@lightless 2014-10-17T08:36:43.000000Z 字数 4798 阅读 5884

Homework 1

computer network

Computer Networks - Chaper 1

Problem 1

Q:

Imagine that you have trained your St. Bernard, Bernie, to carry a box of three 8mm tapes instead of a flask of brandy. (When your disk fills up, you consider that an emergency.) These tapes each contain 7 gigabytes. The dog can travel to your side, wherever you may be, at 18 km/hour. For what range of distances does Bernie have a higher data rate than a transmission line whose data rate (excluding overhead) is 150 Mbps?

A:

18 k m / h o u r = 0.005 m / s

$18km/hour = 0.005m/s$

3 * 7 g i g a b y t e s / (x / 0.005) > 150 / 1024 G b p s

$3*7gigabytes/(x/0.005) > 150/1024Gbps$
so, we can get:

x > 5.73

$x > 5.73$

Problem 6

Q:

A client-server system uses a satellite network, with the satellite at a height of 40,000 km. What is the best-case delay in response to a request?

A:

Because of request and response both via satellite, so the sum of transfer path length is 160,000km. The speed of transfer is 300,000km/s.
so, the bese-case delay is:

4 * 40000 k m 300 , 000 k m / s = 533 m s e c

$\frac{4*40000km}{300,000km/s} = 533 msec$

Problem 10

Q:

A disadvantage of a broadcast subnet is the capacity wasted when multiple hosts attempt to access the channel at the same time. As a simplistic example, suppose that time is divided into discrete slots, with each of the n hosts attempting to use the channel with probability p during each slot. What fraction of the slots are wasted due to collisions?

A:

The probability of free is $(1-p)^n$
The probability of used but not congestion is $np(1-p)^{n-1}$
So, the fraction of the slots are wasted due to collisions is

P = 1 - (1 - p) n - n p (1 - p) n - 1

$P=1-(1-p)^n-np(1-p)^{n-1}$

Prbblem 20

Q:

A system has an n-layer protocol hierarchy. Applications generate messages of length M bytes. At each of the layers, an h-byte header is added. What fraction of the network bandwidth is filled with headers?

A:

From the question we know "each of the layer will add an h-byte header to the data", so we can get the answer is:

n h n h + M

$\frac{nh}{nh+M}$

Problem 27

Q:

How long was a bit on the original 802.3 standard in meters? Use a transmission speed of 10 Mbps and assume the propagation speed in coax is 2/3 the speed of light in vacuum.

A:

The waves speed in coaxial line is $200m/\mu sec$ ,so the answer is $20m$ .

Problem 28

Q:

An image is 1024 x 768 pixels with 3 bytes/pixel. Assume the image is uncompressed. How long does it take to transmit it over a 56-kbps modem channel? Over a 1-Mbps cable modem? Over a 10-Mbps Ethernet? Over 100-Mbps Ethernet?

A:

The image have $1024*768*3*8bit$ in totally.
If transmit it over a 56-kbps channel, the answer is:

t = 1024 * 768 * 3 * 8 b i t 56 * 10 3 b i t / s = 337.04 s

$t = \frac{1024*768*3*8bit}{56 * 10^3 bit/s} = 337.04s$
If transmit it over a 1-Mbps cable modem, the answer is:

t = 1024 * 768 * 3 * 8 b i t 1 * 10 6 b i t / s = 18.87 s

$t = \frac{1024*768*3*8bit}{1 * 10^6 bit/s} = 18.87s$
If transmit it over a 10-Mbps Ethernet, the answer is:

t = 1024 * 768 * 3 * 8 b i t 10 * 10 6 b i t / s = 1.89 s

$t = \frac{1024*768*3*8bit}{10 * 10^6 bit/s} = 1.89s$
If transmit it over a 100-Mbps Ethernet, the answer is:

t = 1024 * 768 * 3 * 8 b i t 100 * 10 6 b i t / s = 0.19 s

$t = \frac{1024*768*3*8bit}{100 * 10^6 bit/s} = 0.19s$

Data And Computer Communication - Chapter2

Problem 2

Q:

a. The French and Chinese prime ministers need to come to an agreement by telephone, but neither speaks the other’s language. Further, neither has on hand a translator that can translate to the language of the other. However, both prime ministers have English translators on their staffs. Draw a diagram similar to Figure 2.12 to depict the situation, and describe the interaction and each level.

b. Now suppose that the Chinese prime minister’s translator can translate only into Japanese and that the French prime minister has a German translator available.A translator between German and Japanese is available in Germany. Draw a new
diagram that reflects this arrangement and describe the hypothetical phone conversation.

A:

China	MITM	France
Chinese prime minister	none	French prime minister
Chinese translator	none	French translator
telphone	talk here!	telphone

Chinese prime minister talk to Chinese translator via Chinese, and French prime minister talk to French prime minister in French.
And the translator can talk to prime minister in their own language.
So the translators can speak to each other in English with telphone.

China	MITM	France
Chinese prime minister	none	French prime minister
Chinese translator	none	French translator
telphone	Janpanese -- German	telphone

Chinese must translate Chinese prime minister's language to Japanese and translate Japanese to Chinese.
German translator must translate German to Janpanese and translate Janpanese to German.
French translator must translate prime minister's language to German, and translate German to French.

Problem 3

Q:

List the major disadvantages with the layered approach to protocols.

A:

Layered protocols willcause delay or some ranges of distortion in the messages which will be transmitted.

Problem 4

Q:

Two blue armies are each poised on opposite hills preparing to attack a single red army in the valley. The red army can defeat either of the blue armies separately but will fail to defeat both blue armies if they attack simultaneously.The blue armies communicate via an unreliable communications system (a foot soldier). The commander with one of the blue armies would like to attack at noon. His problem is this: If he sends a message to the other blue army, ordering the attack, he cannot be sure it will get through. He could ask for acknowledgment, but that might not get through. Is there a protocol that the two blue armies can use to avoid defeat?

A:

No. Blue can not ensure the infomation be transfered in safely.