Midterm Examination, 1/8/2007

MODD / Mr. Hansen	Name: _______________________________________
1/8/2007	1-point bonus for having a calculator: _______________

Midterm Examination

Please read:

· There are 100 points possible. Your score on this examination will determine approximately 20% of your course grade.

· You will probably need a calculator. If you have forgotten to bring yours, ask Mr. Hansen to lend you one. Important: DO NOT SHARE CALCULATORS WITH OTHER STUDENTS DURING THE EXAM. Doing so would be a potential honor offense.

· Clues to some questions may be found elsewhere in the examination. If you are having trouble with a question, come back to it later.

· Point values are indicated in parentheses, except for the multiple-choice questions, which are 2 points each.

· Please raise your hand if you have an emergency or a question that is not addressed here. When you stand up, you are finished.

· Thank you for taking this course! It has been my pleasure working with you, and I hope that you will find some of what we covered in MODD to be useful in your future life.

1. (9½ pts.)	Draw lines to connect the first and last names of each person with the accomplishment for which he or she is noted. Several examples have already been done for you. Each connection you make of a first name to a last name is worth ½ point, and each connection you make of a last name to an accomplishment is worth 1½ points.


	First Name	Last Name	Accomplishment

	Hedy	Nyquist	the MODD course

	Gordon	Shannon	famous “law” or rule concerning microprocessor chips

	Kurt	Moore	famous theorem concerning sampling rate

	Harry	Gödel	information theory

	Claude	Turing	cryptanalysis and foundations of computer science

	Michael	Lamarr	Incompleteness Theorem

	Alan	Hansen	patent that eventually led to modern CDMA (code division multiple access) cell phone technology

	Fun fact of the day: Hedy Lamarr was an Austrian actress born in 1914. She escaped from the Nazis and later became a U.S. citizen. She made more than 30 movies during her career, including a scandalous film called Ecstasy (1933) that was one of the first general release movies with nudity. The movie was banned for a time in the U.S. and was condemned by the Pope. However, Hedy Lamarr also found time to co-invent the concept of frequency-hopping spread spectrum telecommunication (U.S. Patent #2,292,387). She died in January 2000.

2.___	Which of the following statements are true?

	I. A byte consists of 1 hex digit. II. A byte consists of 2 hex digits. III. A byte consists of 8 bits. IV. A byte represents a single character in ASCII. V. A byte represents a single character in Unicode.

	(A) I, III, and IV (B) II, III, and IV (C) I, III, and V			(D) II, III, and V (E) II and IV only

3.___	Why is the CD sampling rate set at 44.1 KHz?

	(A) It isn’t. CD audio requires sampling at 44.1 MHz, thus generating 176.4 MB of data per second.
	(B) A popular AM radio station had a frequency 20 times greater than this rate.
	(C) Harry Nyquist (1889-1976) specified this rate.
	(D) There is no good reason. The selection of 44.1 KHz was completely arbitrary.
	(E) CD audio uses a 44.1 KHz sampling rate because 44.1 KHz is slightly higher than the Nyquist rate for capturing frequencies up to the limit of human hearing (about 20 KHz).

4.___	Entropy is a topic from . . .

	(A) networking (B) nanotechnology (C) information theory			(D) cryptography (E) steganography

5.___	Entropy is measured in units of . . .

	(A) bits (B) bits per symbol (C) symbols per bit			(D) Hz per kilobyte (E) kilobytes per Hz

6.___	Which data stream has the greatest entropy?

	(A) uncompressed English text (B) uncompressed bitmap (.RAW or .BMP) (C) JPEG image (.JPG)			(D) uncompressed audio (.WAV) (E) a long sequence of hex FF characters

7.___	Which data stream is the easiest to encode with RLE compression?

	(A) uncompressed English text (B) uncompressed bitmap (.RAW or .BMP) (C) JPEG image (.JPG)			(D) uncompressed audio (.WAV) (E) a long sequence of hex FF characters

8.___	Noise in the digital setting refers to . . .

	(A) static (B) any loud sound, whether intentional or unintentional (C) any loud sound, but only if intentional (D) any loud sound, but only if unintentional (E) the mathematical difference between the signal that is actually stored and the intended signal

9.___	Sampling with 12-bit samples will make the S/N ratio be approximately . . .

	(A) 60 dBV (B) 72 dBV (C) 78 dBV			(D) 90 dBV (E) 96 dBV

10.___	A signal that is 3 dB down from an original power level of 200 watts would have what level?

	(A) 100 watts (B) 50 watts (C) 25 watts			(D) 10 watts (E) 5 watts

11.___	A signal that is 60 dB down from an original power level of 200 watts would have what level?

	(A) 2 milliwatts (B) 0.2 milliwatts (C) 0.02 milliwatts			(D) 0.002 milliwatts (E) 0.0002 milliwatts

12.___	The abbreviation dB stands for . . .

	(A) delta bravo (B) decibels (C) deleted bytes			(D) direct broadcasting (E) downband

13.___	“Muxing” and “demuxing” are . . .

	(A) technologies for expanding channel capacity by using multiple data streams on one channel
	(B) abbreviations for “multiplexing” and “de-multiplexing”
	(C) both A and B
	(D) legal but rarely used
	(E) illegal

14.___	A point-to-point network with 1700 nodes requires how many links?

	(A) 2,890,000 (B) 1,445,850 (C) 1,445,000			(D) 1,444,150 (E) 1,443,300

15.___	Add BABB + 0B0E in hex.

	(A) C559 (B) D5C9 (C) C5CA			(D) C5C9 (E) D6C9

16.___	All modern commercial computers use . . .

	(A) ternary (B) hex (C) decimal			(D) EBCDIC (Extended Binary Coded Decimal Interchange Code) (E) binary internally, but decimal or hex for display purposes

17.___	The integer 90 (in base 10) is equivalent to what 2-byte hex constant?

	(A) 005A (B) 006A (C) 0510			(D) 0050 (E) none of these

18.___	The term nybble usually refers to . . .

	(A) one hex digit (B) 2 bits (half of a hex digit) (C) one byte			(D) 1000 bytes (E) 1024 bytes

19.___

One difference between KHz and KB is that . . .

(A) KHz refers to digital storage, while KB refers to frequency
(B) KHz means 1024 cycles per second, while KB means 1000 bytes
(C) KHz means 1000 cycles per second, while KB means 1024 bytes
(D) the S/N ratios are different
(E) none of these

20.___

Which strategy for encryption and compression is most sensible, and why?

(A) Encrypt first, then compress. This will achieve the best compression ratio.

(B) Compress first, then encrypt. This will achieve the best compression ratio.

(D) Compress first; then skip the encryption step since packet sniffing never occurs anyway.

(E) Compress first, then encrypt to produce a low-entropy data stream. Finally, compress one more time to produce a high-entropy data stream.

21.___

“Quantization” is a synonym for . . .

(A) A/D conversion
(B) D/A conversion
(C) muxing

(D) S/N ratio
(E) none of these

22.___

Aliasing is caused by . . .

(A) defective camera lenses
(B) noise
(C) lack of noise

(D) oversampling
(E) none of these

23.___

In a Nissan TV ad, the car is whizzing along at a good clip, but the alloy wheels appear to be almost motionless. This visual artifact is an example of . . .

(A) noise
(B) aliasing
(C) buffer underrun

(D) buffer overflow
(E) numeric overflow

24.___

What type of compression protocol relies exclusively on repeat counts?

(A) JPEG
(B) MPEG
(C) MP3

(D) RLE
(E) ZIP

25.___

What do JPEG, MPEG, and MP3 have in common?

(A) all are lossy compression protocols
(B) all are lossless compression protocols
(C) all are video protocols

(D) all are analog
(E) none of these

26.___

The big-endian hex integer 0ABB5DCAF037 has what little-endian representation?

(A) 0ABB5DCAF037
(B) 730FACD5BBA0
(C) 37F0CA5DBB0A

(D) F0375DCA0ABB
(E) none of these

27.
(15 pts.)

Instructions: Match each phrase to its most closely associated term by writing the appropriate letter in the blank provided. Please do not draw lines this time, because there will be way too many lines to keep track of. Each term is used once and only once.

A. Written procedure for creating or interpreting a data stream

B. Method of error correction

C. Numbering system in which all place values are powers of 16

D. Boolean operation useful in cryptography for one-time pads

E. Method of error detection

F. Configuration (for example, of a network)

G. Falsification of header data, usually the sender’s IP address

H. The larger this is, the stronger the encryption will generally be

J. Extremely lightweight substance composed of a network of interconnected nanoparticles

K. Use of phony websites and/or e-mail to solicit unwilling victims to enter their passwords, bank account numbers, or other personal information

___ aerogel

___ spoofing

___ phishing

___ XOR

___ topology

___ protocol

___ hexadecimal (hex)

___ parity

___ Hamming code

___ key space

28.
(24½ pts.)

Course capstone question.

The time is July 1979, and Mr. Hansen has been out of high school for about a month. The Knack’s only Top 10 hit, “My Sharona,” is climbing to the top of the pop charts. Everyone is complaining about the outrageously high cost of gasoline (about $2.30 a gallon in 2007 dollars). Personal computers have been sold for a few years, but only hobbyists and geeks own them. The first IBM PC is still more than 2 years away. Hard drives are not yet available for personal computers; the first one, when it comes out next year, will be a 5 MB system that costs about $12,500 in 2007 dollars, or $2500 per MB. By 2007, the cost will have dropped to a twentieth of a cent per MB, which is a cost reduction by a factor of 5 million.

As you can see, some things in 1979 are almost exactly the same (the cost of gasoline) and some things are very different (the price and availability of computer gear). Do you have the picture? Good.

Pretend that you are employed as a senior electrical engineer and project leader for XYZW Corporation. Your firm wishes to transmit encrypted high-fidelity audio and video content, 24 hours per day, on behalf of its clients. This is to be a point-to-point link between New York City and Los Angeles only, with no other nodes required. The daily volume of data is estimated to be the equivalent of 30 MB uncompressed. In 2007, this would not be much of a challenge, something easily handled in a few seconds on a fast DSL connection, but in 1979, 30 MB seems like a large volume of data.

A compression ratio of approximately 5:1 can be achieved using the primitive digital compression technologies available. (Analog signals cannot be significantly compressed.) Instantaneous or real-time (“streaming”) throughput is not a requirement of the system.

There are three debates currently raging in the XYZW executive boardroom. These debate questions are labeled I, II, and III below.

I. Should XYZW use

(a) satellite uplink/downlink time leased from a satellite owner,

(b) microwave relay towers at several dozen mountain ridges across the U.S.,

(c) lower-frequency radio communication that would not require relay towers, or

(d) a leased telephone line?

Some background information may help. For choice (a), you need to know that satellite time is prohibitively expensive in 1979 and is usually used only for short connections (e.g., a network TV news report or a football game). For (b), using microwave links would require millions of dollars for tower construction and maintenance, or rental of existing towers, but the signals could be transmitted on a high-capacity channel in the GHz range with low probability of interference and a moderate probability of interception. For (c), XYZW can use a frequency band from 5.33 KHz to 6.22 KHz (usable bandwidth 0.89 KHz), but this would involve more potential for both interference (sunspots, lightning, etc.) and interception. Signals in this lower frequency range follow the curvature of the earth and are easily intercepted by anyone with a shortwave radio receiver, but the cost would be much lower than for choices (a) and (b) because there would be no need to pay for satellite time or to build relay towers. For choice (d), interference and interception would both be much less likely.

II. Should the transmission system design be analog or digital? At this point in American history, analog electronics technology is well developed and has decades of refinement behind it. Digital technologies are still somewhat unreliable, they are much more expensive than analog, and since all of XYZW’s audio and video content is still stored on analog tape, any use of digital transmission would require A/D and D/A conversion. Nobody in America owns a CD, and the “Red Book” engineering standard for audio CD production has not even been released yet.

III. How can the transmissions be protected from piracy (i.e., interception followed by illegal reselling)? The content is all copyrighted and must be protected from eavesdroppers. If any of XYZW’s clients discover that there is widespread piracy occurring, they will probably fire XYZW and may even sue the company for negligence. Interception is unlikely with choice I(d), somewhat likely with choice I(b), and very likely with choices I(a) and I(c).

The following table summarizes these complicated tradeoffs.

Choice (a)

(b)

(c)

(d)

Recurring infrastructure cost

extremely high

high

low

high

Engineering cost (analog)

low

low*

low

Implementation risk (analog)

high

medium

high

low

Engineering cost (digital)

medium

high

medium

Implementation risk (digital)

medium

high

medium

* However, results will probably be unacceptable because of low S/N ratio.

Implementation risks for an analog system are driven by the problem of piracy. While it is true that analog pirated copies are not as marketable as digital pirated copies, they are still a huge potential problem. There are some weak techniques to make analog signals harder for pirates to copy.

For a digital system, the risk of piracy can be reduced by encryption, but since this is 1979, there is a substantial risk that the system cannot be implemented at all. In other words, XYZW might spend several million dollars developing a system that ultimately has to be scrapped because the complexity overwhelms the ability of the engineers to manage the project. This risk is reduced when using choice (a) (satellite) or (d) (leased telephone line), because many of the implementation details would be handled by a telecommunications provider.

Try to decide what you will recommend to the XYZW executives. Of course, in 2007, the questions are all fairly easy to answer: I is a non-issue (we would use IP packets on the Internet, thus completely bypassing (a), (b), (c), and (d)), II is blindingly obvious (digital), and III is answered by using digital encryption with a sufficiently large key space. However, try to put yourself back in the year 1979 and imagine the engineering tradeoffs that would need to be weighed (the primary theme of our textbook).

Address all three broad questions (I, II, and III) on the reverse side after filling in the blanks below. The blanks are worth 1 point each, and the essay questions are worth 2½ points each.

IV. If high-fidelity audio has frequencies going all the way up to 20 KHz, what is the minimum sampling frequency needed to avoid aliasing errors? __________ Give a reason for your answer. __________ __________

V. Is it possible to transmit high-fidelity audio in real time using choice I(c)? __________ Keep in mind that the bandwidth is extremely narrow, leaving a maximum data throughput of only about 890 baud, which even with sophisticated encoding would be unlikely to exceed 3560 bits per second in 1979. If the samples are each 32 bits (16 bits times 2 channels), what is the maximum number of samples per second that could be transmitted? __________

VI. If you were to recommend I(c) with II(digital), remember that one additional problem you must face is noise caused by interference. After allocating some of the 3560 bits per second for error ____________ and/or error ____________ , there might be (for example) about 3000 bits per second left over to be used for transmitting the intended signal. To determine the actual number of usable bits, we would need to know the __________ to __________ ratio and the transmission power of the communications channel.

VII. The table above has a footnote saying that I(c) with II(analog) may produce unacceptable results. Are analog data streams compressible (circle one)? yes no not very well
Remember, we have a usable bandwidth of only 890 Hz with choice (c). With demuxing, we could send a 20 KHz baseband stereo audio signal at about 1/45 real time. (Translation into English: 45 seconds of transmission time would be needed for each second of content.) In 24 hours, we could therefore transmit __________ seconds of stereo audio content, which is the equivalent of about 300 MB. This meets the 30 MB daily requirement with plenty of room to spare, but analog transmissions in the low-frequency band are easy to intercept and highly susceptible to noise. You might have to send each selection 10 times in order to get a usable version!

VIII. One of your coworkers suggests using choice I(c) with II(digital). For question III, he (since almost all engineers are male in 1979) proposes sending a carrier signal at 3560 bps that appears to be a low-resolution digital audio loop of “My Sharona,” 24 hours a day, except with every 9th bit altered to carry the actual signal of interest. This is a feeble attempt at ________________ (a vocabulary word that starts with an “S”). The “My Sharona” protocol is doomed to failure, because the maximum signal rate would be only _________ bits per second, or about 4 MB per day, much too slow to meet the 30 MB requirement. A better plan would be to ____________ the signal at a ratio of 5:1, then ____________ it so that it cannot be pirated, forget about “My Sharona,” set aside about 560 bps for error correction, and use the remaining 3000 bps (which equals ________ bytes per second) to transmit the signal. The daily net throughput is therefore ________ MB (give answer to nearest tenth), which slightly exceeds the requirement. Note that there is not much capacity for future growth.

Should XYZW use (a) satellite uplink/downlink time leased from a satellite owner, (b) microwave relay towers at several dozen mountain ridges across the U.S., (c) lower-frequency radio communication that would not require relay towers, or (d) a leased telephone line? Explain your reasoning briefly.

II.

Should the transmission system design be analog or digital? Justify your answer.

III.

How can the transmissions be protected from piracy (i.e., interception followed by illegal reselling)?