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1 Yarmouk University Hijjawi Faculty for Engineering Technology Computer Engineering Department CPE-462 Digital Data Communications Final Exam: A Date: 20/05/09 Student Name : Instructor Student Number: Section INSTRUCTIONS (please read carefully) 1. TURN OFF YOUR CELL PHONE NOW 2. Time allowed 120 minutes 3. Provide your answers on this page. Answers elsewhere will not be looked at. 4. Return all pages. 5. Provide only one answer to each question. Giving more than one answer for the same question will result in zero credit for that question. 6. Mark the right answer with ( ). A B C D E A B C D E A B C D E /10

2 1. Which multiplexing technique transmits analog signals? a. FDM b. TDM c. WDM d. (a) and (c) e. (b) and (c) 2. The DS-1 is the basis of: a. AT&T analog carrier systems b. ITU-T analog carrier systems c. AT&T digital carrier systems d. ITU-T digital carrier systems e. All of the given choices 3. What is the lowest level in the SDH hierarchy? a. STS-1 b. STM-1 c. OC-1 d. Group 4. What is the type of multiplexing used in ADSL? a. FDM b. Synchronous TDM c. Statistical TDM d. A mix of FDM and synchronous TDM e. A mix of FDM and tatistical TDM 5. Which of the following variations of xdsl has the highest data rate? a. ADSL b. HDSL c. VDSL d. SDSL 6. Suppose that it is desirable to convert an analog baseband signal with a bandwidth of 5 khz into digital using a 4-bit A/D converter. What is the output data rate? a. 5 kbps b. 10 kbps c. 20 kbps d. 40 kbps e. 80 kbps 7. In a synchronous TDM system, the data rate of the link must be the sum of the signal sources data rates. a. Less than or equal to b. Exactly equal to c. Unequal to d. Greater than or equal to e. Less than 2/10

3 8. Which of the following statements is correct? a. Synchronous TDM is more efficient than statistical TDM. Synchronous TDM has higher complexity than statistical TDM b. Synchronous TDM is more efficient than statistical TDM. Synchronous TDM has lower complexity than statistical TDM c. Statistical TDM is more efficient than synchronous TDM. Statistical TDM has higher complexity than synchronous TDM d. Statistical TDM is more efficient than synchronous TDM. Statistical TDM has lower complexity than synchronous TDM e. Statistical TDM is as efficient as synchronous TDM. Statistical TDM is as complex as synchronous TDM 9. Which of the following statements is wrong? a. The Asymmetrical digital subscriber line uses currently installed twisted pair cable b. The discrete multitone uses multiple carrier signals at different frequencies c. In theory, if the discrete multitone uses 256 downstream subchannels at 60 kbps each, then a data rate of Mbps can be achieved d. The Asymmetrical digital subscriber line is asymmetrical because it has a bigger downstream than upstream 10. In cable modems, a cable TV provider dedicates channels. Each channel is shared by a number of subscribers. a. 2 b. 12 c. 24 d. 48 e What does HDLC stand for? a. None of the given choices b. High-level digital link control c. High-level digital link code d. Hierarchical-link digital level control e. Hierarchical-link digital level code 12. Suppose that it takes 200 ms to send a frame and get an ACK to that frame by the sender. What should the window size be in order to the sender to transmit frames continuously if it takes 14 ms to transmit one frame? a. 4 b. 5 c. 7 d. 15 e Consider a 800 kbps link between two ground stations that communicate via a satellite relay with an altitude of km. What is the transmission time of a frame of 1500 octets? a ms b ms c. 15 ms d. 20 ms e. 60 ms 3/10

4 14. What is the maximum window size for Go-Back-N ARQ if a 5 bit sequence number field is used? a. 5 b. 31 c. 32 d. 15 e Consider a 300 m coaxial cable link operating at 10 Mbps. What is the bit length of this link if the velocity of propagation is m/s? a. 15 b. 20 c d The figure below represents the sliding window at the sender s side. Use this figure for answering questions 16 18: 16. What ARQ is being used? a. Stop and wait b. Go back N c. Selective reject d. Either stop and wait or go back N e. The information provided by the figure is inadequate to tell 17. How many frames have been sent but not yet acknowledged? a. 2 b. 4 c. 6 d What is the maximum window size? a. 4 b. 7 c. 2 d. 3 e. 6 4/10

5 19. Suppose that the following data bits were received in an HDLC frame: What was the original bit pattern? a b c d e What data rate should be provided to transmit a video at 30 frames per second if each frame has a size of 33.2 mega bytes? a. 1.1 Mbps b Mbps c. 0.9 Mbps d. 996 Mbps e Gbps Consider the frequency domain representation shown below then answer the questions that follow. Amplitude Frequency (MHz) 21. The composite analog signal may be represented as a. s(t) = sin(8 x10 6 π t ) + sin(24 x10 6 π t) sin(40 x10 6 π t ) b. s(t) = sin(8 x10 6 π t ) + sin(24 x10 6 π t) sin(40 x10 6 π t ) c. s(t) = sin(4 x10 6 π t ) + sin(12 x10 6 π t) sin(20 x10 6 π t) d. s(t) = 4 +4 sin(4 x10 6 π t ) + sin(12 x10 6 π t) sin(20 x10 6 π t) e. s(t) = 3 sin(4 x10 6 π t ) + sin(12 x10 6 π t) sin(20 x10 6 π t) 22. Assume that the propagation speed is 3x10 8 m/s. What is the shortest wavelength of the signal s(t) a. 30 m b. 75 m c. 15 m d. 150 e m 23. Calculate the power of a signal (in watts) if its dbw = 20. a. 50 b. 100 c. 200 d. 300 e /10

6 24. The power of a signal is 10 watt and the power of the noise is 1 watt. What is SNR db? a b. 0.1 c. 1 d. 10 e What is the maximum bit rate of a noiseless channel with a bandwidth of 1 khz transmitting a signal with two signal levels? a. 500 bps b. 1 kbps c. 2 kbps d. 4 kbps e. 8 kbps 26. Which of the following statements is wrong? a. All other factors remaining constant, higher bandwidth gives higher data rate b. Impairments, such as attenuation, limit the distance that a signal can travel c. Interference has higher impact on unguided media than on guided media d. More receivers introduces more attenuation 27. Which of the following statements is correct? a. Shielded twisted pair cables are less expensive than unshielded twisted pair cables b. External electromagnetic interference has higher impact on unshielded twisted pair cables than on shielded twisted pair cables c. Shielded twisted pair cables are easier to handle than unshielded twisted pair cables d. Unshielded twisted pair cables achieve higher data rates than shielded twisted pair cables 28. Which of the following does not require licensing? a. AM radio b. FM radio c. Infrared d. UHF television e. VHF television 29. Which of the following CRC codes are used in the FCS field of the HDLC frames? a. CRC-12 and CRC-16 b. CRC-16 and CRC-32 c. CRC-CCITT and CRC-12 d. CRC-12 and CRC-32 e. CRC-CCITT and CRC Packet switching has many advantages over circuit switching. Which of the following is not considered an advantage for packet switching? a. Line efficiency b. Data-rate conversion c. No blocking d. Priorities can be used e. Continuous transmission of data 6/10

7 31. Assume that there is a virtual circuit from station X through nodes a and b to station Y. The message to be sent comprises 30 octets, and each packet contains 1 octet of contol information. What is the total transmission time (in octet-times) if the message is broken up into 2 packets? a. 92 b. 64 c. 55 d In a circuit switching network, suppose there are 4 hops between two end systems. If the propagation delay per hop is equal to seconds. What is the total propagation delay in seconds to send a call request signal? a b c d Suppose that a message of size 5000 bits is to be sent through a packet switching network. What is the required number of packets if the packet size is 1000 bits (of which there are 200 overhead bits)? a. 8 b. 7 c. 6 d Identify the wrong statement a. In datagram packet switching, some redundant connections are desirable b. Channel capacity can be wasted in circuit switching c. Softswitches cost significantly less than traditional circuit switches and can provide more functionality d. Packet switching was designed for data 35. Which of the following switching techniques is the most reliable? a. Datagram packet switching b. Virtual circuit packet switching c. Circuit switching d. Datagram and virtual circuit have the same reliability because both belong to packet switching e. Circuit switching and virtual circuit have the same reliability because they have some similiraties 36. Which of the following statements is wrong? a. Packets in datagram switching may arrive out of order b. Packets in virtual circuit switching arrive inorder c. Packets in circuit switching arrive inorder d. Packets in virtual circuits are forwarded more quickly than in in datagram switching e. In datagram switching, there is no call setup phase 7/10

8 37. Identify the wrong statement: a. The call control signaling in frame relay is carried on a separate logical connection from user data b. There are three significant types of delay that affect the performance of packetswitching networks: transmission, processing, and queuing delays c. In frame relay there is no "hop-by-hop" flow control or error control. d. The multiplexing of logical channels in frame relay takes place at Level 3 e. On an end-to-end basis, there is no error control or flow control in frame relay 38. Identify the wrong statement: a. It is possible for a packet to be delivered to the wrong station even if there is no malfunction in any of the stations or nodes of a network. b. In X.25, it is needed to assure that all of the packets are delivered properly. c. The public telecommunications network can be described using four generic architectural components: subscribers, subscriber line, exchanges, and trunks. d. Both layer 2 and layer 3 of X.25 include flow control and error control mechanisms. 39. Choose the correct statement: a. Packets transit the network more rapidly with a virtual cicuit. Virtual cicuits have better congestion control and are more reliable than the datagram approach. b. Packets transit the network more rapidly with the datagram approach. Datagram approach has better congestion control and is more reliable than the virtual cicuit approach. c. Packets transit the network more rapidly with a virtual cicuit. But datagram approach has better congestion control and is more reliable than the virtual cicuit approach. d. Packets transit the network more rapidly with the datagram approach. But the virtual cicuit approach has better congestion control and is more reliable than the datagram approach. e. Packets transit the network more rapidly with a virtual cicuit. Virtual cicuits have better congestion control, but the datagram approach is more reliable. 40. Which of the following uses statistical TDM? a. Analog carrier systems b. Digital carrier systems c. Cable modem d. ADSL 41. DMT (Discrete Multi-Tone) modulation technique used for the ADSL technology combines: a. FDM and TDM b. FDM and QAM c. TDM and QAM d. TDM only e. FDM only 42. The largest portion of the ADSL bandwidth carries: a. Voice communication b. Upstream data c. Downstream data d. Control data e. POTS 8/10

9 43. In the analog hierarchy, a supergroup carries 60 voice channels. Its bandwidth requirement would be a. 144 KHz b. 180 KHz c. 240 KHz d. 480 KHz e MHz 44. Four non-homogenous connections are to be multiplexed over a link using synchronous TDM. The data rates of the four connections are 40 kbps, 80 kbps, 160 kbps, and 200 kbps, respectively. The multiplexed unit is one byte. Each TDM frame has one framing bit. What is the frame size in bits? a. 9 bits/frame b. 12 bits/frame c. 13 bits/frame d. 73 bits/frame e. 97 bits/frame 45. In selective reject, if the sender sent frames 0, 1, 2, and 3 and the receiver sent SREJ 1. What frame(s) will the sender resend? a. 1 b. 2, 3 c. 1, 2, 3 d. A timeout will occur e. It tells the sender to send one frame and after that everything will be rejected. 46. In a multiplexed system, lines share the bandwidth of link. a. N; N b. N; M c. 1; N d. 1; 1 e. N What is the achievable spectral efficiency of a link with 3 Mbps capacity and has a spectrum between 500 khz and 700 khz? a. 1.5 b c. 0.6 d. 15 e The IPv6 source address consists of a. 16 bits b. 32 bits c. 64 bits d. 128 bits e. 256 bits 49. What is the gain in decibels of a parabolic antenna with a 1.2 m diameter at 3 GHz? a b c d. 20 e /10

10 50. Determine the height of an antenna in meters for a TV station that must be able to reach customers up to 25 km away. a b c d e The layer of the Internet stack that provides the TCP and UDP services is the: a. Application layer b. Network layer c. Transport layer d. Data link layer 10/10