p Sends immediately Question 9 The 1-persistent approach can be considered a special case of the P-persistent approach with a P equal to………… 0.1 0.5 1.0 2.0
- 3. Question 10 …………………..is a controlled access method. Slotted ALOHA Pure ALOHA CSMA/CD Token passing Question 11 At the CRC checker, if the remainder is…………………., then the data unit is damaged. A string of 0s A string of 1s A string of alternating 1s and 0s Not a string of 0s Question 12 In go-back-n ARQ, if frames 4, 5, and 6 are received successfully, the receiver may send an ACK…………….to the sender. 4 5 6 7 Question 13 In stop and-wait ARQ, if data 1 has an error, the receiver sends a ……….frame. NAK 0 NAK 1 NAK 2 NAK
- 4. Question 14 In sliding window flow control (window size of 7), an ACK 3 means the receiver next expects frame …………. 2 3 4 8 Question 15 In sliding window protocol the left wall of the sender sliding window moves to the ……………….. when it ……………….a data frame. right; receives right; sends left; receives left; sends Question 16 The following numbers are used in a sliding window protocol: 0, 1, 2, 3, 4, 5, 6, and 7. The window size is ………. 6 7 8 9 Question 17 If the data unit is 1010101010101010 and the remainder is 1001, what is the dividend at the receiver? 10101010101010100000 10101010101010101010 10101010101010101001
- 5. 1010101010101010 Question 18 The throughput is 10 kbps. How long does it take to send 2000 bits? 0.2 2 200 2000