1. Physical Layer - Tests

Question 1

The amount of time it takes to switch between sending and receiving in half-duplex transmission is called the propagation delay.

Answer 1

False

The amount of time half-duplex communication takes to switch between sending and receiving is called turnaround time (also called retrain time or reclocking time).

One disadvantage of satellite transmission is the propagation delay that occurs because the signal has to travel out into space and back to earth

Question 2

Statistical time division multiplexing does not require the capacity of the circuit to be equal to the sum of the combined circuits.

Answer 2

True

STDM is the exception to the rule that the capacity of the multiplexed circuit must equal the sum of the circuits it combines.

Question 3

In simplex transmission, the data flows in both directions at the same time

Answer 3

False

Data can flow through the circuit in one direction only (simplex)

Question 4

Computers produce digital data whose values are binary: 0 or 1.

Answer 4

True

Computers produce digital data that are binary, either on or off, 0 or 1.

Question 5

Nonreturn to zero is a type of unipolar signaling

Answer 5

False

In bipolar signaling, the ones and zeros vary from a plus voltage to a minus voltage (like an AC current). The first bipolar technique is called nonreturn to zero (NRZ) because the voltage alternates from +5 volts (a symbol indicating a 1) to −5 volts (a symbol indicating a 0) without ever returning to 0 volts.

Question 6

Serial mode transmission is much faster than parallel mode transmission.

Answer 6

False

Serial transmission means that a stream of data is sent over a communication circuit sequentially in a bit-by-bit fashion. In this case, there is only one physical wire inside the bundle, and all data must be transmitted over that one physical wire. The transmitting device sends one bit, then a second bit, and so on, until all the bits are transmitted. It takes n iterations or cycles to transmit n bits.
Thus, serial transmission is considerably slower than parallel transmission—eight times slower in the case of 8-bit ASCII (because there are 8 bits)

Question 7

Manchester encoding is a special type of bipolar signaling in which the signal is changed from high to low or from low to high in the middle of the signal.

Answer 7

True

Ethernet uses Manchester encoding, which is a special type of bipolar signaling in which the signal is changed from high to low or from low to high in the middle of the signal. A change from high to low is used to represent a 0, whereas the opposite (a change from low to high) is used to represent a 1 (see Figure 3-13). Manchester encoding is less susceptible to having errors go undetected, because if there is no transition in midsignal, the receiver knows that an error must have occurred.

Question 8

Coding schemes are groups of bits that translate into the “alphabet” of any given system.

Answer 8

True

A character is a symbol that has a common, constant meaning. A character might be the letter A or B, or it might be a number such as 1 or 2. Characters also may be special symbols such as ? or &. Characters in data communications, as in computer systems, are represented by groups of bits that are binary zeros (0) and ones (1). The groups of bits representing the set of characters that are the “alphabet” of any given system are called a coding scheme, or simply a code.

Question 9

A walkie-talkie is an example of half-duplex transmission.

Answer 9

True

Half-duplex transmission is two-way transmission, but you can transmit in only one direction at a time. A half-duplex communication link is similar to a walkie-talkie link; only one computer can transmit at a time. Computers use control signals to negotiate that will send and that will receive data.

Question 10

Multimode fiber is capable of longer transmission distances than single mode fiber.

Answer 10

False

The earliest fiber-optic systems were multimode, meaning that the light could reflect inside the cable at many different angles. Multimode cables are plagued by excessive signal weakening (attenuation) and dispersion (spreading of the signal so that different parts of the signal arrive at different times at the destination). For these reasons, early multimode fiber was usually limited to about 500 - 1,000 meters.

Single-mode fiber-optic cables transmit a single direct beam of light through a cable that ensures the light reflects in only one pattern, in part because the core diameter has been reduced from 50 microns to about 5–10 microns. This smaller-diameter core allows the fiber to send a more concentrated light beam, resulting in faster data transmission speeds and longer distances, often up to 100 kilometers.

Question 11

Based on the TIA/EIA 568-B structured cabling standard, the cabling that runs from the telecommunications closet to each work area is called backbone cabling.

Answer 11

False

Backbone cabling: the cabling that interconnects telecommunication closets, equipment rooms, and building entrances within a building; also, this refers to cabling between buildings

Work area: the cabling where the computers, printers, patch cables, jacks, and so on, are located

Question 12

Time division multiplexing is more efficient that frequency division multiplexing because the guardbands used in the frequency division multiplexing or not needed.

Answer 12

True

TDM
◦ Divides circuit by having devices take turns
◦ In traditional TDM, all have equal turns
◦ More efficient than FDM, but may have idle time slots

Question 13

Of all the media available, fiber optic provides the highest transmission speeds.

Answer 13

True

One of the main advantages of fiber optics is that it can carry huge amounts of information at extremely fast data rates.
Transmission distance is a related factor. Twisted pair wire coaxial cable and radio can transmit data only a short distance before the signal must be regenerated. Twisted pair wire and radio typically can transmit up to 100–300 meters and coaxial cable typically between 200 and 500 meters. Fiber optics can transmit up to 75 miles, and new types of fiber-optic cable can reach more than 600 miles.

Question 14

A codec converts an incoming digital signal to an analog signal for transmission over an analog network.

Answer 14

False

Codec: A - D
It is possible to translate analog voice data into digital form for transmission over digital computer circuits using a device called a codec.

Question 15

One form of interference with satellite transmission is called raindrop attenuation in which the transmissions are absorbed by heavy rain.

Answer 15

True

Satellite transmission is sometimes also affected by raindrop attenuation when satellite transmissions are absorbed by heavy rain.

Question 16

Newer telephone circuits, built during the last decade, offer higher quality because they were built using analog transmission.

Answer 16

False

For these reasons, most long-distance telephone circuits built by the telephone companies and other common carriers over the past decades use digital transmission. In the future, most transmissions (voice, data, and video) will be sent digitally.

Question 17

A physical circuit refers to the transmission characteristics of the connection, such as the speed at which data is being sent through the connection.

Answer 17

False

Logical circuit used to connect two devices, which refers to the transmission characteristics of the connection,

Question 18

Data compression can increase the throughput of a communication link.

Answer 18

True

A modem’s data transmission rate is the primary factor that determines the throughput rate of data, but it is not the only factor. Data compression can increase throughput of data over a communication link by literally compressing the data.

Question 19

With modem communications, the receiving modem converts the incoming digital signal from the telephone line into an analog signal that can be understood by the computer.

Answer 19

False

Modem: D-A
The modem (an acronym for modulator/demodulator) takes the digital data from a computer in the form of electrical pulses and converts them into the analog signal that is needed for transmission over an analog voice-grade circuit.

Question 20

In a multipoint circuit configuration, each computer has a dedicated circuit between itself and each of the other computers in the network.

Answer 20

False

Point-to-point circuit, which is so named because it goes from one point to another (e.g., one computer to another computer). These circuits sometimes are called dedicated circuits because they are dedicated to the use of these two computers

Multipoint circuit (also called a shared circuit). In this configuration, many computers are connected on the same circuit.

Question 21

The _____________ of a circuit determines a circuit’s capacity.

frequency
bandwidth
phase
amplitude
loudness

Answer 21

Bandwidth

Generally (but not always), the symbol rate is the same as the bandwidth, so bandwidth is often used as a measure of capacity.

Question 22

A coding scheme that uses 10 bits to represent each character is capable of representing _________ different characters.

256
20
200
512
1024

Answer 22

1024

The number of combinations can be determined by taking the number 2 and raising it to the power equal to the number of bits in the code because each bit has two possible values, a 0 or a 1. In this case 27 = 128 characters or 28 = 256 characters.
2 to the power of 10 = 1,024

Question 23

To get more “bandwidth” in a digital transmission, you must:

increase the range of frequencies available for a transmission
increase the loudness per transmission
decrease the bits per second transmission speed
increase the phase shifts per transmission
decrease the baud rate per transmission

Answer 23

Increase the range of frequencies available for a transmission

The bandwidth is the difference between the highest and the lowest frequencies in a band or set of frequencies.

Question 24

Which is not true about point-to-point circuits?

point-to-point circuits are dedicated to the use of the two computers.
point-to-point circuits are more expensive than multipoint circuits.
point-to-point circuits are called dedicated circuits.
point-to-point circuits reduce the amount of cable required and use the circuit more efficiently than multipoint circuits.
point-to-point circuits are used when the computers generate enough traffic to fill the capacity of the communication circuit.

Answer 24

Point-to-point circuits reduce the amount of cable required and use the circuit more efficiently than multipoint circuits.

A point-to-point circuit, which is so named because it goes from one point to another (e.g., one computer to another computer). These circuits sometimes are called dedicated circuits because they are dedicated to the use of these two computers. This type of configuration is used when the computers generate enough data to fill the capacity of the communication circuit. When an organization builds a network using point-to-point circuits, each computer has its own circuit running from itself to the other computers. This can get very expensive, particularly if there is some distance between the computers. Despite the cost, point-to-point circuits are used regularly in modern wired networks to connect clients to switches, switches to switches and routers, and routers to routers.

Question 25

Radio, microwave and satellite are all examples of guided media. True False

Answer 25

False Guided media are those in which the message flows through a physical medium such as a twisted pair wire, coaxial cable, or fiber-optic cable; the medium “guides” the signal. Wireless media are those in which the message is broadcast through the air, such as microwave or satellite.

Question 26

Newer telephone circuits, built during the last decade, offer higher quality because they were built using analog transmission. True False

Answer 26

False

Question 27

Having a conversation with someone using a cellular phone is an example of _________ transmission. simplex full-duplex half-duplex analog

Answer 27

Chose half-duplex, but the correct answer is full duplex Half-duplex transmission is two-way transmission, but you can transmit in only one direction at a time. A half-duplex communication link is similar to a walkie-talkie link; only one computer can transmit at a time. Computers use control signals to negotiate that will send and that will receive data. With full-duplex transmission, you can transmit in both directions simultaneously, with no turnaround time.

Question 28

One form of interference with satellite transmission is called raindrop attenuation in which the transmissions are absorbed by heavy rain. True False

Answer 28

True Satellite transmission is sometimes also affected by raindrop attenuation when satellite transmissions are absorbed by heavy rain.

Question 29

Baud rate is the number of bits transmitted per second. True False

Answer 29

False Baud rate – the number of times per second the signal on the communication circuit changes Bit rate – the number of bits transmitted per second

Question 30

Ethernet, a common LAN technology, uses Manchester encoding. True False

Answer 30

True The most common technology used in LANs is Ethernet. Ethernet uses Manchester encoding, which is a special type of bipolar signaling in which the signal is changed from high to low or from low to high in the middle of the signal. Ethernet uses Manchester encoding, which is a version of unipolar transmission.

Question 31

The "local loop" refers to the wires that run from the customer premises to the telephone switch of the telephone company. True False

Answer 31

True The local loop (sometimes called the last mile), the wires that run from your home or business to the telephone switch that connects your local loop into the telephone network.

Question 32

Which type of digital signaling would experience fewer errors because it has more distinct signals? ``` unipolar serial bipolar attenuation Huffman encoding signal ```

Answer 32

Bipolar In general, bipolar signaling experiences fewer errors than unipolar signaling because the symbols are more distinct.

Question 33

Multitasking means to break one high-speed physical communication circuit into several lower-speed logical circuits so that different devices can simultaneously use it. True False

Answer 33

False Multiplexing means to break one high-speed physical communication circuit into several lower-speed logical circuits so that many different devices can simultaneously use it but still “think” that they have their own separate circuits (the multiplexer is “transparent”). Without multiplexing, the Internet would have collapsed in the 1990s. Multiplexing often is done in multiples of 4 (e.g., 8, 16).

Question 34

Time division multiplexing is more efficient that frequency division multiplexing because the guardbands used in the frequency division multiplexing or not needed. True False

Answer 34

True Time division multiplexing (TDM) ◦ Divides circuit by having devices take turns ◦ In traditional TDM, all have equal turns ◦ More efficient than FDM, but may have idle time slots

Question 35

Another term for the weakening of a signal over distance is: ``` turnaround time propagation delay dispersion insulation attenuation ```

Answer 35

Attenuation Propagation delay is the length of time taken for a signal to reach its destination. It can relate to networking, electronics or physics. Attenuation is a telecommunications term that refers to a reduction in signal strength commonly occurring while transmitting analog or digital signals over long distances. Attenuation is historically measured in dB but it can also be measured in terms of voltage.

Question 36

Amplitude, frequency and phase are all characteristics of a sound wave. True False

Answer 36

True The sound waves transmitted through the voice circuit have three important characteristics (see Figure 3-14). The first is the height of the wave, called amplitude. Amplitude is measured in decibels (dB). Our ears detect amplitude as the loudness or volume of sound. Every sound waves has two parts, half above the zero amplitude point (i.e., positive) and half below (i.e., negative), and both halves are always the same height. The second characteristic is the length of the wave, usually expressed as the number of waves per second, or frequency. Frequency is expressed in hertz (Hz). Our ears detect frequency as the pitch of the sound. Frequency is the inverse of the length of the sound wave, so that a high frequency means that there are many short waves in a 1-second interval, whereas a low frequency means that there are fewer (but longer) waves in 1 second. The third characteristic is the phase, which refers to the direction in which the wave begins. Phase is measured in the number of degrees (°). The wave in Figure 3-14 starts up and to the right, which is defined as a 0° phase wave. Waves can also start down and to the right (a 180° phase wave), and in virtually any other part of the sound wave.

Question 37

Multimode fiber is capable of longer transmission distances than single mode fiber. True False

Answer 37

False The earliest fiber-optic systems were multimode, meaning that the light could reflect inside the cable at many different angles. Multimode cables are plagued by excessive signal weakening (attenuation) and dispersion (spreading of the signal so that different parts of the signal arrive at different times at the destination). For these reasons, early multimode fiber was usually limited to about 500 meters. Single-mode fiber-optic cables transmit a single direct beam of light through a cable that ensures the light reflects in only one pattern, in part because the core diameter has been reduced from 50 microns to about 5–10 microns. This smaller-diameter core allows the fiber to send a more concentrated light beam, resulting in faster data transmission speeds and longer distances, often up to 100 kilometers.

Question 38

Signal bandwidth is the difference between the highest and lowest frequencies in a band. True False

Answer 38

True The bandwidth is the difference between the highest and the lowest frequencies in a band or set of frequencies

Question 39

Statistical time division multiplexing does not require the capacity of the circuit to be equal to the sum of the combined circuits. True False

Answer 39

True STDM is the exception to the rule that the capacity of the multiplexed circuit must equal the sum of the circuits it combines.

Question 40

_____________ refers to the difference between the original analog data and the approximation of that data using the techniques for translating from analog data to digital signals. ``` quantizing error handshaking phase modulating frequency POTS anomaly amplitude Trellis effect ```

Answer 40

Quantizing error Quantizing error: the difference between the original analog signal and the approximated, digital signal The digitized signal is only a rough approximation of the original signal. The original signal had a smooth flow, but the digitized signal has jagged “steps.” The difference between the two signals is called quantizing error. Voice transmissions using digitized signals that have a great deal of quantizing error sound metallic or machinelike to the ear.