Business Data Communications and Networking 7th Edition Test Bank - Chapter 04 (400 Level Course)
TRUE/FALSE
The following are possible True/False questions for tests. The statement is given and the answer is provided in square brackets. The level of difficulty (easy, moderate, difficult) and the page number(s) relevant to the topic are also furnished.
1. The data link layer accepts messages (in packet form) from the network layer. [True; p. 109]
Moderate
2. Only the sender of a data transmission needs to be concerned about the rules or protocols that govern how it communicates with the receiver. [False, both sender and receiver must agree on the protocols; p. 109]
Easy
3. Media access control refers to detecting errors. [False, it refers to the need to control when computers transmit; p. 109]
Easy
4. Message delineation refers to the control that is needed when computers transmit. [False, it is identifying the start and end of a message; p. 109]
Moderate
5. Two fundamental approaches to media access control are: controlled access and multiplexing. [False, it is controlled access and contention; p. 110]
Easy
6. Most computer networks managed by a host mainframe computer use contention media access control. [False, they use controlled access; p. 110]
Moderate
7. Polling is the process of permitting all clients to transmit or receive at any time. [False, it is the process of sending a signal to the client that gives it permission to transmit or receive; p. 110]
Moderate
8. With roll call polling, a server polls clients in a consecutive, pre-arranged priority list. [True; p. 110]
Easy
9. Token passing is a term that refers to hub polling, in which one computer starts a poll and passes it to the next computer on a multipoint circuit. [True; p. 111]
Moderate
10. Contention is commonly used with Ethernet local area networks. [True; p. 111]
Easy
11. Controlled access MAC approaches work better in a large network with high usage. [True; p. 111]
Easy
12. In a network, the type of errors caused during data transmission can be controlled by the network hardware and software. [True; p. 112]
Easy
13. The two categories of network errors are: lost data and delimited data. [False, they are corrupted data and lost data; p. 112]
Moderate
14. Corrupted data refers to data that has been changed. [True; p. 112]
Easy
15. In data transmission, data errors are uniformly distributed in time. [False, they appear in bursts; p. 112]
Easy
16. Noise that is introduced into a data transmission can show up as extra bits. [True; p. 113]
Moderate
17. Gaussian noise is a special type of attenuation. [False, it is the same as white noise; p. 113]
Moderate
18. Eliminating jitter to generate a pure carrier signal in an analog circuit is impossible. [True; p. 114]
Moderate
19. The distance between repeaters or amplifiers on a telephone circuit is determined by the amount of power gained per unit length of the transmission. [False, it is determined by the amount of power lost; p. 115]
Moderate
20. Amplifiers are commonly used on analog circuits. [True; p. 115]
Moderate
21. A conditioned circuit is more expensive than a regular telephone circuit because it has been certified by the carrier to experience fewer errors. [True; p. 116]
Moderate
22. For effective error detection and correction, extra error detection “data” must be included with each message. [True; p. 116]
Moderate
23. In an odd parity-checking scheme, the parity bit is set to make the total number of ones in the byte (including the parity bit) an even number. [False; it makes the total number of ones, including the parity bit, an odd number; p. 116]
Moderate
24. The simplest method for error correction is retransmission. [True; p. 118]
Easy
25. Another term for stop-and-wait ARQ is sliding window. [False, continuous ARQ is also known as sliding-window; p. 118]
Moderate
26. One type of forward error correction is the Hamming code. [True; p. 121]
Easy
27. Forward error correction is commonly used in satellite transmission. [True; p. 120]
Easy
28. The asynchronous file transfer protocol, Kermit, was named after Kermit the Frog. [True; p. 124]
Easy
29. HDLC is very similar to the SDLC synchronous data link protocol. [True; p. 126]
Moderate
30. Ethernet is a character-oriented data link protocol. [False, it is a byte-count-oriented protocol; p. 126]
Moderate
31. Point-to-point Protocol is a byte-count-oriented protocol. [False, it is a byte-oriented protocol; p. 126]
Moderate
MULTIPLE CHOICE
The following are possible multiple-choice questions for tests. The question is posed and the answer is provided under the choices. The level of difficulty (easy, moderate, difficult) and the page number(s) relevant to the topic are also furnished.
1. As part of the five-layer network model used in this textbook, the data link layer sits directly between:
a. the physical and the application layers
b. the network and the application layers
c. the network and transport layers
d. the physical and the application layers
e. the physical and the network layers
Answer: e, Easy, p. 109
2. Which of the following is true with respect to the data link layer?
a. It accepts streams of bits from the application layer.
b. It is responsible for getting a message from one computer to another (one node to another) without errors.
c. It accepts messages from the physical layer.
d. It performs routing functions.
e. It organizes data from the physical layer and passes these coherent messages to the application layer.
Answer: b, Moderate; p. 109
3. ____________ is not a function of a data link protocol.
a. Media access control
b. Message delineation
c. Amplitude shift keying
d. Indicating when a message starts and stops
e. Error control
Answer: c, Moderate, p. 109
4. Media access control:
a. is not very important in point-to-point with full duplex configuration
b. is not very important in local area networks
c. is not very important in a point-to-point with a half duplex configuration
d. is not very important in a multipoint configuration
e. does not control when computers transmit
Answer: a, Moderate; p. 109-110
5. Which of the following is not a controlled access method of media access control?
a. X-ON/X-OFF
b. CSMA/CD
c. polling
d. roll call polling
e. hub polling
Answer: b, Moderate, p. 110-111
6. Which of the following is true about roll call polling?
a. It can not be modified to increase priority of clients or terminals.
b. It does not require a server or host or special device that performs the polling.
c. It is also called token passing.
d. It is a type of contention approach to media access control.
e. It typically involves some waiting because the front-end processor or server has to wait for a response from the polled client or terminal.
Answer: e, Moderate, p. 110
7. With contention:
a. computers wait until the circuit is free before they send data
b. the server or front end processor works consecutively through a list of clients to determine who should have access to the media
c. the front end processor must wait for a response from the polled client or terminal
d. one computer starts the poll and passes it to the next computer on the multipoint circuit
e. there is never a chance for “collision,” or two computers trying to send data at the same time
Answer: a, Moderate, p. 111
8. In general, controlled approaches:
a. work better than contention approaches for small networks that have low usage
b. work better than contention approaches for large networks that have high usage
c. work better than contention approaches for all sizes of networks
d. do not require a host, server, or active monitor to assign media access control
e. have many collisions
Answer: b, Moderate, p. 111
9. Which of the following is not an appropriate choice in this sentence, “Networks should be designed to __________ both corrupted and lost data.”
a. detect
b. prevent
c. aggravate
d. correct
e. all of the above
Answer: c, Easy, p. 112
10. In a _________ , more than one data bit is changed by the error-causing condition.
a. burst error
b. data rate shift
c. Trellis-coded modulation
d. uniform distribution
e. amplitude key shift
Answer: a, Moderate, p. 113
11. Errors on a network can occur:
a. only on dial-up type of circuits
b. because of noise on the line
c. only on poorly maintained networks
d. only due to Gaussian noise
e. only due to lightning strikes
Answer: b, Easy, p. 113
12. In a dial-up network:
a. the error rate will vary because the circuits will change with each dial-up
b. the network is less prone to errors than private dedicated lines
c. users might try to transmit the data at a higher speed to decrease the error rate
d. constant transmission conditions will always create a constant error rate
e. users might try to transmit the data at a lower speed to speed up the data transmission
Answer: a, Moderate, p. 113
13. Which of the following media is least susceptible to noise?
a. fiber optic cable
b. coaxial cable
c. twisted pair
d. unshielded twisted pair
e. shielded twisted pair
Answer: a, Easy, p. 113
14. Optical media is:
a. more likely to suffer from noise than electrical media
b. has about the same likelihood of suffering from noise as electrical media
c. has about the same likelihood of suffering from noise as coaxial cable
d. less likely to suffer from noise than electrical media
e. more prone to noise than twisted pair media
Answer: d, Moderate, p. 113
15. ____________ refers to bits that have been changed, in error, from 1 to 0, or vice versa, in a data transmission.
a. Contracted bits
b. Polled bits
c. Inverse multiplexed bits
d. Flipped bits
e. Bit delineation
Answer: d, Moderate, p. 113
16. In a data communication network, noise can cause:
a. extra bits in an message
b. missing bits in a message
c. flipped bits in a message
d. lost bits in a message
e. all of the above
Answer: e, Moderate, p. 113
17. ____________ is not a major source of data transmission error.
a. Conditioning
b. White noise
c. Line outages
d. Impulse noise
e. Cross-talk
Answer: a, Moderate, p. 113-114
18. Another term for impulse noise is:
a. Gaussian noise
b. spikes
c. attenuation
d. cross-talk
e. intermodulation noise
Answer: b, Easy, p. 114
19. The familiar background static on radios and telephones is called:
a. echoes
b. intermodulation noise
c. line outages
d. cross-talk
e. white noise
Answer: e, Easy, p. 113
20. The primary source of error in data communications is:
a. echoes
b. intermodulation noise
c. spikes
d. jitter
e. cross-talk
Answer: c, Easy, p. 114
21. Cross-talk:
a. occurs when one circuit picks up signals in another
b. is always bothersome because it has a high signal strength
c. decreases with increased proximity of two wires
d. decreases during wet or damp weather
e. increases with lower frequency signals
Answer: a, Moderate, p. 114
22. The loss of power a signal suffers as it travels from the transmitting computer to a receiving computer is:
a. jitter
b. spiking
c. attenuation
d. intermodulation
e. echo
Answer: c, Easy, p. 114
23. A phase hit is likely to be:
a. spikes
b. a short term shift out of phase
c. intermodulation noise
d. white noise
e. Gaussian noise
Answer: b, Moderate, p. 114
24. _____________ is an effective way to prevent impulse noise, cross talk, and intermodulation noise.
a. Shielding wires
b. Adding fluorescent lights
c. Adding repeaters to a circuit
d. Adding amplifiers to a circuit
e. Shorting a circuit
Answer: a, Easy, p. 114-115
25. _____________ is an effective way to prevent attenuation.
a. Shielding wires
b. Adding fluorescent lights
c. Adding repeaters or amplifiers to a circuit
d. Changing multiplexing techniques
e. Shorting a circuit
Answer: c, Easy, p. 115
26. Which of the following is not an error detection method used in the data link layer?
a. parity checking
b. cyclic redundancy checking
c. longitudinal redundancy checking
d. pulse code checking
e. polynomial checking
Answer: d, Moderate, p. 116-118
27. With odd parity (assume that the parity bit has been placed at the end of each of the following) and a 7-bit ASCII code, which of the following is incorrect?
a. 01101011
b. 00011011
c. 00100101
d. 10110110
e. 11111110
Answer: b, Moderate, p. 116
28. The probability of detecting an error, given that one has occurred, using parity checking is about:
a. 100%
b. 0%
c. 50%
d. 75%
e. 98%
Answer: c, Easy, p. 117
29. Using parity, the probability for detecting an error, given that one has occurred, is:
a. about 50% for either even or odd parity
b. about 70% for even parity and 30% for odd parity
c. about 30% for even parity and 70% for odd parity
d. about 0% for either even or odd parity
e. about 100% for either even or odd parity
Answer: a, Moderate, p. 117
30. Longitudinal redundancy checking adds one additional character, called the ____________, to the end of the data packet.
a. parity byte
b. checksum character
c. cyclic redundancy character
d. block check character
e. polynomial check character
Answer: d, Easy, p. 117
31. The probability of detecting an error, provided that one has occurred, using cyclic redundancy checking is about:
a. 75%
b. 0%
c. exactly 100%
d. 50%
e. > 99%
Answer: e, Easy, p. 117
32. ARQ means that:
a. a receiver that detects an error in a message simply asks the sender to retransmit the message until it is received without error
b. the common carrier Automatically Returns Queries to the subscriber upon receipt of such queries
c. a sender is using a data link protocol called Asynchronous Repeating reQuest
d. a fiber optic cable meets the American Registered Quality, a certification standard for use in high-quality data communication transmission lines
e. a sender is using a parity scheme called Array Resource Quality
Answer: a, Moderate, p. 118
33. In ARQ, a NAK:
a. is sent by the recipient if the message was received without error
b. is sent by the sender at the same time as it sends a data packet
c. is sent by the recipient if the message contains an error
d. refers to non-asynchronous Kermit technique
e. means that the sender should continue with sending the next message
Answer: c, Moderate, p. 118
34. Stop-and-wait ARQ uses ___________ type of data flow.
a. full simplex
b. half complex
c. full duplex
d. half duplex
e. full complex
Answer: d, Moderate, p. 118
35. With __________ ARQ, the sender pauses for a response from the receiver to alter each message or packet of data.
a. open window
b. halt and be recognized (HBR)
c. sliding window
d. stop and wait
e. continuous
Answer: d, Moderate, p. 118
36. With __________ ARQ, the sender immediately sends the next message or packet of data.
a. continuous
b. immediate
c. open window
d. stop-and-wait
e. halt and be recognized (HBR)
Answer: a, Moderate, p. 118
37. ______________ controls errors by detecting and correcting them at the receiving end without retransmission of the original message.
a. Hamming code
b. Huffman encoding
c. Front end processing
d. Wave division multiplexing
e. Hub polling
Answer: a, Moderate, p. 120-121
38. One of the characteristics of many forward error correcting codes is that there must be a minimum:
a. of 48 parity bits per byte
b. number of error-free bits between burst of errors
c. line speed of 56,000 bits per second
d. all of the above
e. b and c only
Answer: b, Difficult, p. 120
39. Asynchronous transmission:
a. is used to transmit each character simultaneously with all other characters
b. has a pre-determined, fixed time between sending characters
c. is typically used on multipoint half duplex circuits
d. uses a continuous series of start bits as an idle signal
e. is also known as start-stop transmission
Answer: e, Moderate, p. 122-123
40. Which of the following is not a type of asynchronous file transfer protocol?
a. XMODEM-CRC
b. OSCAR
c. KERMIT
d. XMODEM-1K
e. ZMODEM
Answer: b, Easy, p. 123-124
41. Synchronous transmission:
a. cannot be used on multipoint circuits
b. is used to send one character at a time
c. uses start bits before each character to be sent
d. uses stop bits after each character to be sent
e. is used to transmit a “frame” or “packet” of data at a time
Answer: e, Moderate, p. 124-125
42. Which of the following is true about the data link protocol, SDLC:
a. It is a client-server protocol developed by Compaq in 1996
b. It is a byte-count-oriented protocol
c. It uses a special bit pattern called a flag at the beginning and end of every frame (or packet)
d. It uses a contention media access control protocol
e. It does not have a problem with transparency
Answer: c, Moderate, p. 125
43. Which of the following is true about the data link protocol, Ethernet (IEEE 802.3)?
a. it is a byte-oriented protocol
b. it is a bit-oriented protocol
c. it uses block check characters for error detection
d. it is a byte-count oriented protocol
e. it uses parity bits for error detection
Answer: d, Difficult, p. 126
44. Which of the following is true about the data link protocol, PPP?
a. it is a byte-oriented protocol
b. it is a bit-oriented protocol
c. it uses block check characters for error detection
d. it is a byte-count oriented protocol
e. it uses parity bits for error detection
Answer: a, Difficult, p. 126
45. Which of the following is not a type of synchronous data link protocol?
a. SDLC
b. KERMIT
c. PPP
d. HDLC
e. Ethernet (IEEE 802.3)
Answer: b Moderate, p. 124-127
46. In communication protocols, _________ are used to convey the user’s meaning.
a. information bits
b. overhead bits
c. stop bits
d. start bits
e. flag bits
Answer: a, Easy, p. 127
47. _________________ is defined as the total number of information bits divided by the total number of bits in the transmission.
a. Asynchronous rate
b. Protocol percentage
c. Throughput
d. Transmission Rate of Information Bits
e. Transmission efficiency
Answer: e, Easy, p. 127
48. Throughput of a data communication network is affected by:
a. number of retransmissions
b. the efficiency of the data link protocol
c. error rate on the communication circuit
d. a and b only
e. all of the above
Answer: e, Moderate, p. 128-129
49. Calculating the actual throughput of a data communication network is:
a. not normally required for synchronous networks because they are so fast
b. complex because many factors affect throughput
c. simple because packet size is the primary factor affecting throughput
d. not needed for satellite-based networks
e. far less complicated if the system operates on a contention basis
Answer: b, Easy, p. 129
50. TRIB, an acronym that relates to throughput, stands for:
a. Throughput Reduction of Information Barriers
b. Transmission Rate of Information Bits
c. Throughput Rate of Iso-synchronous Bytes
d. Transmission Regulation for Inverse-multiplexing Bands
e. Trellis-coded Regulation of Information Bits
Answer: b, Moderate, p. 129
Short Answer Questions
1. What is media access control and why is it important?
Answer: Media access control refers to the need to control when computers transmit. This becomes important when several computers share the same communication circuit, such as a multipoint circuit. It is critical to ensure that no two computers attempt to transmit data at the same time, or there must be a way to recover. Two fundamental approaches to MAC are controlled access and contention.
2. Compare and contrast hub polling and contention-based approaches to media access control.
Answer: Hub polling is often used in LAN multipoint configurations (i.e., token ring) that do not have a central host computer. One computer starts the poll and passes it to the next computer on the multipoint circuit, which sends its message and passes the poll to the next. That computer then passes the poll to the next, and so on, until it reaches the first computer, which restarts the process again.
Contention is the opposite of controlled access. Computers wait until the circuit is free (i.e., no other computers are transmitting), and then transmit whenever they have data to send. Contention is commonly used in Ethernet local area networks.
Most computer networks managed by a host mainframe computer use controlled access. In this case, the mainframe or its front-end processor controls the circuit and determines which clients can access media at what time.
3. Under what conditions do contention-based media access control techniques outperform controlled-access techniques (i.e., have lower response time)? Explain.
Answer: In general, contention approaches work better than controlled approaches for small networks that have low usage. In this case, each computer can transmit when necessary, without waiting for permission. In high volume networks, many computers want to transmit at the same time, and a well-controlled circuit prevents collisions.
4. What are two major sources of error and how can you prevent them?
Answer: Two major sources of error are from humans and networks. Human errors, such as a mistake in typing a number, usually are controlled through the application program. Network errors, such as those that occur during transmission, are controlled by the network hardware and software.
5. Briefly define noise.
Answer: Noise is undesirable electrical signals, or undesirable light in the case of fiber optics. Noise is introduced by equipment or natural disturbances, and it degrades the performance of a communication circuit. Noise manifests itself as extra bits, missing bits, or bits that have been “flipped”.
6. How do amplifiers differ from repeaters?
Answer: An amplifier takes an incoming signal, increases its strength, and retransmits it on the next section of the circuit. They are typically used on analog circuits such as the telephone company’s voice circuits. On the other hand, repeaters are commonly used on digital circuits. A repeater receives the incoming signal, translates it into a digital message, and retransmits the message. Because the message is re-created at each repeater, noise and distortion from the previous circuit are not amplified. This produces a much cleaner signal with a lower error rate for digital circuits.
7. How does even and odd parity work?
Answer: The sender and recipient agree to use either an odd or even parity scheme. If they agree to use an odd scheme, then all the ones in that byte (including the additional parity bit) must add up to be an odd number of ones. If they agree to use an even scheme, then all the ones in that byte (including the additional parity bit) must add up to be an even number of ones.
8. What is transparency, and why is this a problem with SDLC?
Answer: SDLC cannot automatically send all types of data with any bit patterns. It is possible that the user’s data to be transmitted contains the same bit pattern as the start/stop flag, 01111110. If this is not prevented, the receiver will mistakenly believe that this data marks the end of the frame and ignore all the data that follows it. The solution is called bit stuffing, a way to enable the recipient to differentiate between the start/stop flag and user’s data.
9. How would a message of THE be sent using odd LRC if the ASCII codes for the letters are: T 0010011 H 0001111 E 0000101
Answer:
Letter ASCII Parity bit
T 0010011 0
H 0001111 1
E 0000101 1
BCC 1100110 1
10. How would the bit pattern 0110110 be sent using odd parity?
Answer: 01101101
11. Which is better, CRC-16 or LRC? Explain.
Answer: CRC-16 is better than LRC. LRC is usually used in conjunction with parity, producing an error detection rate above 98 percent for typical burst errors of ten bits or more; while CRC-16 will detect about 99.998% of all burst errors longer than 16 bits.
12. Suppose two computers transferred a series of two messages using Stop and Wait ARQ. Suppose the first message is initially received with an error, and the second message initially not received at all. Draw the pattern of messages, ACKs, etc. that would flow between the two to ensure successful transfer of both messages.
13. Describe how Ethernet and SDLC mark the end of a message. Which is "better?"
Ethernet is a byte-count protocol because instead of using special characters or bit patterns to mark the end of a packet, it includes a field that specifies the length of the message portion of the packet. The packet ends with a CRC-32 frame check sequence used for error detection.
SDLC is a bit-oriented protocol, because the data contained in the frame do not have to be in 8-bit bytes. SDLC is therefore more flexible than byte-oriented protocols. Each SDLC frame begins and ends with a special bit pattern, known as the flag. It is possible that the user’s data to be transmitted contains the same bit pattern as the flag. If this is not prevented, the receiver will mistakenly believe that this data marks the end of the frame and ignore all the data that follows it. Therefore, Ethernet is “better” for detecting and not causing errors.
14. Which is better for file transfer, large packet sizes or small packet sizes? Justify your answer.
Answer: The general rule is that the larger the packet, the more efficient the protocol and the greater the throughput. However, the probability that a packet contains an error increases with the size of the packet; larger packets are more likely to contain errors than smaller ones, simply due to the laws of probability, and require retransmissions. Thus in designing a protocol, there is a trade-off between large and small packets. Small packets are less efficient, but are less likely to contain errors and “cost” less (in terms of circuit capacity) to retransmit if there is an error.
Throughput is the total number of information bits received per second, after taking into account the overhead bits and the need to retransmit packets containing errors. Generally speaking, small packets provide better throughput for circuits with more errors, while larger packets provide better throughput in less error-prone networks. Packet sizes vary greatly among different networks.
15. Thought question: Could 10-bit asynchronous transmission protocols be used on a multipoint circuit? Explain.
Answer: No, 10-bit asynchronous transmission protocols cannot be used on a multipoint circuit. Multipoint circuits require source and destination addresses and asynchronous transmission does not contain them.
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