Review Questions for Test-2 (Partial List) Part-A

End-of-Chapter-4 Questions (match questions, overlook number mismatch)

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).

1. 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.

4. What is coding?

Coding is the representation of one set of symbols by another set of symbols. In data communications, this coding is a specific arrangement of binary 0s and 1s used to represent letters, numbers, and other symbols that have meaning.

5. Briefly describe the two most important coding schemes.

There are two predominant coding schemes in use today. United States of America Standard Code for Information Interchange (USASCII), or more commonly ASCII, is the most popular code for data communications and is the standard code on most terminals and microcomputers. There are two types of ASCII; one is a 7-bit code that has 128 valid character combinations, and the other is an 8-bit code that has 256 combinations. Extended Binary Coded Decimal Interchange Code (EBCDIC) is IBM's standard information code. This code has 8 bits, giving 256 valid character combinations.

6. How is data transmitted in parallel?

Parallel mode is the way the internal transfer of binary data takes place inside a computer. If the internal structure of the computer is 8-bit, then all eight bits of the data element are transferred between main memory and the central processing unit simultaneously on 8 separate connections. The same is true of computers that use a 32-bit structure; all 32 bits are transferred simultaneously on 32 connections.

7. What feature distinguishes serial mode from parallel mode?

Serial mode is distinguished from parallel mode by the time cycle in which the bits are transmitted. Parallel implies that all bits of a character are transmitted, followed by a time delay, and then all bits of the next character are transmitted, followed by a time delay. Serial implies that characters are sent one bit at a time, with each bit followed by a time delay. Put another way, parallel is character-by-character and serial is bit-by-bit.

8. How does bipolar signaling differ from unipolar signaling? Why is Manchester encoding more popular than either?

With unipolar signaling, the voltage is always positive or negative (like a dc current). In bipolar signaling, the 1’s and 0’s vary from a plus voltage to a minus voltage (like an ac current). In general, bipolar signaling experiences fewer errors than unipolar signaling because the signals are more distinct.

Manchester encoding is a special type of unipolar signaling in which the signal is changed from a 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 1 (or a 0), while the opposite (a change low to high) is used to represent a 0 (or a 1). Manchester encoding is less susceptible to having errors go undetected, because if there is no transition in mid-signal the receive knows that an error must have occurred.

9. Describe how the North American telephone network is organized.

Your house or office is connected to a telephone company end office (also called central office class 5) by a set of two twisted pair wires (i.e., four wires). The wires from your house to the end office are called the local loop, and usually run 2-5 miles. In most cases, telephones only use one set of twisted pair wires so it is easy to add a second telephone number to most homes by using this second, spare set of wires.

The end office is connected to a central office class 4 by a trunk line. The central offices are arranged in a hierarchy: a class 4 office is connected to a class 3 office which is connected to a class 2 office, which is connected to a class 1 office. Often several end offices are interconnected as well. Each end office and central office has a switch that routes your telephone call through the telephone system based on the number you dialed. Local calls stay within the same end office or go between end offices in the same geographic area. Long distance calls go up the hierarchy to class 4 or higher offices.

The telephone system was originally designed as an analog system. Today in North America, the local loop remains almost entirely analog but the trunk lines among the end offices and central offices are almost entirely digital.

11. What are three important characteristics of a sound wave?

Sounds waves have three important characteristics. The first is the height of the wave, called amplitude. Our ears detect amplitude as the loudness or volume of sound. 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. Human hearing ranges from about 20 hertz to about 14,000 hertz, although some people can hear up to 20,000 hertz. The third characteristic is the phase, refers to the direction in which the wave begins.

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.

1. Computers produce analog data whose electrical signals are shaped like sound waves.

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

3. Analog signals can be converted to digital signals by using a special device called a modem.

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

5. Another term for analog is baseband.

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

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

8. The predominant method of transferring information in data communications is via parallel mode.

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

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.

11. POTS refers to plain old telephone system.

12. Pitch of an analog signal is the same as loudness.

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

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

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

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

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.

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

19. Fast retrain refers to keeping data rates constant during transmission with a modem.

20. Data compression reduces the throughput of a communication link by compressing the data.

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.

22. Pulse Amplitude Modulation refers to a method of translating analog data into a series of binary bits before transmission over a digital communication medium.

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. a. top a. application b. physical c. data link d. transport

2. The two fundamentally different types of data are: a. digital and analog a. text and numeric b. ASCII and EBCDIC c. local area and wide area d. Microsoft and IBM

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 ________. a. slow, fast a. digital, analog b. level 1, level 2 c. direct, indirect d. numeric, musical

4. Digital computer data can be transmitted over a traditional telephone network by using a special device called a: a. digitizer a. analog converter b. protocol converter c. modem d. codec

5. Which of the following is not a key advantage of digital transmission? a. It is more secure. a. It is simpler to integrate voice, video, and data on the same circuit. b. It permits minimum transmission rates. c. It produces fewer errors. d. It is more efficient.

6. Most newer long distance telephone circuits built by the common carriers over the past decade use: a. digital transmission a. copper wire b. analog transmission c. infrared linking d. spread spectrum transmission

7. Analog is also called: a. baseband a. broadband b. loudness c. asynchronous d. synchronous

8. Digital is also called: a. baseband a. broadband b. asynchronous c. synchronous d. pitch

9. In which of the following systems is data represented by measurements on a continuous scale? a. analog a. digital b. asynchronous c. synchronous d. integrated

10. In which of the following systems is data represented by the transmission of discrete signals over a transmission channel? a. analog a. broadband b. digital c. integrated d. synchronous

11. Which of the following is a byte, under the normal definition? a. 0 a. 1 b. 8 consecutive bits, such as 000111000, that represent one character c. 325 d. a special character that is used for Apple computers only

12. Typically, one bit is comprised of _______ bytes to represent one character. a. 8 a. 7 b. 64 c. 1 d. bits are not comprised of bytes

13. ____________ is one of the most commonly used coding schemes used in data transmission. a. ATM a. ISDN b. MAU c. ASCII d. FDM

14. Under EBCDIC, each character is represented by 8 bits. That means that there are ________ valid combinations which can represent different characters. a. 23 a. 256 b. 128 c. 1092 d. 8

15. The representation of the character A by the group of bits, 10000001, is an example of: a. analog transmission a. digital coding b. binary modulation c. amplitude modulation d. pitch variation

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. a. serial a. frequency division b. multiplexing c. parallel d. full complex

17. The predominant method of transferring information in data communication is _________ mode transmission. a. parallel a. serial b. broadbased c. monospaced d. bisynchronous

18. Which of the following statements is correct? a. Serial transmission is considerably slower than parallel transmission. a. 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. b. Parallel transmission is only used for analog data. c. Parallel transmission is the same as serial transmission. d. Parallel transmission is most often used for sending telegraph messages.

19. ___________ is not a type of digital signaling technique. a. bipolar signaling a. unipolar signaling b. Manchester encoding c. broadband signaling d. none of the above are appropriate choices

20. With which type of digital signaling is the voltage always positive or negative (like a dc current)? a. unipolar a. bipolar b. parallel c. serial d. Huffman encoding signal

21. Which type of digital signaling experiences fewer errors? a. unipolar a. bipolar b. direct c. alternating d. Huffman encoding signal

22. The ____________ is the agreement between sender and receiver on how often the sender can transmit data. a. frequency division a. non-return-to-zero technique b. data coding scheme c. broadband signaling d. data rate

23. Generally, in North America, your home telephone connection is as follows: your house is connected to a telephone company __________ by a set of two twisted pair wires. a. central office a. trunk line b. front-end processor c. end office d. gateway office

24. Which of the following is not an important characteristic of sound waves? a. phase a. frequency b. amplitude c. bandwidth d. all of the above are important characteristics of sound waves

25. The length of a sound wave is expressed as: a. amplitude, which our ears detect as loudness a. frequency, which our ears detect as pitch b. phase c. bandwidth d. furlong

26. The height of a sound wave is called its: a. amplitude, which our ears detect as loudness a. frequency, which our ears detect as pitch b. phase c. bandwidth d. furlong

27. The _____________ of a circuit determines a circuit’s capacity. a. frequency a. bandwidth b. phase c. amplitude d. loudness

28. The typical range of human hearing is: a. 0 to 1,200 Hz a. 0 to 4,000 Hz b. 0 to 300 Hz c. 20 to 20,000 Hz d. 0 to 56,000 Hz

29. __________ is the approximate bandwidth of traditional telephone signal. a. 300 - 3400 Hz a. 30,000 - 40,000 Hz b. 3-4 Hz c. 300 Hz d. the note that is one octave above middle C

30. Which of the following statements is true? a. the longer the distance of a telephone wire between sender and recipient, the higher the bandwidth across that wire a. the longer the distance of a telephone wire between sender and recipient, the lower the bandwidth across that wire b. the shorter the distance of a telephone wire between sender and recipient, the lower the bandwidth across that wire c. bandwidth is constant across any wire, therefore distance is not relevant d. bandwidth is not a term that can be used when discussing telephone transmissions

32. _____________ is not a form of modulation used to transform digital data into analog signals. a. amplitude shift keying a. statistical time division b. amplitude modulation c. frequency modulation d. phase modulation

33. AM stands for: a. Analog Modem a. Anomaly Multiplexing b. Asynchronous Modem c. Amplitude Modulation d. Analytical Modem

34. In which type of modulation is a 1 distinguished from a 0 by the number of waves transmitted in the same unit of time? a. bandwidth modulation a. amplitude modulation b. frequency modulation c. phase modulation d. codec modulation

35. When sending four bits at a time using amplitude modulation, the number of different amplitude levels that would be needed would be _______ . a. 2 a. 24 b. 16 c. 8 d. 4

36. ____________ is a modulation technique that combines amplitude and phase modulation to send four bits per wave, or symbol. a. time division multiplexing a. pulse code modulation b. synchronous digital line control c. quadrature amplitude modulation d. baseband signaling

37. __________ indicates how often the signal changes on a communication circuit. a. baud rate a. bits per second rate b. symbol rate c. data rate d. a and b

38. TCM stands for: a. Trellis coded modulation, a technique that combines phase modulation and amplitude modulation a. Tandem Control Microprocessor, a hardware feature of front end processors used to manage and control data communications b. Telephone Call Manager, a software package to monitor and report on telephone usage c. Threaded Code Messaging, a data communications transmission technique in which all redundant bits are stripped before sending and replaced upon receipt of the message d. True Calling Measures, one telephone company’s method for changing customers

39. Using TCM (6 bits per signal) and a baud rate of 3000 bauds per second, the maximum data rate would be ___________ bps on a traditional voice grade line. a. 50,000 a. 6 b. 24,000 c. 18,000 d. 56K

40. Modem stands for _________________. a. multiplexing/demultiplexing a. modulator/demodulator b. more/demote c. Mode-M technique d. Mod emulation technique

41. Which statement is false about modems? a. modems must always be internal to the computer a. a receiving modem demodulates and a sending modem modulates b. modems are usually used in pairs (2 at a time) c. most modems support several modem standards so that they can communicate with a variety of modems d. modems may operate at a lower speed that that for which they are rated, especially if they detect noise in the communication line

44. Using __________ , several users can share one or more modems. a. pulse code modulation a. modem pooling b. amplitude shift keying c. handshaking d. point of presence

46. Codec refers to: a. compression/decompression a. code/decode b. co- Digital Equipment Corporation c. Codd’s laws for data relations d. continuous operational digital energy communication

Short Answer Questions

1. Use a drawing to show how the bit pattern 11100010 would be sent using 1-bit Frequency Modulation (FM) (i.e., one bit per signal).

2. Use a drawing to show how the bit pattern 11100100 would be sent using 1-bit Amplitude Modulation (AM) (i.e., one bit per signal).

1. Compare and contrast data rate (i.e., bits per second) and symbol rate (also called baud rate).

2. A bit is a unit of information. A baud is a unit of signaling speed used to indicate the number of times per second the signal on the communication circuit changes. 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.