Chapter 6: Errors, Error Detection, and Error Control true/false



Download 221.68 Kb.
Date18.06.2017
Size221.68 Kb.





Chapter 6: Errors, Error Detection, and Error Control
TRUE/FALSE
1. Because white noise is relatively continuous, it can be reduced significantly but never completely eliminated.

ANS: T PTS: 1


2. White noise is dependent on the temperature of the medium.

ANS: T PTS: 1


3. If the impulse spike interferes with an analog signal, removing it without affecting the original signal is extremely easy.

ANS: F PTS: 1


4. If impulse noise interferes with a digital signal, often the original digital signal cannot be recognized and recovered.

ANS: F PTS: 1


5. Noise is a problem for both analog and digital signals.

ANS: T PTS: 1


6. Telephone signal crosstalk was a more common problem before telephone companies used fiber-optic cables and other well-shielded wires.

ANS: T PTS: 1


7. Echo error rarely occurs at junctions where wires are connected.

ANS: F PTS: 1


8. If jitter becomes too great, correcting it can require the transmitting devices to speed up their transmission rates.

ANS: F PTS: 1


9. Attenuation can be eliminated with the use of repeaters for analog systems.

ANS: F PTS: 1


10. An unfortunate side effect of noise during a transmission is that the transmitting station has to slow down its transmission rate.

ANS: T PTS: 1


11. Installing special filters for analog signals and digital signal regeneration for digital signals helps in preventing white noise.

ANS: T PTS: 1


12. TCP performs error detection at the end points of the connection.

ANS: T PTS: 1


13. Simple parity can detect only an even number of erroneous bits per character.

ANS: F PTS: 1


14. The CRC error-detection method treats the packet of data to be transmitted as a large polynomial.

ANS: T PTS: 1


15. The CRC method is almost foolproof.

ANS: T PTS: 1


16. Returning a message was one of the first error-control techniques developed.

ANS: T PTS: 1


17. NAK represents a positive acknowledgment.

ANS: F PTS: 1


18. Sliding window protocols have been around since the 1970s.

ANS: T PTS: 1


19. When a parity bit is assigned to ASCII, the Hamming distance becomes 4.

ANS: F PTS: 1


20. In a stop-and-wait error control system, a receiver may acknowledge multiple packets with a single ACK.

ANS: F PTS: 1


21. In a sliding-winow error control system, a receiver may acknowledge multiple packets with a single ACK.

ANS: T PTS: 1



MULTIPLE CHOICE
1. White noise is also called thermal noise or ____ noise.

a.

crosstalk

c.

Gaussian

b.

jitter

d.

spike

ANS: C PTS: 1


2. Impulse noise is also known as ____.

a.

noise spike

c.

attenuation

b.

jitter

d.

crosstalk

ANS: A PTS: 1


3. Typically, impulse noise is a(n) ____ burst of energy.

a.

digital

c.

binary

b.

analog

d.

logical

ANS: B PTS: 1


4. High humidity and wet weather can cause an increase in electrical ____ over a telephone system.

a.

compression

c.

jitter

b.

attenuation

d.

crosstalk

ANS: D PTS: 1


5. To minimize the effect of echo, a device called a(n) ____ can be attached to a line.

a.

echo suppressor

c.

amplifier

b.

repeater

d.

hub

ANS: A PTS: 1


6. ____ is the result of small timing irregularities that become magnified during the transmission of digital signals as the signals are passed from one device to another.

a.

White noise

c.

Attenuation

b.

Jitter

d.

Echo

ANS: B PTS: 1


7. Causes of jitter can include electromagnetic interference, ____, passing the signal through too many repeaters, and the use of lower-quality equipment.

a.

coding

c.

compression

b.

sampling

d.

crosstalk

ANS: D PTS: 1


8. ____ is not necessarily a form of error, but can indirectly lead to an increase in errors affecting the transmitted signal.

a.

Impulse noise

c.

Attenuation

b.

Crosstalk

d.

White noise

ANS: C PTS: 1


9. ____ means that if the transmitting modem sends data and the data arrives garbled, the receiving modem may ask the transmitting modem to fall back to a slower transmission speed.

a.

Impulse negotiation

c.

Attenuation

b.

Fallback negotiation

d.

Downlink

ANS: B PTS: 1


10. Twisted pair Category 5e/6 cable should not be longer than the recommended 100-meter (300-foot) distance when it’s transmitting at ____ Mbps.

a.

100

c.

500

b.

250

d.

1000

ANS: A PTS: 1


11. Installing proper shielding on cables prevents ____.

a.

white noise

c.

attenuation

b.

impulse noise

d.

crosstalk

ANS: D PTS: 1


12. Error detection can be performed in several places within a communications model. One of the most common places is the TCP/IP ____ layer.

a.

network

c.

network access/data link

b.

application

d.

physical

ANS: C PTS: 1


13. The most basic error-detection techniques are parity checks, which are used with ____ connections.

a.

synchronous

c.

statistical

b.

asynchronous

d.

fiber-optic

ANS: B PTS: 1


14. With ____ parity, the 0 or 1 added to the string produces an even number of binary 1s.

a.

even

c.

longitudinal

b.

odd

d.

binary

ANS: A PTS: 1


15. Isolated single-bit errors occur ____ percent of the time.

a.

30 to 40

c.

50 to 60

b.

40 to 50

d.

60 to 70

ANS: C PTS: 1


16. Error bursts occur ____ percent of the time.

a.

2 to 5

c.

8 to 10

b.

5 to 10

d.

10 to 20

ANS: D PTS: 1

17. Longitudinal parity is sometimes called longitudinal redundancy check or ____ parity.

a.

vertical

c.

random

b.

horizontal

d.

binary

ANS: B PTS: 1


18. The cyclic redundancy checksum (CRC) is also called ____.

a.

horizontal parity

c.

bit parity

b.

vertical cyclic parity

d.

cyclic checksum

ANS: D PTS: 1


19. CRC-____ is found in Ethernet LANs.

a.

5

c.

32

b.

12

d.

64

ANS: C PTS: 1


20. ____ for error control is becoming a mode of operation for some modern wide area network transmission techniques.

a.

Returning a message

c.

Correcting the error

b.

Doing nothing

d.

Fixing the error

ANS: B PTS: 1


21. ____ is offered by telephone companies to transfer data over wide areas.

a.

RS-232

c.

SSL

b.

EIA-232F

d.

Frame Relay

ANS: D PTS: 1


22. ____ cable is the medium least prone to generating errors.

a.

Fiber-optic

c.

Twisted-pair

b.

Coaxial

d.

Copper-based

ANS: A PTS: 1


23. ____ is probably the most common form of error control.

a.

Do nothing

b.

Correct the error without retransmission

c.

Sending a message back to the transmitter

d.

Correct the error with retransmission

ANS: C PTS: 1


24. ____ error control is a technique usually associated with the Stop-and-wait flow control protocol.

a.

Stop-and-wait

c.

Sliding window

b.

Timeout

d.

ACK

ANS: A PTS: 1

25. When working with stop-and-wait error control, the amount of time a station waits before retransmitting a packet is called ____.

a.

stop time

c.

NAK time

b.

ACK time

d.

timeout

ANS: D PTS: 1


26. ____ is a flow control scheme that allows a station to transmit a number of data packets at one time before receiving some form of acknowledgment.

a.

Stop-and-wait

c.

Timeout

b.

Sliding window

d.

ACK window

ANS: B PTS: 1


27. With the sliding window protocol, a station that had a maximum window size of 7 could transmit only ____ data packets at one time before it had to stop and wait for an acknowledgment.

a.

5

c.

7

b.

6

d.

8

ANS: C PTS: 1


28. A ____ protocol’s function is simply to inform the transmitter what piece of data is expected next.

a.

CRC-16

c.

stop-and-wait

b.

CRC-32

d.

sliding window

ANS: D PTS: 1


29. For a receiver to be able to fix an error—in a process called ____—redundant information must be present so that the receiver knows which bit or bits are in error and what their original values were.

a.

forward error correction

c.

backward error correction

b.

forward error detection

d.

backward error detection

ANS: A PTS: 1


30. If a receiver correctly receives packets 2, 3, and 4, it will respond with a(an) ____ .

a.

ACK 3

c.

ACK 5

b.

ACK 4

d.

NAK

ANS: C PTS: 1


31. If a receiver has just received one packet of data with bytes numbered 600 to 700, it will respond immediately with ____.

a.

an ACK of 700

c.

an ACK of 800

b.

an ACK of 701

d.

it will not respond just yet

ANS: B PTS: 1



COMPLETION
1. ____________________ is a relatively continuous type of noise and is much like the static you hear when a radio is being tuned between two stations.

ANS: White noise

PTS: 1
2. ____________________ is a noncontinuous noise and one of the most difficult errors to detect, because it can occur randomly.

ANS: Impulse noise

PTS: 1
3. ____________________ is an unwanted coupling between two different signal paths.

ANS: Crosstalk

PTS: 1
4. ____________________ is the reflective feedback of a transmitted signal as the signal moves through a medium.

ANS: Echo

PTS: 1
5. A(n) ____________________ is essentially a filter that allows the signal to pass in one direction only.

ANS: echo suppressor

PTS: 1
6. When a digital signal is being transmitted, the rises and falls of the signal can start to shift, or become blurry, and thus produce ____________________.

ANS: jitter

PTS: 1
7. ____________________ is the continuous loss of a signal’s strength as it travels though a medium.

ANS: Attenuation

PTS: 1

8. When a modem first makes a connection with another modem, the two modems participate in ____________________ negotiation.



ANS: fallback

PTS: 1
9. The simplest error-detection technique is ____________________ parity.

ANS: simple

PTS: 1
10. Simple parity comes in two basic forms: even parity and ____________________ parity.

ANS: odd

PTS: 1
11. With ____________________ parity, the 0 or 1 added to the string produces an odd number of binary 1s.

ANS: odd

PTS: 1
12. ____________________ parity tries to solve the main weakness of simple parity—that all even numbers of errors are not detected.

ANS: Longitudinal

PTS: 1
13. The ____________________ method typically adds 8 to 32 check bits to potentially large data packets and yields an error detection capability approaching 100 percent.

ANS:

cyclic redundancy checksum (CRC)



cyclic redundancy checksum

CRC


cyclic checksum

PTS: 1
14. A(n) _________________________ is an industry-approved bit string that is used to create the cyclic checksum remainder.

ANS: generating polynomial

PTS: 1
15. Once an error in the received data transmission stream is detected, the action that the receiver takes is called ____________________.

ANS: error control

PTS: 1
16. ____________________ is a process that keeps a transmitter from sending too much data to a receiver.

ANS: Flow control

PTS: 1
17. Sliding window error control is based on the ____________________ protocol.

ANS: sliding window

PTS: 1
18. In TCP/IP, if a receiver just received some data and wishes to send data back to the sender, then the receiver should include an ACK with the data it’s about to send. This is called ____________________.

ANS: piggybacking

PTS: 1
19. A(n) ____________________ code is a specially designed code in which special check bits have been added to data bits such that, if an error occurs during transmission, the receiver may be able to correct the error using the included check and data bits.

ANS: Hamming

PTS: 1
20. The ____________________ of a code is the smallest number of bits by which character codes differ.

ANS: Hamming distance

PTS: 1


ESSAY
1. List and briefly explain some error-prevention techniques.

ANS:


You can prevent the occurrence of many types of transmission errors by applying proper error-prevention techniques, including those listed below:

* Install wiring with the proper shielding to reduce electromagnetic interference and crosstalk.

* Use telephone line conditioning or equalization (provided by the telephone company), in which filters are used to help reduce signal irregularities. For an additional charge, the telephone company will provide various levels of conditioning to leased lines. This conditioning provides a quieter line, which minimizes data transmission errors.

* Replace older equipment with more modern, digital equipment; although initially expensive, this technique is often the most cost-effective way to minimize transmission errors in the long run.

* Use the proper number of digital repeaters and analog amplifiers to increase signal strength, thus decreasing the probability of errors.

* Observe the stated capacities of a medium, and, to reduce the possibility of errors, avoid pushing transmission speeds beyond their recommended limits.

PTS: 1
2. How does simple parity work?

ANS:


If the 7-bit ASCII character set is used, a parity bit is added as the eighth bit. Suppose, for example, that the character “k”—which is 1101011 in binary—is transmitted and even parity is being applied. In this case, a parity bit of 1 would be added to the end of the bit stream, as follows: 11010111. There is now an even number (six) of 1s. (If odd parity were used, a 0 would be added at the end, resulting in 11010110.)
Now, if a transmission error causes one of the bits to be flipped (the value is erroneously interpreted as a 0 instead of a 1, or vice versa), the error can be detected if the receiver understands that it needs to check for even parity. Returning to the example of the character “k” sent with even parity, if you send 11010111 but 01010111 is received, the receiver will count the 1s, see that there is an odd number, and know there is an error. What happens if 11010111 with even parity is sent and two bits are corrupted? For example, 00010111 is received. Will an error be detected? The answer is no, an error will not be detected, because the number of 1s is still even.

PTS: 1


3. How does longitudinal parity work?

ANS:


The first step of this parity scheme involves grouping individual characters together in a block. Each character (also called a row) in the block has its own parity bit. In addition, after a certain number of characters are sent, a row of parity bits, or a block character check, is also sent. Each parity bit in this last row is a parity check for all the bits in the column above it. If one bit is altered in Row 1, the parity bit at the end of Row 1 signals an error. In addition, the parity bit for the corresponding column also signals an error. If two bits in Row 1 are flipped, the Row 1 parity check will not signal an error, but two column parity checks will signal errors. This is how longitudinal parity is able to detect more errors than simple parity. Note, however, that if two bits are flipped in Row 1 and two bits are flipped in Row 2, and the errors occur in the same column, no errors will be detected.

PTS: 1
4. How does the cyclic redundancy checksum work?

ANS:

The CRC error-detection method treats the packet of data to be transmitted (the message) as a large polynomial. The rightmost bit of the data becomes the x0 term, the next data bit to the left is the x1 term, and so on. When a bit in the message is 1, the corresponding polynomial term is included.


The transmitter takes this message polynomial and, using polynomial arithmetic, divides it by a given generating polynomial, and produces a quotient and a remainder. The quotient is discarded, but the remainder (in bit form) is appended to the end of the original message polynomial, and this combined unit is transmitted over the medium. When the data plus remainder arrive at the destination, the same generating polynomial is used to detect an error.
The receiver divides the incoming data (the original message polynomial plus the remainder) by the exact same generating polynomial that was used by the transmitter. If no errors were introduced during data transmission, the division should produce a remainder of zero. If an error was introduced during transmission, the arriving original message polynomial plus the remainder will not divide evenly by the generating polynomial and will produce a nonzero remainder, signaling an error condition.

PTS: 1


5. How does frame relay control errors?

ANS:


Frame relay, which has only been in existence since 1994 and is offered by telephone companies to transfer data over wide areas, supports the “do nothing” approach to error control. If a data frame arrives at a frame relay switch and an error is detected after the cyclic checksum is performed, the frame is simply discarded. The rationale behind this action is twofold. Frame relay networks are created primarily of fiber-optic cable. Because fiber-optic cable is the medium least prone to generating errors, it is assumed that the rate of errors is low and that error control is unnecessary. If a frame is in error and is discarded, frame relay assumes that either the transport layer or the higher-layer application that is using frame relay to transmit data will keep track of the frames and will notice that a frame has been discarded. It would then be the responsibility of the application to request that the dropped frame be retransmitted. Consider the example in which a company has a database application that sends database records across the country between two corporate locations. The database application (at the application layer) is using frame relay at the data link layer to transfer the actual records. If a record or part of a record is dropped by frame relay because of a transmission error, frame relay does not inform the application. Instead, the database application has to keep track of all records sent and received, and if one record does not arrive at the destination, the database application has to ask for a retransmission.

PTS: 1


© 2011 Cengage Learning. All Rights Reserved. This edition is intended for use outside of the U.S. only, with content that may be different from the U.S. Edition. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in part.


Download 221.68 Kb.

Share with your friends:




The database is protected by copyright ©sckool.org 2020
send message

    Main page