Chapter 4
Data Link Layer
True-False Questions
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, medium, and hard) and the page
number(s) relevant to the topic are also furnished.
1.
Media access controls refer to the need to control when computers transmit.
2.
The data link layer accepts messages from the network layer and controls the hardware that
transmits them.
3.
Only the sender of a data transmission needs to be concerned about the rules or protocols that
govern how it communicates with the receiver.
4.
Most computer networks managed by a host mainframe computer use contention media
access control.
5.
Polling is the process of permitting all clients to transmit or receive at any time.
6.
With roll-call polling, a server polls clients in a consecutive, pre-arranged priority list.
7.
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.
8.
With contention, a computer does not have to wait before it can transmit. A computer can
transmit at anytime.
9.
Contention is commonly used with Ethernet local area networks.
10.
Controlled access MAC approaches work better in a large network with high usage.
11.
12.
The two categories of network errors are: lost data and delimited data.
13.
If a computer transmits a message containing “ABC” and the destination computer receives
“abc” as the message, the message is corrupted.
14.
Data transmission errors are typically uniformly distributed in time.
15.
Undesirable stray electrical voltage can cause data communication errors.
16.
Gaussian noise is a special type of attenuation.
17.
Impulse noise is caused by the thermal agitation of electrons.
18.
Crosstalk occurs when the signal transmitted on one circuit or channel of a transmission
system creates an undesired effect in another circuit or channel.
19.
Attenuation refers to the loss of signal strength.
20.
When the signals from two circuits combine to form a new signal that falls into a frequency
band reserved for another signal, this is called intermodulation noise.
21.
Ethernet has been refined over the years and is now formally standardized as IEEE802.3ac.
22.
The distance between repeaters or amplifiers on a telephone circuit is determined by the
amount of power gained per unit length of the transmission.
23.
When we amplify the signal on an analog circuit, we also amplify any noise that is present on
the circuit.
24.
For effective error detection and correction, extra error detection “data” must be included
with each message
25.
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.
26.
Parity checking can only detect an error when an even number of bits are switched.
27.
Cyclical redundancy check is one of the most popular polynomial error-checking schemes.
28.
The simplest method for error correction is retransmission.
29.
Another term for stop-and-wait ARQ is sliding window.
30.
One type of forward error correction is the Hamming code.
31.
Forward error correction is commonly used in satellite transmission.
32.
HDLC is very similar to the SDLC synchronous data link protocol.
33.
Overhead bits are used for error checking and marking the start and end of characters and
packets.
34.
Transmission efficiency refers to the percentage of bits transmitted without errors.
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, medium, and hard) and the
page number(s) relevant to the topic is 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
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 encoding the bit-stream as a series of electronic voltages.
c. It performs error detection.
d. It performs routing functions.
e. It organizes data from the physical layer and passes these coherent messages directly
to the application layer.
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
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
5.
Which of the following is not a controlled access method of media access control?
a. Token passing
b. CSMA/CD
c. polling
d. roll call polling
e. hub polling
6.
________ is the process of sending a signal to a client that gives its permission to transmit or
asks it to receive.
a. Contention
b. Polling
c. Pooling
d. Carrier sense
e. CRC
7.
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 server has to wait for a response from
the polled client or terminal.
8.
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
9.
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
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
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
12.
The two sublayers of the data link layer consist of the media access control sublayer and
the________ .
a. logical link control sublayer
b. network control sublayer
c. session sublayer
d. physical sublayer
e. transport sublayer
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
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
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
16.
Another term for impulse noise is:
a. Gaussian noise
b. spikes
c. attenuation
d. cross-talk
e. intermodulation noise
17.
The familiar background static on radios and telephones is called:
a. echoes
b. intermodulation noise
c. impulse noise
d. cross-talk
e. white noise
18.
The primary source of error in data communications is:
a. echoes
b. intermodulation noise
c. spikes
d. attenuation
e. cross-talk
19.
__________can obliterate a group of bits, causing a burst error.
a. Cross-talk
b. Attenuation
c. Impulse noise
d. Intermodulation noise
e. White noise
20.
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
21.
When a signal is reflected back to the transmitting equipment, it is called a(n) _________.
a. white noise
b. echo
c. crosstalk
d. attenuation
e. impulse noise
22.
The loss of power a signal suffers as it travels from the transmitting computer to a receiving
computer is:
a. white noise
b. spiking
c. attenuation
d. intermodulation noise
e. echo
23.
If a signal with a frequency of 500 MHz combines with a another signal of 1500 MHz and
they form a new signal of 2000 MHz; this is an example of:
a. intermodulation noise
b. attenuation
c. echo
d. white noise
e. cross-talk
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
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
26.
Which of the following is way to reduce or eliminate crosstalk on a circuit?
a. physically separating the cables from other communication cables
b. adding repeaters or amplifiers to a circuit
c. reducing the length of the cable
d. adding fluorescent lights
e. shorting a circuit
27.
On digital circuits, we use _________ to reshape the incoming signal and prevent
attenuation.
a. amplifiers
b. repeaters
c. multiplexers
d. digitizers
e. modems
28.
Which of the following is not an error detection method used in the data link layer?
a. parity checking
b. cyclic redundancy checking
c. CRC-32
d. pulse code checking
e. odd parity
29.
With odd parity (assume that the parity bit (in bold) 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
30.
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%
31.
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
32.
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%
33.
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
34.
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
35.
Stop-and-wait ARQ uses ___________ type of data flow.
a. full simplex
b. half complex
c. full duplex
d. half duplex
e. full complex
36.
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
37.
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)
38.
______________ 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
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. each character is transmitted independently of all other characters
40.
Which of the following is not a type of synchronous protocol?
a. SDLC
b. VT100
c. HDLC
d. Ethernet
e. PPP
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
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 transmits each character independently of one another and includes start and stop
bits for each character sent.
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 and is used in most LANs today.
e. It does not have any error detection capabilities.
43.
Which of the following is true about the data link protocol, Ethernet (IEEE 802.3)?
a. it is not very popular
b. it is a bit-oriented protocol
c. it uses block check characters for error detection
d. newer version permits jumbo frames
e. it uses parity bits for error detection
44.
Which of the following is true about the data link protocol, PPP?
a. it is commonly used in WANs
b. it is a bit-oriented protocol
c. it uses block check characters for error detection
d. it was developed in the 1960s
e. it uses parity bits for error detection
45.
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
46.
_________________ 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
47.
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
48.
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
Essay and Short Answer Questions
1. What does the data link layer do? What are its primary responsibilities? Where
does the data link layer sit in terms of the simplified five layer network model?
2. What is media access control and why is it important? What are two examples of
controlled access methods and contention based media access methods? When
might one access method be preferred over another in a network, and why? Under
what conditions do contention-based media access control techniques outperform
controlled-access techniques (i.e., have lower response time)? Explain.
3. Compare and contrast roll call polling, hub polling (or token passing), and
contention. Which is better, hub polling or contention? Explain.
4. Errors normally appear in ______________________________, which is when
more than one data bit is changed by the error-causing condition. Is there any
difference in the error rates of lower speed lines and of higher speed lines? What
kinds of lines are more prone to errors?
5. Briefly define noise. Describe five types of noise and the underlying causes of this
noise. Which type of noise is likely to pose the greatest problem to network
managers? What does error look like in a data network?
6. How do amplifiers differ from repeaters?
7. What are three ways of reducing errors and the types of noise they affect?
8. Describe three approaches to detecting errors, including how they work, the
probability of detecting an error, and any other benefits or limitations.
9. Briefly describe how even parity and odd parity work. Give an example of even
parity with a 7-bit ASCII code, for the following: 0110110, using a 0 start bit and
a 1 stop bit.
10. How does cyclical redundancy checking work?
11. How does forward error correction work? How is it different from other error
correction methods? Under what circumstances is forward error correction
desirable? What is one type of forward error correction by name? Given a
diagram like that in the text of forward error correction, be able to explain how
the system works.
12. Compare and contrast stop-and-wait ARQ and continuous ARQ.
13. Describe from a high level perspective the packet layout for SDLC, Ethernet, and
PPP.
14. What is transparency?
15. What is transmission efficiency? How do information bits differ from overhead
bits? What are three issues which might effect packet throughput rates? Which is
better for file transfer, large packet sizes or small packet sizes?
16. What is asynchronous transmission v. synchronous transmission? Describe one
protocol of each type.
17. Think about a recent world impact event and how useful the technologies of
communications might have been related to helping people impacted by that event
(Hurricane Katrina, Pakistan earthquakes, or some other large world event). How
might telecom have a positive role in large scale human events? Use insight
gained from the technological underpinnings provided in the course. Be creative.