Chapter 4

Review Questions for Lecture-18 : Analog Transmission and Modulation

1.  How does analog data differ from digital data?

Computers produce digital data that are binary, either on or off. In contrast, telephones produce analog data whose electrical signals are shaped like the sound waves they transfer. Analog data are signals that vary continuously within a range of values (e.g., temperature is analog).

2.  Clearly explain the differences between analog data, analog transmission, digital data, and digital transmission.

Data can be transmitted through a circuit in the same form they are produced. Most computers, for example, transmit their data through digital circuits to printers and other attached devices. Likewise, analog voice data can be transmitted through telephone networks in analog form. In general, networks designed primarily to transmit digital computer data tend to use digital transmission, and networks designed primarily to transmit analog voice data tend to use analog transmission (at least for some parts of the transmission).

4.  How does baseband differ from broadband?

Baseband is the digital transmission of binary electrical pulses. Broadband is the analog waveform used for transmission.

11.  What is bandwidth? What is the bandwidth in a traditional North American telephone circuit?

Bandwidth refers to a range of frequencies. It is the difference between the highest and the lowest frequencies in a band; thus the bandwidth of human voice is from 20 Hz to 14,000 Hz or 13,880 Hz.

The bandwidth of a voice grade telephone circuit is from 0 to 4000 Hz, or 4000 Hz; however, not all of this is available for use by telephone or data communications equipment. To start, there is a 300 hertz guardband at the bottom of the bandwidth and a 700 hertz guardband at the top. These prevent data transmissions from interfering with other transmissions when these circuits are multiplexed using frequency division multiplexing. This leaves the bandwidth from 300 to 3300 hertz or a total of 3000 Hz for voice or data transmission.

12.  Describe how data could be transmitted using amplitude modulation.

With amplitude modulation (AM) (also called amplitude shift keying (ASK)), the amplitude or height of the wave is changed. One amplitude is defined to be zero, and another amplitude is defined to be a one.

13.  Describe how data could be transmitted using frequency modulation.

Frequency modulation (FM) (also called frequency shift keying (FSK)), is a modulation technique whereby each 0 or 1 is represented by a number of waves per second (i.e., a different frequency). In this case, the amplitude does not vary. One frequency (i.e., a certain number of waves per second) is defined to be a one, and a different frequency (a different number of waves per second) is defined to be a one.

14.  Describe how data could be transmitted using phase modulation.

Phase modulation (PM) (also called phase shift keying (PSK)), is the most difficult to understand. Phase refers to the direction in which the wave begins. Until now, the waves we have shown start by moving up and to the right (this is called a 0º phase wave). Waves can also start down and to the right. This is called a phase of 180º. With phase modulation, one phase is defined to be a zero and the other phase is defined to be a one.

15.  Describe how data could be transmitted using a combination of modulation techniques.

It is possible to use amplitude modulation, frequency modulation, and phase modulation techniques on the same circuit. For example, we could combine amplitude modulation with four defined amplitudes (capable of sending two bits) with frequency modulation with four defined frequencies (capable of sending two bits) to enable us to send four bits on the same symbol.

16.  Is the bit rate the same as the symbol rate? Explain.

The terms bit rate (i.e., the number bits per second transmitted) and baud rate are used incorrectly much of the time. They often are used interchangeably, but they are not the same. In reality, the network designer or network user is interested in bits per second because it is the bits that are assembled into characters, characters into words and, thus, business information.

Because of the confusion over the term baud rate among the general public, ITU-T now recommends the term baud rate be replaced by the term symbol rate. The bit rate and the symbol rate (or baud rate) are the same only when one bit is sent on each symbol. For example, if we use amplitude modulation with two amplitudes, we send one bit on one symbol. Here the bit rate equals the symbol rate. However, if we use QAM, we can send four bits on every symbol; the bit rate would be four times the symbol rate.

17.  What is a modem?

Modem is an acronym for MOdulator/DEModulator. A modem takes the digital electrical pulses received from a computer, terminal, or microcomputer and converts them into a continuous analog signal that is needed for transmission over an analog voice grade circuit. Modems are either internal (i.e., inside the computer) or external (i.e., connected to the computer by a cable).

18.  What is QAM?

One popular technique is quadrature amplitude modulation (QAM). QAM involves splitting the symbol into eight different phases (three bits) and two different amplitudes (one bit), for a total of 16 different possible values. Thus, one symbol in QAM can represent four bits.

19.  Explain the importance of Trellis Coded Modulation.

Trellis coded modulation (TCM) is an enhancement of QAM that combines phase modulation and amplitude modulation. TCM is unique in that it can transmit a different number of bits on each symbol. TCM can transmit six, seven, eight or ten bits per symbol. The fastest version of TCM can average a maximum of 9.8 bits per symbol. The problem with high speed modulation techniques such as TCM that send many bits per symbol is that they are more sensitive to imperfections in the communication circuit.

20.  How are modems classified?

Modems are classified by the general public by the speed at which they operate. “Experts” use the standard numbers.

21.  Define retrain time.

Most modems support several standards (some high speed, some low speed) so that they can communicate with a variety of different modems. When these modems connect to another modem, they attempt to use the highest speed standard available and, if unsuccessful, keep trying lower speeds until they find one that works. Some modems can change data rates during transmission, so if a circuit is noisy, they can slow down to reduce the effects of errors. The time for this changing of data rates during transmission is called the retrain time.

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) is also furnished.

1. Computers produce analog data whose electrical signals are shaped like sound waves. [False, computers produce digital data.]

Easy

2. Networks designed for voice data (traditional telephone networks, for example) tend to use analog transmission for at least some parts of transmission. [True]

Moderate

3. Analog signals can be converted to digital signals by using a special device called a modem. [False, a codec is required.]

Moderate

4. Newer telephone circuits, built during the last decade, offer higher quality because they were built using analog transmission. [False, they use digital transmission.]

Easy

5. Another term for analog is baseband. [False, it is broadband.]

Moderate

6. A bit is a single digit binary number, and has the value of either 0 or 1. [True]

Easy

8. The predominant method of transferring information in data communications is via parallel mode. [False, it is via serial mode.]

Moderate

9. Unipolar signaling is one method used to transmit baseband signals. [True]

Easy

10. Modems are generally used in pairs: one modem is used by the sender to produce analog signals from digital data, and one modem is used by the recipient to translate the analog signals back into digital data. [True]

Moderate

12. Pitch of an analog signal is the same as loudness. [False, these are two separate things, pitch = frequency.]

Easy

13. In an analog signal, bandwidth is the difference between the highest and lowest frequencies in a band. [True]

Easy

14. Some of the bandwidth of traditional telephone circuits is taken up by guardbands. [True]

Moderate

15. Modulation is a technique that transforms an electrical signal so that it can transmitted on a particular type of communication medium. [True]

Moderate

16. Four different amplitude possibilities are required to transmit two bits per wave using amplitude modulation. [True]

Moderate

17. Although it is theoretically possible to send an infinite number of bits per wave using one-type (not combined, as in QAM) modulation, it is only practical to send about five bits per wave. [True]

Moderate

18. According to the ITU-T, baud rate is the same as symbol rate. [True]

Moderate

19. Fast retrain refers to keeping data rates constant during transmission with a modem. [False, it refers to changing of data rates.]

Moderate

20. Data compression reduces the throughput of a communication link by compressing the data. [False, it increases throughput.]

Easy

21. One drawback to the use of data compression in today’s Internet environment is that many data files are already compressed; therefore, in these instances, compressing already compressed data provides little advantage to users of a modem that uses sophisticated data compression techniques. [True]

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) is also furnished.

1. Data is transmitted from one computer or terminal to another in the ______layer.

  1. top
  1. application
  1. physical
  1. data link
  1. transport

Answers: c, Easy

2. The two fundamentally different types of data are:

  1. digital and analog
  1. text and numeric
  1. ASCII and EBCDIC
  1. local area and wide area
  1. Microsoft and IBM

Answers: a, Easy

3. In general, networks designed to transmit primarily computer data are likely to be ______, wile networks designed to transmit voice data are likely to be ______.

  1. slow, fast
  1. digital, analog
  1. level 1, level 2
  1. direct, indirect
  1. numeric, musical

Answers: b, Easy

4. Digital computer data can be transmitted over a traditional telephone network by using a special device called a:

  1. digitizer
  1. analog converter
  1. protocol converter
  1. modem
  1. codec

Answers: d, Easy

5. Which of the following is not a key advantage of digital transmission?

  1. It is more secure.
  1. It is simpler to integrate voice, video, and data on the same circuit.
  1. It permits minimum transmission rates.
  1. It produces fewer errors.
  1. It is more efficient.

Answers: c, Moderate

6. Most newer long distance telephone circuits built by the common carriers over the past decade use:

  1. digital transmission
  1. copper wire
  1. analog transmission
  1. infrared linking
  1. spread spectrum transmission

Answers: a, Moderate

7. Analog is also called:

  1. baseband
  1. broadband
  1. loudness
  1. asynchronous
  1. synchronous

Answers: b, Easy

8. Digital is also called:

  1. baseband
  1. broadband
  1. asynchronous
  1. synchronous
  1. pitch

Answers: a, Easy

9. In which of the following systems is data represented by measurements on a continuous scale?

  1. analog
  1. digital
  1. asynchronous
  1. synchronous
  1. integrated

Answers: a, Moderate

10. In which of the following systems is data represented by the transmission of discrete signals over a transmission channel?

  1. analog
  1. broadband
  1. digital
  1. integrated
  1. synchronous

Answers: c, Moderate

11. Which of the following is a byte, under the normal definition?

  1. 0
  1. 1
  1. 8 consecutive bits, such as 000111000, that represent one character
  1. 325
  1. a special character that is used for Apple computers only

Answers: c, Moderate

12. Typically, one bit is comprised of ______bytes to represent one character.

  1. 8
  1. 7
  1. 64
  1. 1
  1. bits are not comprised of bytes

Answers: e, Moderate

13. ______is one of the most commonly used coding schemes used in data transmission.

  1. ATM
  1. ISDN
  1. MAU
  1. ASCII
  1. FDM

Answers: d, Moderate

15. The representation of the character A by the group of bits, 10000001, is an example of:

  1. analog transmission
  1. digital coding
  1. binary modulation
  1. amplitude modulation
  1. pitch variation

Answers: b, Moderate

16. When all bits (usually 8) of a character are transferred simultaneously, as in a printer cable connection, for example, the bits are transferred in ______mode.

  1. serial
  1. frequency division
  1. multiplexing
  1. parallel
  1. full complex

Answers: d, Moderate

17. The predominant method of transferring information in data communication is ______mode transmission.

  1. parallel
  1. serial
  1. broadbased
  1. monospaced
  1. bisynchronous

Answers: b, Easy

18. Which of the following statements is correct?

  1. Serial transmission is considerably slower than parallel transmission.
  1. Parallel transmission is distinguished from serial transmission by the fact that the transmitting device sends a single bit, then a second bit, and so on, until all the bits are transmitted.
  1. Parallel transmission is only used for analog data.
  1. Parallel transmission is the same as serial transmission.
  1. Parallel transmission is most often used for sending telegraph messages.

Answers: a, Moderate

19. ______is not a type of digital signaling technique.

  1. bipolar signaling
  1. unipolar signaling
  1. Manchester encoding
  1. broadband signaling
  1. none of the above are appropriate choices

Answers: d, Moderate

20. With which type of digital signaling is the voltage always positive or negative (like a dc current)?

  1. unipolar
  1. bipolar
  1. parallel
  1. serial
  1. Huffman encoding signal

Answers: a, Easy

21. Which type of digital signaling experiences fewer errors?

  1. unipolar
  1. bipolar
  1. direct
  1. alternating
  1. Huffman encoding signal

Answers: b, Moderate