MODD / Mr. Hansen	Name: _______________________________________
12/16/2010

Second Quarter Test (100 points)

1.	Multiplexing (the process of merging multiple data streams into a single broadband channel) is called __________ for short. The reverse process (decoding a broadband stream at, usually, gigabits per second into multiple data streams) is called __________ .
2.	Bandwidth is measured in units of ______________________________________ .
3.	Entropy refers to the amount of “real” _________________ that is present in a file or data stream. ZIP, JPG, and MPEG files all contain (circle one) less  more  the same  entropy as an English text file of the same byte size.
4.	Convert from big-endian to little-endian, assuming 16-bit words: FACE099148AF9BC03118.
5.	What does PKI stand for? ______________________________________________ Explain briefly what PKI refers to.
6.	In a PKI, every user in the user community must have both a _________ key and a _________ key.
7.	A ____________________ hash function is an example of a mathematical one-way function.
8.	What is the approximate effective throughput of a 10 megabit-per-second channel? Be sure to provide units in your answer.
9.	Explain why you do not divide by 8 to get the answer to #8.
10.	Hamming coding is an example of an algorithm for _______________  _______________ .
11.	Explain why simple parity checks do not allow us to detect the occurrence of two bit errors within the same byte.

12.	If we wish to send a file efficiently, with high security, is it better to (a) compress first, then encrypt, or (b) encrypt first, then compress? Explain in one or two sentences. For full credit, correctly use the word entropy at least once in your answer.
13.	What is a hash collision? (If you cannot recall, raise your hand, since you need to know in order to answer the next question. There will be a small point penalty.)
14.	Explain briefly what the “birthday problem” is and why it is related, in some way, to the notion of hash collisions. A technical discussion is not expected.
15.	A digital signature is a bitstream that proves (that is, with very high probability) that the sender of the message is indeed who he claims to be. Explain how a properly chosen hash function can make it essentially impossible for a hacker to forge a digital signature by changing, say, some of the bits in Karl’s digital signature so that the message appears to come from President Obama.
16.	The general technique of falsifying return-address information, alluded to in #15, is called ___________ . Is regular e-mail, that is, without digital signatures, susceptible to this problem? ____________
17.	Convert each of the following power ratios to dB:
	(a) 15,000 : 1
	(b) 4000 : 1
	(c) 20,000,000,000,000 : 1
	(d) 8 : 1
18.	Moore’s Law can be thought of as a _________ dB annual increase in computing capability at the same price level. Use whatever number you put in the blank to estimate the factor (not decibels) by which computing capability increases over a period of
	(a) 10 years
	(b) 15 years
	(c) 33 years (at which point all of you will be my current age or slightly older)
	(d) 45 years (the length of time since Mr. Morris entered the computer field)
19.	What is noise? Give the MODD definition, not the common definition used by non-MODDians (muggles).
20.	What does “S/N” stand for in the term “S/N ratio”? _____________________
21.	Compute the S/N ratio, in dBV, for audio sampling conducted at 32 bits per sample. ________
22.	In #21, if an audio source is sampled at 32 bps but later converted to an MP3 file, the file size will be _____________ by a factor of about 10 or 12. Explain very briefly (one short sentence) why the S/N ratio will decrease in the process.
23.	A/D conversion, also called ___________________________ (one word), is the process of converting ________ data into numbers. List 3 advantages of doing this:
	1.
	2.
	3.
24.	Compute the number of links in a full mesh network of 5 nodes.