CHAPTER 14 (BOOK) MULTIPLE CHOICE QUESTIONS 23. a 24. b 25. c 26. d 27. b 28. c 29. b 30. d 31. a 32. d 33. b 34. a 35. d 36. d 37.

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1 CHAPTER 14 (BOOK) REVIEW QUESTIONS 1. Virtual circuit switching is more efficient because the segments follow a created route, the links of which might be shared by other connections, while in circuit switching the different segments of a message follow a dedicated path that cannot be shared by other connections. 2. A switched network consists of a series of interlinked nodes (switches). These switches are capable of creating temporary connections between two or more devices linked to the switch but not to each other. Some of these nodes are con-nected to the communicating device, others are only used for routing. 3. Circuit switching, packet switching, and message switching 4. Space-division and time-division 5. A crosspoint is a microswitch at the junction of an input and an output line in a crossbar switch. 6. The major limitation of a crossbar switch is the number of crosspoints required. In multistage switch, combining crossbar switches in several stages can reduce the number of crosspoints. 7. In a single crossbar switch, every combination of input and output has its own indi-vidual crosspoint. Therefore blocking does not occur. 8. In multistage switching blocking may occur when all of the possible intermediate switches are occupied. 9. In a space-division switch, the path from one device to another is spatially separate from other paths. The inputs and the outputs are connected using a grid of elec-tronic microswitches. In a time-division switch, the inputs are divided in time using TDM. A control unit sends the input to the correct output device. 10. Time slot interchange and TDM bus 11. A TSI consists of RAM with several memory locations. The size of each location is equal to the size of a single time slot (TDM). There are as many locations as input devices. The RAM fills up with incoming data from the time slot in the order received. The control unit in the TSI sends out the slots to the correct output device. In a TDM bus, the input and output lines are connected to a high-speed bus through input and output gates. The control unit opens and closes the gates accord-ing to switching needed. 12. In TSI, the control unit makes decisions on the destination of the slots sent out, while the control unit in the TDM bus opens and closes the input and output gates according to the switching needed. 13. Space division is instantaneous, which means there are no delays when processing a connection. 14. Time-division switching does not need crosspoints. 15. Regional offices, sectional offices, primary offices, toll offices, and end offices 16. Non-voice transmission tends to be bursty and causes idle gaps between data spurts. This means the line is often idle, which causes waste of the facility. 17. Circuit switching means creating a dedicated physical connection between two end devices. Packet switching means dividing the data into possibly variable length packets and sending them through possibly different paths to the destination. 18. Datagram approach and virtual circuit approach 19. Each packet is treated independently from the others. Each packet may go a differ-ent path to reach the destination. This might cause packets arriving out of order. 20. All three packets of the message travel the same route using a virtual circuit estab-lished permanently between the sender and the receiver 21. A virtual circuit is created first and then all three packets of the message travel the same route. 22. Message switching was created to provide high-level network service for unintelli-gent devices. These devices have been replaced and so has the message switching method. CHAPTER 14 (BOOK) MULTIPLE CHOICE QUESTIONS 23. a 24. b 25. c 26. d 27. b 28. c 29. b 30. d 31. a 32. d 33. b 34. a 35. d 36. d 37. b CHAPTER 14 (BOOK) EXERCISES 38. 1,000,000 crosspoints crosspoints 1

2 40. 20??20 = 400 crosspoints 41. Efficiency is improved by 58%. 42. Three users connected to each first stage switch can access the system at the same time. 12 users can access the whole system at the same time. Three users per first stage switch and four first stage switches are used; 3 x 4 = Yes, more second stage switches allow more output lines at the first-stage switches, which requires more crosspoints. The more crosspoints, the less block-ing. 44. c, each switch has 4 input as well as 4 output lines (N X?L) + K 2 X?L 46. Output line 1: C Output line 2: D Output line 3: A Output line 4: B 50 Digit First frequency Second frequency Hz 1336 Hz Hz 1477 Hz Hz 1209 Hz Hz 1336 Hz Hz 1477 Hz Hz 1336 Hz Hz 1336 Hz 51. Issue Circuit-switched Packet-switched Dedicated path Yes No Store and forward No No Need for connection establishment Yes No and yes Routing table No Yes Delay No Yes 52. Issue Datagram Virtual circuit All packets follow the same route Not necessarily Yes Table lookup Yes Yes and no Connection establishment No Yes Packet may arrive out of order Yes No 53. Issue PVC SVC Connection and disconnection No Yes Payment Pay per month Pay per connection Table lookup No Yes Duration of an entry in a table Permanent Temporary Chapter 14 (Web Quizzes) 1. switching has been phased out in general communication. c. Message 2. The switching method best suited for voice communication is switching. a. circuit 3. Circuit switching can involve the use of. d. all of the above 4. The time-slot interchange is a device used in switching. a. circuit 5. The TDM bus is a device used in switching. a. circuit 6. There are two popular approaches to switching: the datagram approach and the virtual circuit approach. b. packet 7. Switched virtual circuits or permanent virtual circuits implement the approach to packet switching. b. virtual circuit 8. circuits are implementations to the virtual circuit approach to packet switching. d. a and b 2

3 9. Store and forward are terms used to describe switching. c. message 10. A crossbar switch is a switch. a. space-division 11. A switch usually has a higher efficiency than a switch. d. multistage; single crossbar 12. A crossbar switch with x inputs and y outputs requires crosspoints. a. xy 13. A crossbar switch with 100 inputs and outputs requires 20,000 crosspoints. b A multistage switch usually has crosspoints than a corresponding single crossbar switch. b. less 15. In a multistage switch, there is usually path(s) between input xand output y. c. multiple 16. A three-stage switch has. d. multiple switches for each of three stages 17. Blocking is not a problem in a switch. a. single-stage 18. occurs when there is no path available between an input and an output. b. Blocking 19. The time slot interchange uses multiplexing to achieve switching. c. time division 20. The TDM bus uses multiplexing to achieve switching. c. time division 21. The of a TSI holds the incoming input data. b. RAM 22. In a TDM bus, the opens an input gate and an output gate to allow data transfer. a. control unit 23. The advantage of space-division switching is that it. a. is instantaneous 24. The disadvantage of switching is the processing delay. a. time division 25. In a TST switch, if there are x first stage switches, one second stage switch, and y third stage switches, the second stage must have crosspoints. c. xy 26. In the PSTN hierarchy, the class one switching centers are known as the offices. c. regional 27. In the PSTN hierarchy, the class two switching centers are known as the offices. b. sectional 28. In the PSTN hierarchy, the class three switching centers are known as the offices. d. primary 29. In the PSTN hierarchy, the class four switching centers are known as the offices. b. toll 30. In the PSTN hierarchy, the class five switching centers are known as the offices. a. end 31. Subscriber telephones in the PSTN are connected through to end offices. b. local loops 32. Subscriber telephones in the PSTN are connected through local loops to offices. a. end 33. In the PSTN hierarchy, there are more offices than offices. b. sectional; regional 34. At the top of the PSTN hierarchy are the offices. c. regional 35. At the bottom of the PSTN hierarchy are the offices. d. end 36. In touch-tone dialing, when the 9 key is pressed, bursts of analog signals are sent to the end office. d If analog signals with frequencies of 770 Hz and 1209 Hz are received by the end office, a has been dialed. d If analog signals with frequencies of 941 Hz and 1336 Hz are received by the end office, a has been dialed. a switching is well suited for voice communication while switching is better suited for data and other non-voice communication. d. Circuit; packet 40. switching assumes that the data rate in both directions is the same. a. Circuit 41. An SVC is a transmission path in switching. b. packet 42. A PVC is a transmission path in switching. b. packet 43. In the virtual circuit approach to packet switching, packets of a message follow the same route from sender to receiver. c. all 3

4 44. In the approach to packet switching, all packets of a message follow the same route from sender to receiver. b. virtual circuit 45. In the approach to packet switching, packets of the same message follow the same route from sender to receiver. b. virtual circuit 46. A message from device A consists of packet X and packet Y. In the datagram approach to packet switching, packet Y's path packet X's. c. is independent of 47. A message from device A consists of packet X and packet Y. In the virtual circuit approach to packet switching, packet Y's path packet X's. a. is the same as 48. exists only for the duration of the specific exchange. a. An SVC 49. is a permanent connection for the duration of a lease. b. A PVC 50. An SVC is comparable to a line in circuit switching, while a PVC is comparable to a line. c. dialup; leased 51. An SVC requires. d. a and b 52. Connection establishment is needed for. b. an SVC 53. In a connection, links between sender and receiver are dedicated. b. circuit-switched 54. In a connection, the link between any two switches can also be used by other connections. a. virtual circuit CHAPTER 8 (BOOK) REVIEW QUESTIONS 1. FDM, WDM, and TDM. 2. In FDM each signal modulates a different carrier frequency. The modulated carri-ers are combined to form a new signal that is then sent across the link. 3. A guard band keeps modulated signals from overlapping and interfering with one another. 4. A demultiplexer uses a series of filters to decompose the multiplexed signal into its constituent component signals. 5. WDM is conceptually the same as FDM. Both are combining different signals of different frequencies. In WDM the frequencies are very high and the energy source is light signals transmitted through fiber optic channels. 6. Synchronous and asynchronous 7. In TDM digital signals from n devices are interleaved with one another forming a frame of data. 8. Synchronous and asynchronous. In synchronous TDM each frame contains at least one time slot dedicated to each device. The order in which each device sends its data is fixed. In asynchronous TDM the number of time slots is less than the num-ber of devices and the slot order depends on which devices have data to send. Addressing of each time slot is necessary. 9. In synchronous TDM the demultiplexer at the receiver decomposes each frame by discarding the framing bits and extracting each data unit in turn. As a data unit is removed from the frame it is passed to the appropriate receiving device. In asyn-chronous TDM the multiplexer at the receiver decomposes each frame by checking the local address of each data unit. The extracted data unit is removed from the frame and passed to the appropriate receiving device. 10. Inverse multiplexing splits a data stream from one high speed line onto multiple lower speed lines. 11. Analog switched service requires dialing, while analog leased service is a perma-nent dedicated link between two customers; no dialing is needed. 12. Voice channels (12 x 44 KHz) are multiplexed onto a higher bandwidth line to cre-ate a group (48 KHz). Up to five groups (5 x 48 KHz) can be multiplexed to create a super group (240 KHz). Ten super groups (10 x 240 KHz) are multiplexed to cre-ate a master group (2.52 MHz). Six master groups are multiplexed to create a jumbo group with MHz. 13. Switched/56, DDS, and DS. 14. The DSU changes the rate of the digital data created by the subscriber s device to 56 Kbps and encodes it in the format used by the service provider. 15. DS-0: single digital channel (64 Kbps) DS-1: 24 DS-0 channels multiplexed = Mbps DS-2: 4 DS-1 channels multiplexed = Mbps DS-3: 7 DS-2 channels multiplexed = Mbps DS-4: 6 DS-3 channels multiplexed = Mbps 16. DS is the name of the service, which is implemented by T-lines. The capacity of the lines precisely matches the data rate of DS-services. 17. In order to use T lines for analog transmission the analog signal needs to be sam-pled first. 4

5 18. ADSL divides the bandwidth of a twisted pair cable into three bands. The first band, 0-25 KHz is used for regular telephone service. The second band ( KHz) is for upstream communication and the third band (250 KHz 1 MHz) for downstream communication. 19. ADSL uses either carrierless amplitude phase (CAP) or the discrete multitone modulation technique (DMT). 20. FTTC means fiber to the curb. Optical fiber is the medium from the central office of the telephone/cable company to the curb of the users premises. 21. Digital services are less sensitive to noise and have a wider bandwidth. 22. A DSU is used in digital services and because the service is already digital a modem is not needed to transform analog data into digital. The DSU changes the rate of the digital data created by the subscriber s device to 56 Kbps and encodes it in the format used by the service provider. A modem takes a digital signal and changes it to an analog signal and vice versa. 23. For synchronous TDM, the number of slots is the same as the number of input lines; for asynchronous TDM, the number of slots can be less than the number of input lines. 24. For DS-0 the sampling rate is 8000 samples/second; with 8 bits per sample, this means 8000??8 bps or 64 Kbps. CHAPTER 8 (BOOK) MULTIPLE CHOICE QUESTIONS 25. d 26. a 27. d 28. a 29. a 30. b 31. a 32. a 33. b 34. d 35. a 36. a 37. b 38. c 39. b 40. d 41. b 42. d 43. d 44. a 45. c 46. b 47. a 48. d 49. d 50. c 51. b 52. a 53. c 54. c 55. a 56. c 57. c 58. d CHAPTER 8 (BOOK) EXERCISES 59. (4000 X?5) + (200 X?4) = 20.8 KHz 60. (7900 Hz (200 X 2)) / 3 = 2.5 KHz 61. FDM: n is frequency of signal; TDM: n is time (s) bits per frame X?100 = 4.1 Kbps 63. Number of slots is derived by statistical method (analysis) of the number of input lines that are likely to be transferring at any given time. 64. E F - - T I - - E L A G 65. E4F4 T1I4E1 L4E2G us Kbps 68. n (n 1) / 2 = 500 (499)/2 = 124,750 lines; multiplexing can reduce the number of lines. 69. The original telephone lines were designed for voice (0 to 4000 Hz). 70. Nyquist theorem dictates that the sampling rate must be twice the highest fre-quency; 2??4000 Hz or 8000 Hz. 71. Theoretically, 2,000,000, 000 / 64,000 or channels. However, we need fram-ing bits for multiplexing. Therefore, the practical number of channels is a little bit less than T1 line =>?(1,544, X?64000) / 24 = 333 bits /channel =>?0.5% T2 line =>?(6,312,00 96 X?64000) / 96 = 1750 bits /channel =>2.7% T3 line =>?(44,736, X 64000) / 672 = 2571 bits /channel =>4.0% T4 line =>?(274,176, X?64000) / 4032 = 4000 bits /channel =>?6.2% 73. Bw = 20KHz 4KHz = 16 KHz X?20 = 99 KHz 75. See Figure See Figure Kbps X?100 = 1.44 Mbps; assuming the overhead is not too big; a T1 line could handle the situation (1.544 Mbps) 78. Thirty percent of the bandwidth is wasted X?566 Kbps = Mbps 80. See Figure 8.6 Output bit rate: 45 bits/second; duration; 22 ms; 45 slots per second 81. See Figure Mbps; T1 is not appropriate in this case (1.544 Mbps) 83. Data rate of each line: 40 Kbps; number of stations sending at full capacity: See Figure Kbps 86. See Figure

6 CHAPTER 6 (BOOK) MULTIPLE CHOICE QUESTIONS 33. c 34. d 35. a 36. b 37. d 38. a 39. b 40. d 41. b 42. d 43. c 44. a 45. a 46. a 47. d 48. d 49. d 50. d 51. d 52. b 53. c 54. c 55. b 56. c 57. c 58. a 59. a 60. b 61. a 62. c 63. c 64. b 65. b 66. b 67. d 68. b 69. a 70. d 71. b 72. d 73. d 74. c 75. a 76. a 77. a 78. d 79. d 80. c 81. d CHAPTER 7 (BOOK) MULTIPLE CHOICE QUESTIONS 32. b 33. b 34. a 35. b 36. a 37. b 38. b 39. d 40. c 41. c 42. b 43. a 44. a 45. d 46. c 47. b 48. a 49. b 50. a 51. b 52. a 53. a 54. a 55. c 56. b 57. a 58. d 59. a 60. b 61. c 62. c 63. a 64. b 65. d 66. c 67. b 68. a 69. b 70. c 71. d 72. b 73. a 74. c 75. d 76. b 77. d 78. d 79. b 80. c 6